exceptionfactory commented on a change in pull request #5319:
URL: https://github.com/apache/nifi/pull/5319#discussion_r700546896



##########
File path: 
nifi-nar-bundles/nifi-standard-services/nifi-oauth2-provider-bundle/nifi-oauth2-provider-service/src/main/java/org/apache/nifi/oauth2/StandardOauth2AccessTokenProvider.java
##########
@@ -0,0 +1,300 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.nifi.oauth2;
+
+import com.fasterxml.jackson.databind.DeserializationFeature;
+import com.fasterxml.jackson.databind.ObjectMapper;
+import com.fasterxml.jackson.databind.PropertyNamingStrategies;
+import okhttp3.FormBody;
+import okhttp3.OkHttpClient;
+import okhttp3.Request;
+import okhttp3.RequestBody;
+import okhttp3.Response;
+import org.apache.nifi.annotation.documentation.CapabilityDescription;
+import org.apache.nifi.annotation.documentation.Tags;
+import org.apache.nifi.annotation.lifecycle.OnDisabled;
+import org.apache.nifi.annotation.lifecycle.OnEnabled;
+import org.apache.nifi.components.AllowableValue;
+import org.apache.nifi.components.PropertyDescriptor;
+import org.apache.nifi.components.ValidationContext;
+import org.apache.nifi.components.ValidationResult;
+import org.apache.nifi.components.Validator;
+import org.apache.nifi.controller.AbstractControllerService;
+import org.apache.nifi.controller.ConfigurationContext;
+import org.apache.nifi.expression.ExpressionLanguageScope;
+import org.apache.nifi.processor.exception.ProcessException;
+import org.apache.nifi.processor.util.StandardValidators;
+import org.apache.nifi.ssl.SSLContextService;
+
+import javax.net.ssl.SSLContext;
+import javax.net.ssl.X509TrustManager;
+import java.io.IOException;
+import java.io.UncheckedIOException;
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.Collection;
+import java.util.Collections;
+import java.util.List;
+
+@Tags({"oauth2", "provider", "authorization", "access token", "http"})
+@CapabilityDescription("Provides OAuth 2.0 access tokens that can be used as 
Bearer authorization header in HTTP requests." +
+    " Uses Resource Owner Password Credentials Grant.")
+public class StandardOauth2AccessTokenProvider extends 
AbstractControllerService implements OAuth2AccessTokenProvider {
+    public static final PropertyDescriptor AUTHORIZATION_SERVER_URL = new 
PropertyDescriptor.Builder()
+        .name("authorization-server-url")
+        .displayName("Authorization Server URL")
+        .description("The URL of the authorization server that issues access 
tokens.")
+        .required(true)
+        .addValidator(StandardValidators.URL_VALIDATOR)
+        .expressionLanguageSupported(ExpressionLanguageScope.VARIABLE_REGISTRY)
+        .build();
+
+    public static AllowableValue 
RESOURCE_OWNER_PASSWORD_CREDENTIALS_GRANT_TYPE = new AllowableValue(
+        "password",
+        "User Password",
+        "Resource Owner Password Credentials Grant. Used to access resources 
available to users. Requires username and password and usually Client ID and 
Client Secret"
+    );
+    public static AllowableValue CLIENT_CREDENTIALS_GRANT_TYPE = new 
AllowableValue(
+        "client_credentials",
+        "Client Credentials",
+        "Client Credentials Grant. Used to access resources available to 
clients. Requires Client ID and Client Secret"
+    );
+
+    public static final PropertyDescriptor GRANT_TYPE = new 
PropertyDescriptor.Builder()
+        .name("grant-type")
+        .displayName("Grant Type")
+        .description("The OAuth2 Grant Type to be used when acquiring an 
access token.")
+        .required(true)
+        .allowableValues(RESOURCE_OWNER_PASSWORD_CREDENTIALS_GRANT_TYPE, 
CLIENT_CREDENTIALS_GRANT_TYPE)
+        
.defaultValue(RESOURCE_OWNER_PASSWORD_CREDENTIALS_GRANT_TYPE.getValue())
+        .build();
+
+    public static final PropertyDescriptor USERNAME = new 
PropertyDescriptor.Builder()
+        .name("service-user-name")
+        .displayName("Username")
+        .description("Username on the service that is being accessed.")
+        .dependsOn(GRANT_TYPE, RESOURCE_OWNER_PASSWORD_CREDENTIALS_GRANT_TYPE)
+        .required(true)
+        .addValidator(StandardValidators.NON_BLANK_VALIDATOR)
+        .expressionLanguageSupported(ExpressionLanguageScope.VARIABLE_REGISTRY)
+        .build();
+
+    public static final PropertyDescriptor PASSWORD = new 
PropertyDescriptor.Builder()
+        .name("service-password")
+        .displayName("Password")
+        .description("Password for the username on the service that is being 
accessed.")
+        .dependsOn(GRANT_TYPE, RESOURCE_OWNER_PASSWORD_CREDENTIALS_GRANT_TYPE)
+        .required(true)
+        .sensitive(true)
+        .addValidator(StandardValidators.NON_BLANK_VALIDATOR)
+        .build();
+
+    public static final PropertyDescriptor CLIENT_ID = new 
PropertyDescriptor.Builder()
+        .name("client-id")
+        .displayName("Client ID")
+        .required(false)
+        .addValidator(StandardValidators.NON_BLANK_VALIDATOR)
+        .expressionLanguageSupported(ExpressionLanguageScope.VARIABLE_REGISTRY)
+        .build();
+
+    public static final PropertyDescriptor CLIENT_SECRET = new 
PropertyDescriptor.Builder()
+        .name("client-secret")
+        .displayName("Client secret")
+        .dependsOn(CLIENT_ID)
+        .required(true)
+        .sensitive(true)
+        .addValidator(StandardValidators.NON_BLANK_VALIDATOR)
+        .build();
+
+    public static final PropertyDescriptor SSL_CONTEXT = new 
PropertyDescriptor.Builder()
+        .name("ssl-context")
+        .displayName("SSL Context")
+        .addValidator(Validator.VALID)
+        .identifiesControllerService(SSLContextService.class)
+        .required(false)
+        .build();
+
+    private static final List<PropertyDescriptor> PROPERTIES = 
Collections.unmodifiableList(Arrays.asList(
+        AUTHORIZATION_SERVER_URL,
+        GRANT_TYPE,
+        USERNAME,
+        PASSWORD,
+        CLIENT_ID,
+        CLIENT_SECRET,
+        SSL_CONTEXT
+    ));
+
+    public static final ObjectMapper ACCESS_DETAILS_MAPPER = new ObjectMapper()
+        .configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false)
+        .setPropertyNamingStrategy(PropertyNamingStrategies.SNAKE_CASE);
+
+    private volatile String authorizationServerUrl;
+    private volatile OkHttpClient httpClient;
+
+    private volatile String grantType;
+    private volatile String username;
+    private volatile String password;
+    private volatile String clientId;
+    private volatile String clientSecret;
+
+    private volatile AccessToken accessDetails;
+
+    @Override
+    public List<PropertyDescriptor> getSupportedPropertyDescriptors() {
+        return PROPERTIES;
+    }
+
+    @OnEnabled
+    public void onEnabled(ConfigurationContext context) {
+        authorizationServerUrl = 
context.getProperty(AUTHORIZATION_SERVER_URL).evaluateAttributeExpressions().getValue();
+
+        httpClient = createHttpClient(context);
+
+        grantType = context.getProperty(GRANT_TYPE).getValue();
+        username = 
context.getProperty(USERNAME).evaluateAttributeExpressions().getValue();
+        password = context.getProperty(PASSWORD).getValue();
+        clientId = 
context.getProperty(CLIENT_ID).evaluateAttributeExpressions().getValue();
+        clientSecret = context.getProperty(CLIENT_SECRET).getValue();
+    }
+
+    @OnDisabled
+    public void onDisabled() {
+        accessDetails = null;
+    }
+
+    @Override
+    protected Collection<ValidationResult> customValidate(ValidationContext 
validationContext) {
+        final List<ValidationResult> validationResults = new 
ArrayList<>(super.customValidate(validationContext));
+
+        if (
+            
validationContext.getProperty(GRANT_TYPE).getValue().equals(CLIENT_CREDENTIALS_GRANT_TYPE.getValue())
+                && !validationContext.getProperty(CLIENT_ID).isSet()
+        ) {
+            validationResults.add(new 
ValidationResult.Builder().subject(CLIENT_ID.getDisplayName())
+                .valid(false)
+                .explanation(String.format(
+                    "When '%s' is set to '%s', '%s' is required",
+                    GRANT_TYPE.getDisplayName(),
+                    CLIENT_CREDENTIALS_GRANT_TYPE.getDisplayName(),
+                    CLIENT_ID.getDisplayName())
+                )
+                .build());
+        }
+
+        return validationResults;
+    }
+
+    protected OkHttpClient createHttpClient(ConfigurationContext context) {
+        OkHttpClient.Builder clientBuilder = new OkHttpClient.Builder();
+
+        SSLContextService sslService = 
context.getProperty(SSL_CONTEXT).asControllerService(SSLContextService.class);
+        if (sslService != null) {
+            final X509TrustManager trustManager = 
sslService.createTrustManager();
+            SSLContext sslContext = sslService.createContext();
+            clientBuilder.sslSocketFactory(sslContext.getSocketFactory(), 
trustManager);
+        }
+
+        return clientBuilder.build();
+    }
+
+    @Override
+    public AccessToken getAccessDetails() {
+        if (this.accessDetails == null) {
+            acquireAccessDetails();
+        } else if (this.accessDetails.isExpired()) {
+            if (this.accessDetails.getRefreshToken() == null) {
+                acquireAccessDetails();
+            } else {
+                try {
+                    refreshAccessDetails();
+                } catch (Exception e) {
+                    getLogger().info("Couldn't refresh access token.", e);
+                    acquireAccessDetails();
+                }
+            }
+        }
+
+        return accessDetails;
+    }
+
+    private void acquireAccessDetails() {
+        getLogger().debug("Getting a new access token.");
+
+        FormBody.Builder acquireTokenBuilder = new FormBody.Builder();
+
+        if 
(grantType.equals(RESOURCE_OWNER_PASSWORD_CREDENTIALS_GRANT_TYPE.getValue())) {
+            acquireTokenBuilder.add("grant_type", "password")
+                .add("username", username)
+                .add("password", password);
+        } else if (grantType.equals(CLIENT_CREDENTIALS_GRANT_TYPE.getValue())) 
{
+            acquireTokenBuilder.add("grant_type", "client_credentials");
+        }
+
+        if (clientId != null) {
+            acquireTokenBuilder.add("client_id", clientId);
+            acquireTokenBuilder.add("client_secret", clientSecret);
+        }
+
+        RequestBody acquireTokenRequestBody = acquireTokenBuilder.build();
+
+        Request acquireTokenRequest = new Request.Builder()
+            .url(authorizationServerUrl)
+            .post(acquireTokenRequestBody)
+            .build();
+
+        this.accessDetails = getAccessDetails(acquireTokenRequest);
+    }
+
+    private void refreshAccessDetails() {
+        getLogger().debug("Refreshing access token.");
+
+        FormBody.Builder refreshTokenBuilder = new FormBody.Builder()
+            .add("grant_type", "refresh_token")
+            .add("refresh_token", this.accessDetails.getRefreshToken());
+
+        if (clientId != null) {
+            refreshTokenBuilder.add("client_id", clientId);
+            refreshTokenBuilder.add("client_secret", clientSecret);
+        }
+
+        RequestBody refreshTokenRequestBody = refreshTokenBuilder.build();
+
+        Request refreshRequest = new Request.Builder()
+            .url(authorizationServerUrl)
+            .post(refreshTokenRequestBody)
+            .build();
+
+        this.accessDetails = getAccessDetails(refreshRequest);
+    }
+
+    private AccessToken getAccessDetails(Request newRequest) {
+        try {
+            Response response = httpClient.newCall(newRequest).execute();
+            String responseBody = response.body().string();

Review comment:
       Access to the ResponseBody should also be wrapped in a 
try-with-resources according to the OkHttp documentation.
   ```suggestion
               try (final ResponseBody responseBody : response.body()) {
                   final String responseBodyContent = responseBody.string();
               }
   ```

##########
File path: 
nifi-nar-bundles/nifi-standard-services/nifi-oauth2-provider-bundle/nifi-oauth2-provider-service/src/main/java/org/apache/nifi/oauth2/StandardOauth2AccessTokenProvider.java
##########
@@ -0,0 +1,300 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.nifi.oauth2;
+
+import com.fasterxml.jackson.databind.DeserializationFeature;
+import com.fasterxml.jackson.databind.ObjectMapper;
+import com.fasterxml.jackson.databind.PropertyNamingStrategies;
+import okhttp3.FormBody;
+import okhttp3.OkHttpClient;
+import okhttp3.Request;
+import okhttp3.RequestBody;
+import okhttp3.Response;
+import org.apache.nifi.annotation.documentation.CapabilityDescription;
+import org.apache.nifi.annotation.documentation.Tags;
+import org.apache.nifi.annotation.lifecycle.OnDisabled;
+import org.apache.nifi.annotation.lifecycle.OnEnabled;
+import org.apache.nifi.components.AllowableValue;
+import org.apache.nifi.components.PropertyDescriptor;
+import org.apache.nifi.components.ValidationContext;
+import org.apache.nifi.components.ValidationResult;
+import org.apache.nifi.components.Validator;
+import org.apache.nifi.controller.AbstractControllerService;
+import org.apache.nifi.controller.ConfigurationContext;
+import org.apache.nifi.expression.ExpressionLanguageScope;
+import org.apache.nifi.processor.exception.ProcessException;
+import org.apache.nifi.processor.util.StandardValidators;
+import org.apache.nifi.ssl.SSLContextService;
+
+import javax.net.ssl.SSLContext;
+import javax.net.ssl.X509TrustManager;
+import java.io.IOException;
+import java.io.UncheckedIOException;
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.Collection;
+import java.util.Collections;
+import java.util.List;
+
+@Tags({"oauth2", "provider", "authorization", "access token", "http"})
+@CapabilityDescription("Provides OAuth 2.0 access tokens that can be used as 
Bearer authorization header in HTTP requests." +
+    " Uses Resource Owner Password Credentials Grant.")
+public class StandardOauth2AccessTokenProvider extends 
AbstractControllerService implements OAuth2AccessTokenProvider {
+    public static final PropertyDescriptor AUTHORIZATION_SERVER_URL = new 
PropertyDescriptor.Builder()
+        .name("authorization-server-url")
+        .displayName("Authorization Server URL")
+        .description("The URL of the authorization server that issues access 
tokens.")
+        .required(true)
+        .addValidator(StandardValidators.URL_VALIDATOR)
+        .expressionLanguageSupported(ExpressionLanguageScope.VARIABLE_REGISTRY)
+        .build();
+
+    public static AllowableValue 
RESOURCE_OWNER_PASSWORD_CREDENTIALS_GRANT_TYPE = new AllowableValue(
+        "password",
+        "User Password",
+        "Resource Owner Password Credentials Grant. Used to access resources 
available to users. Requires username and password and usually Client ID and 
Client Secret"
+    );
+    public static AllowableValue CLIENT_CREDENTIALS_GRANT_TYPE = new 
AllowableValue(
+        "client_credentials",
+        "Client Credentials",
+        "Client Credentials Grant. Used to access resources available to 
clients. Requires Client ID and Client Secret"
+    );
+
+    public static final PropertyDescriptor GRANT_TYPE = new 
PropertyDescriptor.Builder()
+        .name("grant-type")
+        .displayName("Grant Type")
+        .description("The OAuth2 Grant Type to be used when acquiring an 
access token.")
+        .required(true)
+        .allowableValues(RESOURCE_OWNER_PASSWORD_CREDENTIALS_GRANT_TYPE, 
CLIENT_CREDENTIALS_GRANT_TYPE)
+        
.defaultValue(RESOURCE_OWNER_PASSWORD_CREDENTIALS_GRANT_TYPE.getValue())
+        .build();
+
+    public static final PropertyDescriptor USERNAME = new 
PropertyDescriptor.Builder()
+        .name("service-user-name")
+        .displayName("Username")
+        .description("Username on the service that is being accessed.")
+        .dependsOn(GRANT_TYPE, RESOURCE_OWNER_PASSWORD_CREDENTIALS_GRANT_TYPE)
+        .required(true)
+        .addValidator(StandardValidators.NON_BLANK_VALIDATOR)
+        .expressionLanguageSupported(ExpressionLanguageScope.VARIABLE_REGISTRY)
+        .build();
+
+    public static final PropertyDescriptor PASSWORD = new 
PropertyDescriptor.Builder()
+        .name("service-password")
+        .displayName("Password")
+        .description("Password for the username on the service that is being 
accessed.")
+        .dependsOn(GRANT_TYPE, RESOURCE_OWNER_PASSWORD_CREDENTIALS_GRANT_TYPE)
+        .required(true)
+        .sensitive(true)
+        .addValidator(StandardValidators.NON_BLANK_VALIDATOR)
+        .build();
+
+    public static final PropertyDescriptor CLIENT_ID = new 
PropertyDescriptor.Builder()
+        .name("client-id")
+        .displayName("Client ID")
+        .required(false)
+        .addValidator(StandardValidators.NON_BLANK_VALIDATOR)
+        .expressionLanguageSupported(ExpressionLanguageScope.VARIABLE_REGISTRY)
+        .build();
+
+    public static final PropertyDescriptor CLIENT_SECRET = new 
PropertyDescriptor.Builder()
+        .name("client-secret")
+        .displayName("Client secret")
+        .dependsOn(CLIENT_ID)
+        .required(true)
+        .sensitive(true)
+        .addValidator(StandardValidators.NON_BLANK_VALIDATOR)
+        .build();
+
+    public static final PropertyDescriptor SSL_CONTEXT = new 
PropertyDescriptor.Builder()
+        .name("ssl-context")
+        .displayName("SSL Context")
+        .addValidator(Validator.VALID)
+        .identifiesControllerService(SSLContextService.class)
+        .required(false)
+        .build();
+
+    private static final List<PropertyDescriptor> PROPERTIES = 
Collections.unmodifiableList(Arrays.asList(
+        AUTHORIZATION_SERVER_URL,
+        GRANT_TYPE,
+        USERNAME,
+        PASSWORD,
+        CLIENT_ID,
+        CLIENT_SECRET,
+        SSL_CONTEXT
+    ));
+
+    public static final ObjectMapper ACCESS_DETAILS_MAPPER = new ObjectMapper()
+        .configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false)
+        .setPropertyNamingStrategy(PropertyNamingStrategies.SNAKE_CASE);
+
+    private volatile String authorizationServerUrl;
+    private volatile OkHttpClient httpClient;
+
+    private volatile String grantType;
+    private volatile String username;
+    private volatile String password;
+    private volatile String clientId;
+    private volatile String clientSecret;
+
+    private volatile AccessToken accessDetails;
+
+    @Override
+    public List<PropertyDescriptor> getSupportedPropertyDescriptors() {
+        return PROPERTIES;
+    }
+
+    @OnEnabled
+    public void onEnabled(ConfigurationContext context) {
+        authorizationServerUrl = 
context.getProperty(AUTHORIZATION_SERVER_URL).evaluateAttributeExpressions().getValue();
+
+        httpClient = createHttpClient(context);
+
+        grantType = context.getProperty(GRANT_TYPE).getValue();
+        username = 
context.getProperty(USERNAME).evaluateAttributeExpressions().getValue();
+        password = context.getProperty(PASSWORD).getValue();
+        clientId = 
context.getProperty(CLIENT_ID).evaluateAttributeExpressions().getValue();
+        clientSecret = context.getProperty(CLIENT_SECRET).getValue();
+    }
+
+    @OnDisabled
+    public void onDisabled() {
+        accessDetails = null;
+    }
+
+    @Override
+    protected Collection<ValidationResult> customValidate(ValidationContext 
validationContext) {
+        final List<ValidationResult> validationResults = new 
ArrayList<>(super.customValidate(validationContext));
+
+        if (
+            
validationContext.getProperty(GRANT_TYPE).getValue().equals(CLIENT_CREDENTIALS_GRANT_TYPE.getValue())
+                && !validationContext.getProperty(CLIENT_ID).isSet()
+        ) {
+            validationResults.add(new 
ValidationResult.Builder().subject(CLIENT_ID.getDisplayName())
+                .valid(false)
+                .explanation(String.format(
+                    "When '%s' is set to '%s', '%s' is required",
+                    GRANT_TYPE.getDisplayName(),
+                    CLIENT_CREDENTIALS_GRANT_TYPE.getDisplayName(),
+                    CLIENT_ID.getDisplayName())
+                )
+                .build());
+        }
+
+        return validationResults;
+    }
+
+    protected OkHttpClient createHttpClient(ConfigurationContext context) {
+        OkHttpClient.Builder clientBuilder = new OkHttpClient.Builder();
+
+        SSLContextService sslService = 
context.getProperty(SSL_CONTEXT).asControllerService(SSLContextService.class);
+        if (sslService != null) {
+            final X509TrustManager trustManager = 
sslService.createTrustManager();
+            SSLContext sslContext = sslService.createContext();
+            clientBuilder.sslSocketFactory(sslContext.getSocketFactory(), 
trustManager);
+        }
+
+        return clientBuilder.build();
+    }
+
+    @Override
+    public AccessToken getAccessDetails() {
+        if (this.accessDetails == null) {
+            acquireAccessDetails();
+        } else if (this.accessDetails.isExpired()) {
+            if (this.accessDetails.getRefreshToken() == null) {
+                acquireAccessDetails();
+            } else {
+                try {
+                    refreshAccessDetails();
+                } catch (Exception e) {
+                    getLogger().info("Couldn't refresh access token.", e);
+                    acquireAccessDetails();
+                }
+            }
+        }
+
+        return accessDetails;
+    }
+
+    private void acquireAccessDetails() {
+        getLogger().debug("Getting a new access token.");
+
+        FormBody.Builder acquireTokenBuilder = new FormBody.Builder();
+
+        if 
(grantType.equals(RESOURCE_OWNER_PASSWORD_CREDENTIALS_GRANT_TYPE.getValue())) {
+            acquireTokenBuilder.add("grant_type", "password")
+                .add("username", username)
+                .add("password", password);
+        } else if (grantType.equals(CLIENT_CREDENTIALS_GRANT_TYPE.getValue())) 
{
+            acquireTokenBuilder.add("grant_type", "client_credentials");
+        }
+
+        if (clientId != null) {
+            acquireTokenBuilder.add("client_id", clientId);
+            acquireTokenBuilder.add("client_secret", clientSecret);
+        }
+
+        RequestBody acquireTokenRequestBody = acquireTokenBuilder.build();
+
+        Request acquireTokenRequest = new Request.Builder()
+            .url(authorizationServerUrl)
+            .post(acquireTokenRequestBody)
+            .build();
+
+        this.accessDetails = getAccessDetails(acquireTokenRequest);
+    }
+
+    private void refreshAccessDetails() {
+        getLogger().debug("Refreshing access token.");
+
+        FormBody.Builder refreshTokenBuilder = new FormBody.Builder()
+            .add("grant_type", "refresh_token")
+            .add("refresh_token", this.accessDetails.getRefreshToken());
+
+        if (clientId != null) {
+            refreshTokenBuilder.add("client_id", clientId);
+            refreshTokenBuilder.add("client_secret", clientSecret);
+        }
+
+        RequestBody refreshTokenRequestBody = refreshTokenBuilder.build();
+
+        Request refreshRequest = new Request.Builder()
+            .url(authorizationServerUrl)
+            .post(refreshTokenRequestBody)
+            .build();
+
+        this.accessDetails = getAccessDetails(refreshRequest);
+    }
+
+    private AccessToken getAccessDetails(Request newRequest) {

Review comment:
       ```suggestion
       private AccessToken getAccessDetails(final Request newRequest) {
   ```

##########
File path: 
nifi-nar-bundles/nifi-standard-services/nifi-oauth2-provider-bundle/nifi-oauth2-provider-service/src/main/java/org/apache/nifi/oauth2/StandardOauth2AccessTokenProvider.java
##########
@@ -0,0 +1,300 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.nifi.oauth2;
+
+import com.fasterxml.jackson.databind.DeserializationFeature;
+import com.fasterxml.jackson.databind.ObjectMapper;
+import com.fasterxml.jackson.databind.PropertyNamingStrategies;
+import okhttp3.FormBody;
+import okhttp3.OkHttpClient;
+import okhttp3.Request;
+import okhttp3.RequestBody;
+import okhttp3.Response;
+import org.apache.nifi.annotation.documentation.CapabilityDescription;
+import org.apache.nifi.annotation.documentation.Tags;
+import org.apache.nifi.annotation.lifecycle.OnDisabled;
+import org.apache.nifi.annotation.lifecycle.OnEnabled;
+import org.apache.nifi.components.AllowableValue;
+import org.apache.nifi.components.PropertyDescriptor;
+import org.apache.nifi.components.ValidationContext;
+import org.apache.nifi.components.ValidationResult;
+import org.apache.nifi.components.Validator;
+import org.apache.nifi.controller.AbstractControllerService;
+import org.apache.nifi.controller.ConfigurationContext;
+import org.apache.nifi.expression.ExpressionLanguageScope;
+import org.apache.nifi.processor.exception.ProcessException;
+import org.apache.nifi.processor.util.StandardValidators;
+import org.apache.nifi.ssl.SSLContextService;
+
+import javax.net.ssl.SSLContext;
+import javax.net.ssl.X509TrustManager;
+import java.io.IOException;
+import java.io.UncheckedIOException;
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.Collection;
+import java.util.Collections;
+import java.util.List;
+
+@Tags({"oauth2", "provider", "authorization", "access token", "http"})
+@CapabilityDescription("Provides OAuth 2.0 access tokens that can be used as 
Bearer authorization header in HTTP requests." +
+    " Uses Resource Owner Password Credentials Grant.")
+public class StandardOauth2AccessTokenProvider extends 
AbstractControllerService implements OAuth2AccessTokenProvider {
+    public static final PropertyDescriptor AUTHORIZATION_SERVER_URL = new 
PropertyDescriptor.Builder()
+        .name("authorization-server-url")
+        .displayName("Authorization Server URL")
+        .description("The URL of the authorization server that issues access 
tokens.")
+        .required(true)
+        .addValidator(StandardValidators.URL_VALIDATOR)
+        .expressionLanguageSupported(ExpressionLanguageScope.VARIABLE_REGISTRY)
+        .build();
+
+    public static AllowableValue 
RESOURCE_OWNER_PASSWORD_CREDENTIALS_GRANT_TYPE = new AllowableValue(
+        "password",
+        "User Password",
+        "Resource Owner Password Credentials Grant. Used to access resources 
available to users. Requires username and password and usually Client ID and 
Client Secret"
+    );
+    public static AllowableValue CLIENT_CREDENTIALS_GRANT_TYPE = new 
AllowableValue(
+        "client_credentials",
+        "Client Credentials",
+        "Client Credentials Grant. Used to access resources available to 
clients. Requires Client ID and Client Secret"
+    );
+
+    public static final PropertyDescriptor GRANT_TYPE = new 
PropertyDescriptor.Builder()
+        .name("grant-type")
+        .displayName("Grant Type")
+        .description("The OAuth2 Grant Type to be used when acquiring an 
access token.")
+        .required(true)
+        .allowableValues(RESOURCE_OWNER_PASSWORD_CREDENTIALS_GRANT_TYPE, 
CLIENT_CREDENTIALS_GRANT_TYPE)
+        
.defaultValue(RESOURCE_OWNER_PASSWORD_CREDENTIALS_GRANT_TYPE.getValue())
+        .build();
+
+    public static final PropertyDescriptor USERNAME = new 
PropertyDescriptor.Builder()
+        .name("service-user-name")
+        .displayName("Username")
+        .description("Username on the service that is being accessed.")
+        .dependsOn(GRANT_TYPE, RESOURCE_OWNER_PASSWORD_CREDENTIALS_GRANT_TYPE)
+        .required(true)
+        .addValidator(StandardValidators.NON_BLANK_VALIDATOR)
+        .expressionLanguageSupported(ExpressionLanguageScope.VARIABLE_REGISTRY)
+        .build();
+
+    public static final PropertyDescriptor PASSWORD = new 
PropertyDescriptor.Builder()
+        .name("service-password")
+        .displayName("Password")
+        .description("Password for the username on the service that is being 
accessed.")
+        .dependsOn(GRANT_TYPE, RESOURCE_OWNER_PASSWORD_CREDENTIALS_GRANT_TYPE)
+        .required(true)
+        .sensitive(true)
+        .addValidator(StandardValidators.NON_BLANK_VALIDATOR)
+        .build();
+
+    public static final PropertyDescriptor CLIENT_ID = new 
PropertyDescriptor.Builder()
+        .name("client-id")
+        .displayName("Client ID")
+        .required(false)
+        .addValidator(StandardValidators.NON_BLANK_VALIDATOR)
+        .expressionLanguageSupported(ExpressionLanguageScope.VARIABLE_REGISTRY)
+        .build();
+
+    public static final PropertyDescriptor CLIENT_SECRET = new 
PropertyDescriptor.Builder()
+        .name("client-secret")
+        .displayName("Client secret")
+        .dependsOn(CLIENT_ID)
+        .required(true)
+        .sensitive(true)
+        .addValidator(StandardValidators.NON_BLANK_VALIDATOR)
+        .build();
+
+    public static final PropertyDescriptor SSL_CONTEXT = new 
PropertyDescriptor.Builder()
+        .name("ssl-context")
+        .displayName("SSL Context")
+        .addValidator(Validator.VALID)
+        .identifiesControllerService(SSLContextService.class)
+        .required(false)
+        .build();
+
+    private static final List<PropertyDescriptor> PROPERTIES = 
Collections.unmodifiableList(Arrays.asList(
+        AUTHORIZATION_SERVER_URL,
+        GRANT_TYPE,
+        USERNAME,
+        PASSWORD,
+        CLIENT_ID,
+        CLIENT_SECRET,
+        SSL_CONTEXT
+    ));
+
+    public static final ObjectMapper ACCESS_DETAILS_MAPPER = new ObjectMapper()
+        .configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false)
+        .setPropertyNamingStrategy(PropertyNamingStrategies.SNAKE_CASE);
+
+    private volatile String authorizationServerUrl;
+    private volatile OkHttpClient httpClient;
+
+    private volatile String grantType;
+    private volatile String username;
+    private volatile String password;
+    private volatile String clientId;
+    private volatile String clientSecret;
+
+    private volatile AccessToken accessDetails;
+
+    @Override
+    public List<PropertyDescriptor> getSupportedPropertyDescriptors() {
+        return PROPERTIES;
+    }
+
+    @OnEnabled
+    public void onEnabled(ConfigurationContext context) {
+        authorizationServerUrl = 
context.getProperty(AUTHORIZATION_SERVER_URL).evaluateAttributeExpressions().getValue();
+
+        httpClient = createHttpClient(context);
+
+        grantType = context.getProperty(GRANT_TYPE).getValue();
+        username = 
context.getProperty(USERNAME).evaluateAttributeExpressions().getValue();
+        password = context.getProperty(PASSWORD).getValue();
+        clientId = 
context.getProperty(CLIENT_ID).evaluateAttributeExpressions().getValue();
+        clientSecret = context.getProperty(CLIENT_SECRET).getValue();
+    }
+
+    @OnDisabled
+    public void onDisabled() {
+        accessDetails = null;
+    }
+
+    @Override
+    protected Collection<ValidationResult> customValidate(ValidationContext 
validationContext) {
+        final List<ValidationResult> validationResults = new 
ArrayList<>(super.customValidate(validationContext));
+
+        if (
+            
validationContext.getProperty(GRANT_TYPE).getValue().equals(CLIENT_CREDENTIALS_GRANT_TYPE.getValue())
+                && !validationContext.getProperty(CLIENT_ID).isSet()
+        ) {
+            validationResults.add(new 
ValidationResult.Builder().subject(CLIENT_ID.getDisplayName())
+                .valid(false)
+                .explanation(String.format(
+                    "When '%s' is set to '%s', '%s' is required",
+                    GRANT_TYPE.getDisplayName(),
+                    CLIENT_CREDENTIALS_GRANT_TYPE.getDisplayName(),
+                    CLIENT_ID.getDisplayName())
+                )
+                .build());
+        }
+
+        return validationResults;
+    }
+
+    protected OkHttpClient createHttpClient(ConfigurationContext context) {
+        OkHttpClient.Builder clientBuilder = new OkHttpClient.Builder();
+
+        SSLContextService sslService = 
context.getProperty(SSL_CONTEXT).asControllerService(SSLContextService.class);
+        if (sslService != null) {
+            final X509TrustManager trustManager = 
sslService.createTrustManager();
+            SSLContext sslContext = sslService.createContext();
+            clientBuilder.sslSocketFactory(sslContext.getSocketFactory(), 
trustManager);
+        }
+
+        return clientBuilder.build();
+    }
+
+    @Override
+    public AccessToken getAccessDetails() {
+        if (this.accessDetails == null) {
+            acquireAccessDetails();
+        } else if (this.accessDetails.isExpired()) {
+            if (this.accessDetails.getRefreshToken() == null) {
+                acquireAccessDetails();
+            } else {
+                try {
+                    refreshAccessDetails();
+                } catch (Exception e) {
+                    getLogger().info("Couldn't refresh access token.", e);
+                    acquireAccessDetails();
+                }
+            }
+        }
+
+        return accessDetails;
+    }
+
+    private void acquireAccessDetails() {
+        getLogger().debug("Getting a new access token.");
+
+        FormBody.Builder acquireTokenBuilder = new FormBody.Builder();
+
+        if 
(grantType.equals(RESOURCE_OWNER_PASSWORD_CREDENTIALS_GRANT_TYPE.getValue())) {
+            acquireTokenBuilder.add("grant_type", "password")
+                .add("username", username)
+                .add("password", password);
+        } else if (grantType.equals(CLIENT_CREDENTIALS_GRANT_TYPE.getValue())) 
{
+            acquireTokenBuilder.add("grant_type", "client_credentials");
+        }
+
+        if (clientId != null) {
+            acquireTokenBuilder.add("client_id", clientId);
+            acquireTokenBuilder.add("client_secret", clientSecret);
+        }
+
+        RequestBody acquireTokenRequestBody = acquireTokenBuilder.build();
+
+        Request acquireTokenRequest = new Request.Builder()
+            .url(authorizationServerUrl)
+            .post(acquireTokenRequestBody)
+            .build();
+
+        this.accessDetails = getAccessDetails(acquireTokenRequest);
+    }
+
+    private void refreshAccessDetails() {
+        getLogger().debug("Refreshing access token.");
+
+        FormBody.Builder refreshTokenBuilder = new FormBody.Builder()
+            .add("grant_type", "refresh_token")
+            .add("refresh_token", this.accessDetails.getRefreshToken());
+
+        if (clientId != null) {
+            refreshTokenBuilder.add("client_id", clientId);
+            refreshTokenBuilder.add("client_secret", clientSecret);
+        }
+
+        RequestBody refreshTokenRequestBody = refreshTokenBuilder.build();
+
+        Request refreshRequest = new Request.Builder()
+            .url(authorizationServerUrl)
+            .post(refreshTokenRequestBody)
+            .build();
+
+        this.accessDetails = getAccessDetails(refreshRequest);
+    }
+
+    private AccessToken getAccessDetails(Request newRequest) {
+        try {
+            Response response = httpClient.newCall(newRequest).execute();

Review comment:
       This is also an issue on the current service implementation, but this 
call should be wrapped in a try-with-resources since `Response` implements 
`Closeable` as described in the [OkHttp Response 
documentation](https://square.github.io/okhttp/4.x/okhttp/okhttp3/-response-body/).

##########
File path: 
nifi-nar-bundles/nifi-standard-services/nifi-oauth2-provider-bundle/nifi-oauth2-provider-service/src/main/java/org/apache/nifi/oauth2/StandardOauth2AccessTokenProvider.java
##########
@@ -0,0 +1,300 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.nifi.oauth2;
+
+import com.fasterxml.jackson.databind.DeserializationFeature;
+import com.fasterxml.jackson.databind.ObjectMapper;
+import com.fasterxml.jackson.databind.PropertyNamingStrategies;
+import okhttp3.FormBody;
+import okhttp3.OkHttpClient;
+import okhttp3.Request;
+import okhttp3.RequestBody;
+import okhttp3.Response;
+import org.apache.nifi.annotation.documentation.CapabilityDescription;
+import org.apache.nifi.annotation.documentation.Tags;
+import org.apache.nifi.annotation.lifecycle.OnDisabled;
+import org.apache.nifi.annotation.lifecycle.OnEnabled;
+import org.apache.nifi.components.AllowableValue;
+import org.apache.nifi.components.PropertyDescriptor;
+import org.apache.nifi.components.ValidationContext;
+import org.apache.nifi.components.ValidationResult;
+import org.apache.nifi.components.Validator;
+import org.apache.nifi.controller.AbstractControllerService;
+import org.apache.nifi.controller.ConfigurationContext;
+import org.apache.nifi.expression.ExpressionLanguageScope;
+import org.apache.nifi.processor.exception.ProcessException;
+import org.apache.nifi.processor.util.StandardValidators;
+import org.apache.nifi.ssl.SSLContextService;
+
+import javax.net.ssl.SSLContext;
+import javax.net.ssl.X509TrustManager;
+import java.io.IOException;
+import java.io.UncheckedIOException;
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.Collection;
+import java.util.Collections;
+import java.util.List;
+
+@Tags({"oauth2", "provider", "authorization", "access token", "http"})
+@CapabilityDescription("Provides OAuth 2.0 access tokens that can be used as 
Bearer authorization header in HTTP requests." +
+    " Uses Resource Owner Password Credentials Grant.")
+public class StandardOauth2AccessTokenProvider extends 
AbstractControllerService implements OAuth2AccessTokenProvider {
+    public static final PropertyDescriptor AUTHORIZATION_SERVER_URL = new 
PropertyDescriptor.Builder()
+        .name("authorization-server-url")
+        .displayName("Authorization Server URL")
+        .description("The URL of the authorization server that issues access 
tokens.")
+        .required(true)
+        .addValidator(StandardValidators.URL_VALIDATOR)
+        .expressionLanguageSupported(ExpressionLanguageScope.VARIABLE_REGISTRY)
+        .build();
+
+    public static AllowableValue 
RESOURCE_OWNER_PASSWORD_CREDENTIALS_GRANT_TYPE = new AllowableValue(
+        "password",
+        "User Password",
+        "Resource Owner Password Credentials Grant. Used to access resources 
available to users. Requires username and password and usually Client ID and 
Client Secret"
+    );
+    public static AllowableValue CLIENT_CREDENTIALS_GRANT_TYPE = new 
AllowableValue(
+        "client_credentials",
+        "Client Credentials",
+        "Client Credentials Grant. Used to access resources available to 
clients. Requires Client ID and Client Secret"
+    );
+
+    public static final PropertyDescriptor GRANT_TYPE = new 
PropertyDescriptor.Builder()
+        .name("grant-type")
+        .displayName("Grant Type")
+        .description("The OAuth2 Grant Type to be used when acquiring an 
access token.")
+        .required(true)
+        .allowableValues(RESOURCE_OWNER_PASSWORD_CREDENTIALS_GRANT_TYPE, 
CLIENT_CREDENTIALS_GRANT_TYPE)
+        
.defaultValue(RESOURCE_OWNER_PASSWORD_CREDENTIALS_GRANT_TYPE.getValue())
+        .build();
+
+    public static final PropertyDescriptor USERNAME = new 
PropertyDescriptor.Builder()
+        .name("service-user-name")
+        .displayName("Username")
+        .description("Username on the service that is being accessed.")
+        .dependsOn(GRANT_TYPE, RESOURCE_OWNER_PASSWORD_CREDENTIALS_GRANT_TYPE)
+        .required(true)
+        .addValidator(StandardValidators.NON_BLANK_VALIDATOR)
+        .expressionLanguageSupported(ExpressionLanguageScope.VARIABLE_REGISTRY)
+        .build();
+
+    public static final PropertyDescriptor PASSWORD = new 
PropertyDescriptor.Builder()
+        .name("service-password")
+        .displayName("Password")
+        .description("Password for the username on the service that is being 
accessed.")
+        .dependsOn(GRANT_TYPE, RESOURCE_OWNER_PASSWORD_CREDENTIALS_GRANT_TYPE)
+        .required(true)
+        .sensitive(true)
+        .addValidator(StandardValidators.NON_BLANK_VALIDATOR)
+        .build();
+
+    public static final PropertyDescriptor CLIENT_ID = new 
PropertyDescriptor.Builder()
+        .name("client-id")
+        .displayName("Client ID")
+        .required(false)
+        .addValidator(StandardValidators.NON_BLANK_VALIDATOR)
+        .expressionLanguageSupported(ExpressionLanguageScope.VARIABLE_REGISTRY)
+        .build();
+
+    public static final PropertyDescriptor CLIENT_SECRET = new 
PropertyDescriptor.Builder()
+        .name("client-secret")
+        .displayName("Client secret")
+        .dependsOn(CLIENT_ID)
+        .required(true)
+        .sensitive(true)
+        .addValidator(StandardValidators.NON_BLANK_VALIDATOR)
+        .build();
+
+    public static final PropertyDescriptor SSL_CONTEXT = new 
PropertyDescriptor.Builder()
+        .name("ssl-context")
+        .displayName("SSL Context")
+        .addValidator(Validator.VALID)
+        .identifiesControllerService(SSLContextService.class)
+        .required(false)
+        .build();
+
+    private static final List<PropertyDescriptor> PROPERTIES = 
Collections.unmodifiableList(Arrays.asList(
+        AUTHORIZATION_SERVER_URL,
+        GRANT_TYPE,
+        USERNAME,
+        PASSWORD,
+        CLIENT_ID,
+        CLIENT_SECRET,
+        SSL_CONTEXT
+    ));
+
+    public static final ObjectMapper ACCESS_DETAILS_MAPPER = new ObjectMapper()
+        .configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false)
+        .setPropertyNamingStrategy(PropertyNamingStrategies.SNAKE_CASE);
+
+    private volatile String authorizationServerUrl;
+    private volatile OkHttpClient httpClient;
+
+    private volatile String grantType;
+    private volatile String username;
+    private volatile String password;
+    private volatile String clientId;
+    private volatile String clientSecret;
+
+    private volatile AccessToken accessDetails;
+
+    @Override
+    public List<PropertyDescriptor> getSupportedPropertyDescriptors() {
+        return PROPERTIES;
+    }
+
+    @OnEnabled
+    public void onEnabled(ConfigurationContext context) {
+        authorizationServerUrl = 
context.getProperty(AUTHORIZATION_SERVER_URL).evaluateAttributeExpressions().getValue();
+
+        httpClient = createHttpClient(context);
+
+        grantType = context.getProperty(GRANT_TYPE).getValue();
+        username = 
context.getProperty(USERNAME).evaluateAttributeExpressions().getValue();
+        password = context.getProperty(PASSWORD).getValue();
+        clientId = 
context.getProperty(CLIENT_ID).evaluateAttributeExpressions().getValue();
+        clientSecret = context.getProperty(CLIENT_SECRET).getValue();
+    }
+
+    @OnDisabled
+    public void onDisabled() {
+        accessDetails = null;
+    }
+
+    @Override
+    protected Collection<ValidationResult> customValidate(ValidationContext 
validationContext) {
+        final List<ValidationResult> validationResults = new 
ArrayList<>(super.customValidate(validationContext));
+
+        if (
+            
validationContext.getProperty(GRANT_TYPE).getValue().equals(CLIENT_CREDENTIALS_GRANT_TYPE.getValue())
+                && !validationContext.getProperty(CLIENT_ID).isSet()
+        ) {
+            validationResults.add(new 
ValidationResult.Builder().subject(CLIENT_ID.getDisplayName())
+                .valid(false)
+                .explanation(String.format(
+                    "When '%s' is set to '%s', '%s' is required",
+                    GRANT_TYPE.getDisplayName(),
+                    CLIENT_CREDENTIALS_GRANT_TYPE.getDisplayName(),
+                    CLIENT_ID.getDisplayName())
+                )
+                .build());
+        }
+
+        return validationResults;
+    }
+
+    protected OkHttpClient createHttpClient(ConfigurationContext context) {
+        OkHttpClient.Builder clientBuilder = new OkHttpClient.Builder();
+
+        SSLContextService sslService = 
context.getProperty(SSL_CONTEXT).asControllerService(SSLContextService.class);
+        if (sslService != null) {
+            final X509TrustManager trustManager = 
sslService.createTrustManager();
+            SSLContext sslContext = sslService.createContext();
+            clientBuilder.sslSocketFactory(sslContext.getSocketFactory(), 
trustManager);
+        }
+
+        return clientBuilder.build();
+    }
+
+    @Override
+    public AccessToken getAccessDetails() {
+        if (this.accessDetails == null) {
+            acquireAccessDetails();
+        } else if (this.accessDetails.isExpired()) {
+            if (this.accessDetails.getRefreshToken() == null) {
+                acquireAccessDetails();
+            } else {
+                try {
+                    refreshAccessDetails();
+                } catch (Exception e) {
+                    getLogger().info("Couldn't refresh access token.", e);

Review comment:
       If the refresh request throws an Exception, the object still retains a 
reference to the expired Access Token. If the subsequent call to 
`acquireAccessDetails()` also throws an Exception, it looks like the class will 
retain a reference to a refreshToken that is potentially invalid, does that 
sounds possible? One option would be setting `accessDetails = null` in this 
catch block, which would cause subsequent calls to `getAccessDetails()` to call 
`acquireAccessDetails()` instead of going down this path again.

##########
File path: 
nifi-nar-bundles/nifi-standard-services/nifi-oauth2-provider-bundle/nifi-oauth2-provider-service/src/main/java/org/apache/nifi/oauth2/StandardOauth2AccessTokenProvider.java
##########
@@ -0,0 +1,300 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.nifi.oauth2;
+
+import com.fasterxml.jackson.databind.DeserializationFeature;
+import com.fasterxml.jackson.databind.ObjectMapper;
+import com.fasterxml.jackson.databind.PropertyNamingStrategies;
+import okhttp3.FormBody;
+import okhttp3.OkHttpClient;
+import okhttp3.Request;
+import okhttp3.RequestBody;
+import okhttp3.Response;
+import org.apache.nifi.annotation.documentation.CapabilityDescription;
+import org.apache.nifi.annotation.documentation.Tags;
+import org.apache.nifi.annotation.lifecycle.OnDisabled;
+import org.apache.nifi.annotation.lifecycle.OnEnabled;
+import org.apache.nifi.components.AllowableValue;
+import org.apache.nifi.components.PropertyDescriptor;
+import org.apache.nifi.components.ValidationContext;
+import org.apache.nifi.components.ValidationResult;
+import org.apache.nifi.components.Validator;
+import org.apache.nifi.controller.AbstractControllerService;
+import org.apache.nifi.controller.ConfigurationContext;
+import org.apache.nifi.expression.ExpressionLanguageScope;
+import org.apache.nifi.processor.exception.ProcessException;
+import org.apache.nifi.processor.util.StandardValidators;
+import org.apache.nifi.ssl.SSLContextService;
+
+import javax.net.ssl.SSLContext;
+import javax.net.ssl.X509TrustManager;
+import java.io.IOException;
+import java.io.UncheckedIOException;
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.Collection;
+import java.util.Collections;
+import java.util.List;
+
+@Tags({"oauth2", "provider", "authorization", "access token", "http"})
+@CapabilityDescription("Provides OAuth 2.0 access tokens that can be used as 
Bearer authorization header in HTTP requests." +
+    " Uses Resource Owner Password Credentials Grant.")
+public class StandardOauth2AccessTokenProvider extends 
AbstractControllerService implements OAuth2AccessTokenProvider {
+    public static final PropertyDescriptor AUTHORIZATION_SERVER_URL = new 
PropertyDescriptor.Builder()
+        .name("authorization-server-url")
+        .displayName("Authorization Server URL")
+        .description("The URL of the authorization server that issues access 
tokens.")
+        .required(true)
+        .addValidator(StandardValidators.URL_VALIDATOR)
+        .expressionLanguageSupported(ExpressionLanguageScope.VARIABLE_REGISTRY)
+        .build();
+
+    public static AllowableValue 
RESOURCE_OWNER_PASSWORD_CREDENTIALS_GRANT_TYPE = new AllowableValue(
+        "password",
+        "User Password",
+        "Resource Owner Password Credentials Grant. Used to access resources 
available to users. Requires username and password and usually Client ID and 
Client Secret"
+    );
+    public static AllowableValue CLIENT_CREDENTIALS_GRANT_TYPE = new 
AllowableValue(
+        "client_credentials",
+        "Client Credentials",
+        "Client Credentials Grant. Used to access resources available to 
clients. Requires Client ID and Client Secret"
+    );
+
+    public static final PropertyDescriptor GRANT_TYPE = new 
PropertyDescriptor.Builder()
+        .name("grant-type")
+        .displayName("Grant Type")
+        .description("The OAuth2 Grant Type to be used when acquiring an 
access token.")
+        .required(true)
+        .allowableValues(RESOURCE_OWNER_PASSWORD_CREDENTIALS_GRANT_TYPE, 
CLIENT_CREDENTIALS_GRANT_TYPE)
+        
.defaultValue(RESOURCE_OWNER_PASSWORD_CREDENTIALS_GRANT_TYPE.getValue())
+        .build();
+
+    public static final PropertyDescriptor USERNAME = new 
PropertyDescriptor.Builder()
+        .name("service-user-name")
+        .displayName("Username")
+        .description("Username on the service that is being accessed.")
+        .dependsOn(GRANT_TYPE, RESOURCE_OWNER_PASSWORD_CREDENTIALS_GRANT_TYPE)
+        .required(true)
+        .addValidator(StandardValidators.NON_BLANK_VALIDATOR)
+        .expressionLanguageSupported(ExpressionLanguageScope.VARIABLE_REGISTRY)
+        .build();
+
+    public static final PropertyDescriptor PASSWORD = new 
PropertyDescriptor.Builder()
+        .name("service-password")
+        .displayName("Password")
+        .description("Password for the username on the service that is being 
accessed.")
+        .dependsOn(GRANT_TYPE, RESOURCE_OWNER_PASSWORD_CREDENTIALS_GRANT_TYPE)
+        .required(true)
+        .sensitive(true)
+        .addValidator(StandardValidators.NON_BLANK_VALIDATOR)
+        .build();
+
+    public static final PropertyDescriptor CLIENT_ID = new 
PropertyDescriptor.Builder()
+        .name("client-id")
+        .displayName("Client ID")
+        .required(false)
+        .addValidator(StandardValidators.NON_BLANK_VALIDATOR)
+        .expressionLanguageSupported(ExpressionLanguageScope.VARIABLE_REGISTRY)
+        .build();
+
+    public static final PropertyDescriptor CLIENT_SECRET = new 
PropertyDescriptor.Builder()
+        .name("client-secret")
+        .displayName("Client secret")
+        .dependsOn(CLIENT_ID)
+        .required(true)
+        .sensitive(true)
+        .addValidator(StandardValidators.NON_BLANK_VALIDATOR)
+        .build();
+
+    public static final PropertyDescriptor SSL_CONTEXT = new 
PropertyDescriptor.Builder()
+        .name("ssl-context")
+        .displayName("SSL Context")
+        .addValidator(Validator.VALID)
+        .identifiesControllerService(SSLContextService.class)
+        .required(false)
+        .build();
+
+    private static final List<PropertyDescriptor> PROPERTIES = 
Collections.unmodifiableList(Arrays.asList(
+        AUTHORIZATION_SERVER_URL,
+        GRANT_TYPE,
+        USERNAME,
+        PASSWORD,
+        CLIENT_ID,
+        CLIENT_SECRET,
+        SSL_CONTEXT
+    ));
+
+    public static final ObjectMapper ACCESS_DETAILS_MAPPER = new ObjectMapper()
+        .configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false)
+        .setPropertyNamingStrategy(PropertyNamingStrategies.SNAKE_CASE);
+
+    private volatile String authorizationServerUrl;
+    private volatile OkHttpClient httpClient;
+
+    private volatile String grantType;
+    private volatile String username;
+    private volatile String password;
+    private volatile String clientId;
+    private volatile String clientSecret;
+
+    private volatile AccessToken accessDetails;
+
+    @Override
+    public List<PropertyDescriptor> getSupportedPropertyDescriptors() {
+        return PROPERTIES;
+    }
+
+    @OnEnabled
+    public void onEnabled(ConfigurationContext context) {
+        authorizationServerUrl = 
context.getProperty(AUTHORIZATION_SERVER_URL).evaluateAttributeExpressions().getValue();
+
+        httpClient = createHttpClient(context);
+
+        grantType = context.getProperty(GRANT_TYPE).getValue();
+        username = 
context.getProperty(USERNAME).evaluateAttributeExpressions().getValue();
+        password = context.getProperty(PASSWORD).getValue();
+        clientId = 
context.getProperty(CLIENT_ID).evaluateAttributeExpressions().getValue();
+        clientSecret = context.getProperty(CLIENT_SECRET).getValue();
+    }
+
+    @OnDisabled
+    public void onDisabled() {
+        accessDetails = null;
+    }
+
+    @Override
+    protected Collection<ValidationResult> customValidate(ValidationContext 
validationContext) {
+        final List<ValidationResult> validationResults = new 
ArrayList<>(super.customValidate(validationContext));
+
+        if (
+            
validationContext.getProperty(GRANT_TYPE).getValue().equals(CLIENT_CREDENTIALS_GRANT_TYPE.getValue())
+                && !validationContext.getProperty(CLIENT_ID).isSet()
+        ) {
+            validationResults.add(new 
ValidationResult.Builder().subject(CLIENT_ID.getDisplayName())
+                .valid(false)
+                .explanation(String.format(
+                    "When '%s' is set to '%s', '%s' is required",
+                    GRANT_TYPE.getDisplayName(),
+                    CLIENT_CREDENTIALS_GRANT_TYPE.getDisplayName(),
+                    CLIENT_ID.getDisplayName())
+                )
+                .build());
+        }
+
+        return validationResults;
+    }
+
+    protected OkHttpClient createHttpClient(ConfigurationContext context) {
+        OkHttpClient.Builder clientBuilder = new OkHttpClient.Builder();
+
+        SSLContextService sslService = 
context.getProperty(SSL_CONTEXT).asControllerService(SSLContextService.class);
+        if (sslService != null) {
+            final X509TrustManager trustManager = 
sslService.createTrustManager();
+            SSLContext sslContext = sslService.createContext();
+            clientBuilder.sslSocketFactory(sslContext.getSocketFactory(), 
trustManager);
+        }
+
+        return clientBuilder.build();
+    }
+
+    @Override
+    public AccessToken getAccessDetails() {
+        if (this.accessDetails == null) {
+            acquireAccessDetails();
+        } else if (this.accessDetails.isExpired()) {
+            if (this.accessDetails.getRefreshToken() == null) {
+                acquireAccessDetails();
+            } else {
+                try {
+                    refreshAccessDetails();
+                } catch (Exception e) {
+                    getLogger().info("Couldn't refresh access token.", e);
+                    acquireAccessDetails();
+                }
+            }
+        }
+
+        return accessDetails;
+    }
+
+    private void acquireAccessDetails() {
+        getLogger().debug("Getting a new access token.");
+
+        FormBody.Builder acquireTokenBuilder = new FormBody.Builder();
+
+        if 
(grantType.equals(RESOURCE_OWNER_PASSWORD_CREDENTIALS_GRANT_TYPE.getValue())) {
+            acquireTokenBuilder.add("grant_type", "password")
+                .add("username", username)
+                .add("password", password);
+        } else if (grantType.equals(CLIENT_CREDENTIALS_GRANT_TYPE.getValue())) 
{
+            acquireTokenBuilder.add("grant_type", "client_credentials");
+        }
+
+        if (clientId != null) {
+            acquireTokenBuilder.add("client_id", clientId);
+            acquireTokenBuilder.add("client_secret", clientSecret);
+        }
+
+        RequestBody acquireTokenRequestBody = acquireTokenBuilder.build();
+
+        Request acquireTokenRequest = new Request.Builder()
+            .url(authorizationServerUrl)
+            .post(acquireTokenRequestBody)
+            .build();
+
+        this.accessDetails = getAccessDetails(acquireTokenRequest);
+    }
+
+    private void refreshAccessDetails() {
+        getLogger().debug("Refreshing access token.");
+
+        FormBody.Builder refreshTokenBuilder = new FormBody.Builder()
+            .add("grant_type", "refresh_token")
+            .add("refresh_token", this.accessDetails.getRefreshToken());
+
+        if (clientId != null) {
+            refreshTokenBuilder.add("client_id", clientId);
+            refreshTokenBuilder.add("client_secret", clientSecret);
+        }
+
+        RequestBody refreshTokenRequestBody = refreshTokenBuilder.build();
+
+        Request refreshRequest = new Request.Builder()
+            .url(authorizationServerUrl)
+            .post(refreshTokenRequestBody)
+            .build();
+
+        this.accessDetails = getAccessDetails(refreshRequest);
+    }
+
+    private AccessToken getAccessDetails(Request newRequest) {
+        try {
+            Response response = httpClient.newCall(newRequest).execute();
+            String responseBody = response.body().string();
+            if (response.code() != 200) {

Review comment:
       This could be simplified to `!response.isSuccessful()`.

##########
File path: 
nifi-nar-bundles/nifi-standard-services/nifi-oauth2-provider-bundle/nifi-oauth2-provider-service/src/main/java/org/apache/nifi/oauth2/StandardOauth2AccessTokenProvider.java
##########
@@ -0,0 +1,300 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.nifi.oauth2;
+
+import com.fasterxml.jackson.databind.DeserializationFeature;
+import com.fasterxml.jackson.databind.ObjectMapper;
+import com.fasterxml.jackson.databind.PropertyNamingStrategies;
+import okhttp3.FormBody;
+import okhttp3.OkHttpClient;
+import okhttp3.Request;
+import okhttp3.RequestBody;
+import okhttp3.Response;
+import org.apache.nifi.annotation.documentation.CapabilityDescription;
+import org.apache.nifi.annotation.documentation.Tags;
+import org.apache.nifi.annotation.lifecycle.OnDisabled;
+import org.apache.nifi.annotation.lifecycle.OnEnabled;
+import org.apache.nifi.components.AllowableValue;
+import org.apache.nifi.components.PropertyDescriptor;
+import org.apache.nifi.components.ValidationContext;
+import org.apache.nifi.components.ValidationResult;
+import org.apache.nifi.components.Validator;
+import org.apache.nifi.controller.AbstractControllerService;
+import org.apache.nifi.controller.ConfigurationContext;
+import org.apache.nifi.expression.ExpressionLanguageScope;
+import org.apache.nifi.processor.exception.ProcessException;
+import org.apache.nifi.processor.util.StandardValidators;
+import org.apache.nifi.ssl.SSLContextService;
+
+import javax.net.ssl.SSLContext;
+import javax.net.ssl.X509TrustManager;
+import java.io.IOException;
+import java.io.UncheckedIOException;
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.Collection;
+import java.util.Collections;
+import java.util.List;
+
+@Tags({"oauth2", "provider", "authorization", "access token", "http"})
+@CapabilityDescription("Provides OAuth 2.0 access tokens that can be used as 
Bearer authorization header in HTTP requests." +
+    " Uses Resource Owner Password Credentials Grant.")
+public class StandardOauth2AccessTokenProvider extends 
AbstractControllerService implements OAuth2AccessTokenProvider {
+    public static final PropertyDescriptor AUTHORIZATION_SERVER_URL = new 
PropertyDescriptor.Builder()
+        .name("authorization-server-url")
+        .displayName("Authorization Server URL")
+        .description("The URL of the authorization server that issues access 
tokens.")
+        .required(true)
+        .addValidator(StandardValidators.URL_VALIDATOR)
+        .expressionLanguageSupported(ExpressionLanguageScope.VARIABLE_REGISTRY)
+        .build();
+
+    public static AllowableValue 
RESOURCE_OWNER_PASSWORD_CREDENTIALS_GRANT_TYPE = new AllowableValue(
+        "password",
+        "User Password",
+        "Resource Owner Password Credentials Grant. Used to access resources 
available to users. Requires username and password and usually Client ID and 
Client Secret"
+    );
+    public static AllowableValue CLIENT_CREDENTIALS_GRANT_TYPE = new 
AllowableValue(
+        "client_credentials",
+        "Client Credentials",
+        "Client Credentials Grant. Used to access resources available to 
clients. Requires Client ID and Client Secret"
+    );
+
+    public static final PropertyDescriptor GRANT_TYPE = new 
PropertyDescriptor.Builder()
+        .name("grant-type")
+        .displayName("Grant Type")
+        .description("The OAuth2 Grant Type to be used when acquiring an 
access token.")
+        .required(true)
+        .allowableValues(RESOURCE_OWNER_PASSWORD_CREDENTIALS_GRANT_TYPE, 
CLIENT_CREDENTIALS_GRANT_TYPE)
+        
.defaultValue(RESOURCE_OWNER_PASSWORD_CREDENTIALS_GRANT_TYPE.getValue())
+        .build();
+
+    public static final PropertyDescriptor USERNAME = new 
PropertyDescriptor.Builder()
+        .name("service-user-name")
+        .displayName("Username")
+        .description("Username on the service that is being accessed.")
+        .dependsOn(GRANT_TYPE, RESOURCE_OWNER_PASSWORD_CREDENTIALS_GRANT_TYPE)
+        .required(true)
+        .addValidator(StandardValidators.NON_BLANK_VALIDATOR)
+        .expressionLanguageSupported(ExpressionLanguageScope.VARIABLE_REGISTRY)
+        .build();
+
+    public static final PropertyDescriptor PASSWORD = new 
PropertyDescriptor.Builder()
+        .name("service-password")
+        .displayName("Password")
+        .description("Password for the username on the service that is being 
accessed.")
+        .dependsOn(GRANT_TYPE, RESOURCE_OWNER_PASSWORD_CREDENTIALS_GRANT_TYPE)
+        .required(true)
+        .sensitive(true)
+        .addValidator(StandardValidators.NON_BLANK_VALIDATOR)
+        .build();
+
+    public static final PropertyDescriptor CLIENT_ID = new 
PropertyDescriptor.Builder()
+        .name("client-id")
+        .displayName("Client ID")
+        .required(false)
+        .addValidator(StandardValidators.NON_BLANK_VALIDATOR)
+        .expressionLanguageSupported(ExpressionLanguageScope.VARIABLE_REGISTRY)
+        .build();
+
+    public static final PropertyDescriptor CLIENT_SECRET = new 
PropertyDescriptor.Builder()
+        .name("client-secret")
+        .displayName("Client secret")
+        .dependsOn(CLIENT_ID)
+        .required(true)
+        .sensitive(true)
+        .addValidator(StandardValidators.NON_BLANK_VALIDATOR)
+        .build();
+
+    public static final PropertyDescriptor SSL_CONTEXT = new 
PropertyDescriptor.Builder()
+        .name("ssl-context")
+        .displayName("SSL Context")
+        .addValidator(Validator.VALID)
+        .identifiesControllerService(SSLContextService.class)
+        .required(false)
+        .build();
+
+    private static final List<PropertyDescriptor> PROPERTIES = 
Collections.unmodifiableList(Arrays.asList(
+        AUTHORIZATION_SERVER_URL,
+        GRANT_TYPE,
+        USERNAME,
+        PASSWORD,
+        CLIENT_ID,
+        CLIENT_SECRET,
+        SSL_CONTEXT
+    ));
+
+    public static final ObjectMapper ACCESS_DETAILS_MAPPER = new ObjectMapper()
+        .configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false)
+        .setPropertyNamingStrategy(PropertyNamingStrategies.SNAKE_CASE);
+
+    private volatile String authorizationServerUrl;
+    private volatile OkHttpClient httpClient;
+
+    private volatile String grantType;
+    private volatile String username;
+    private volatile String password;
+    private volatile String clientId;
+    private volatile String clientSecret;
+
+    private volatile AccessToken accessDetails;
+
+    @Override
+    public List<PropertyDescriptor> getSupportedPropertyDescriptors() {
+        return PROPERTIES;
+    }
+
+    @OnEnabled
+    public void onEnabled(ConfigurationContext context) {
+        authorizationServerUrl = 
context.getProperty(AUTHORIZATION_SERVER_URL).evaluateAttributeExpressions().getValue();
+
+        httpClient = createHttpClient(context);
+
+        grantType = context.getProperty(GRANT_TYPE).getValue();
+        username = 
context.getProperty(USERNAME).evaluateAttributeExpressions().getValue();
+        password = context.getProperty(PASSWORD).getValue();
+        clientId = 
context.getProperty(CLIENT_ID).evaluateAttributeExpressions().getValue();
+        clientSecret = context.getProperty(CLIENT_SECRET).getValue();
+    }
+
+    @OnDisabled
+    public void onDisabled() {
+        accessDetails = null;
+    }
+
+    @Override
+    protected Collection<ValidationResult> customValidate(ValidationContext 
validationContext) {
+        final List<ValidationResult> validationResults = new 
ArrayList<>(super.customValidate(validationContext));
+
+        if (
+            
validationContext.getProperty(GRANT_TYPE).getValue().equals(CLIENT_CREDENTIALS_GRANT_TYPE.getValue())
+                && !validationContext.getProperty(CLIENT_ID).isSet()
+        ) {
+            validationResults.add(new 
ValidationResult.Builder().subject(CLIENT_ID.getDisplayName())
+                .valid(false)
+                .explanation(String.format(
+                    "When '%s' is set to '%s', '%s' is required",
+                    GRANT_TYPE.getDisplayName(),
+                    CLIENT_CREDENTIALS_GRANT_TYPE.getDisplayName(),
+                    CLIENT_ID.getDisplayName())
+                )
+                .build());
+        }
+
+        return validationResults;
+    }
+
+    protected OkHttpClient createHttpClient(ConfigurationContext context) {
+        OkHttpClient.Builder clientBuilder = new OkHttpClient.Builder();
+
+        SSLContextService sslService = 
context.getProperty(SSL_CONTEXT).asControllerService(SSLContextService.class);
+        if (sslService != null) {
+            final X509TrustManager trustManager = 
sslService.createTrustManager();
+            SSLContext sslContext = sslService.createContext();
+            clientBuilder.sslSocketFactory(sslContext.getSocketFactory(), 
trustManager);
+        }
+
+        return clientBuilder.build();
+    }
+
+    @Override
+    public AccessToken getAccessDetails() {
+        if (this.accessDetails == null) {
+            acquireAccessDetails();
+        } else if (this.accessDetails.isExpired()) {
+            if (this.accessDetails.getRefreshToken() == null) {
+                acquireAccessDetails();
+            } else {
+                try {
+                    refreshAccessDetails();
+                } catch (Exception e) {
+                    getLogger().info("Couldn't refresh access token.", e);
+                    acquireAccessDetails();
+                }
+            }
+        }
+
+        return accessDetails;
+    }
+
+    private void acquireAccessDetails() {
+        getLogger().debug("Getting a new access token.");

Review comment:
       Although logging is not consistent across the application, recommend 
removing the period `.` character from the message:
   ```suggestion
           getLogger().debug("Getting a new access token");
   ```

##########
File path: 
nifi-nar-bundles/nifi-standard-services/nifi-oauth2-provider-bundle/nifi-oauth2-provider-service/src/main/java/org/apache/nifi/oauth2/StandardOauth2AccessTokenProvider.java
##########
@@ -0,0 +1,300 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.nifi.oauth2;
+
+import com.fasterxml.jackson.databind.DeserializationFeature;
+import com.fasterxml.jackson.databind.ObjectMapper;
+import com.fasterxml.jackson.databind.PropertyNamingStrategies;
+import okhttp3.FormBody;
+import okhttp3.OkHttpClient;
+import okhttp3.Request;
+import okhttp3.RequestBody;
+import okhttp3.Response;
+import org.apache.nifi.annotation.documentation.CapabilityDescription;
+import org.apache.nifi.annotation.documentation.Tags;
+import org.apache.nifi.annotation.lifecycle.OnDisabled;
+import org.apache.nifi.annotation.lifecycle.OnEnabled;
+import org.apache.nifi.components.AllowableValue;
+import org.apache.nifi.components.PropertyDescriptor;
+import org.apache.nifi.components.ValidationContext;
+import org.apache.nifi.components.ValidationResult;
+import org.apache.nifi.components.Validator;
+import org.apache.nifi.controller.AbstractControllerService;
+import org.apache.nifi.controller.ConfigurationContext;
+import org.apache.nifi.expression.ExpressionLanguageScope;
+import org.apache.nifi.processor.exception.ProcessException;
+import org.apache.nifi.processor.util.StandardValidators;
+import org.apache.nifi.ssl.SSLContextService;
+
+import javax.net.ssl.SSLContext;
+import javax.net.ssl.X509TrustManager;
+import java.io.IOException;
+import java.io.UncheckedIOException;
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.Collection;
+import java.util.Collections;
+import java.util.List;
+
+@Tags({"oauth2", "provider", "authorization", "access token", "http"})
+@CapabilityDescription("Provides OAuth 2.0 access tokens that can be used as 
Bearer authorization header in HTTP requests." +
+    " Uses Resource Owner Password Credentials Grant.")
+public class StandardOauth2AccessTokenProvider extends 
AbstractControllerService implements OAuth2AccessTokenProvider {
+    public static final PropertyDescriptor AUTHORIZATION_SERVER_URL = new 
PropertyDescriptor.Builder()
+        .name("authorization-server-url")
+        .displayName("Authorization Server URL")
+        .description("The URL of the authorization server that issues access 
tokens.")
+        .required(true)
+        .addValidator(StandardValidators.URL_VALIDATOR)
+        .expressionLanguageSupported(ExpressionLanguageScope.VARIABLE_REGISTRY)
+        .build();
+
+    public static AllowableValue 
RESOURCE_OWNER_PASSWORD_CREDENTIALS_GRANT_TYPE = new AllowableValue(
+        "password",
+        "User Password",
+        "Resource Owner Password Credentials Grant. Used to access resources 
available to users. Requires username and password and usually Client ID and 
Client Secret"
+    );
+    public static AllowableValue CLIENT_CREDENTIALS_GRANT_TYPE = new 
AllowableValue(
+        "client_credentials",
+        "Client Credentials",
+        "Client Credentials Grant. Used to access resources available to 
clients. Requires Client ID and Client Secret"
+    );
+
+    public static final PropertyDescriptor GRANT_TYPE = new 
PropertyDescriptor.Builder()
+        .name("grant-type")
+        .displayName("Grant Type")
+        .description("The OAuth2 Grant Type to be used when acquiring an 
access token.")
+        .required(true)
+        .allowableValues(RESOURCE_OWNER_PASSWORD_CREDENTIALS_GRANT_TYPE, 
CLIENT_CREDENTIALS_GRANT_TYPE)
+        
.defaultValue(RESOURCE_OWNER_PASSWORD_CREDENTIALS_GRANT_TYPE.getValue())
+        .build();
+
+    public static final PropertyDescriptor USERNAME = new 
PropertyDescriptor.Builder()
+        .name("service-user-name")
+        .displayName("Username")
+        .description("Username on the service that is being accessed.")
+        .dependsOn(GRANT_TYPE, RESOURCE_OWNER_PASSWORD_CREDENTIALS_GRANT_TYPE)
+        .required(true)
+        .addValidator(StandardValidators.NON_BLANK_VALIDATOR)
+        .expressionLanguageSupported(ExpressionLanguageScope.VARIABLE_REGISTRY)
+        .build();
+
+    public static final PropertyDescriptor PASSWORD = new 
PropertyDescriptor.Builder()
+        .name("service-password")
+        .displayName("Password")
+        .description("Password for the username on the service that is being 
accessed.")
+        .dependsOn(GRANT_TYPE, RESOURCE_OWNER_PASSWORD_CREDENTIALS_GRANT_TYPE)
+        .required(true)
+        .sensitive(true)
+        .addValidator(StandardValidators.NON_BLANK_VALIDATOR)
+        .build();
+
+    public static final PropertyDescriptor CLIENT_ID = new 
PropertyDescriptor.Builder()
+        .name("client-id")
+        .displayName("Client ID")
+        .required(false)
+        .addValidator(StandardValidators.NON_BLANK_VALIDATOR)
+        .expressionLanguageSupported(ExpressionLanguageScope.VARIABLE_REGISTRY)
+        .build();
+
+    public static final PropertyDescriptor CLIENT_SECRET = new 
PropertyDescriptor.Builder()
+        .name("client-secret")
+        .displayName("Client secret")
+        .dependsOn(CLIENT_ID)
+        .required(true)
+        .sensitive(true)
+        .addValidator(StandardValidators.NON_BLANK_VALIDATOR)
+        .build();
+
+    public static final PropertyDescriptor SSL_CONTEXT = new 
PropertyDescriptor.Builder()
+        .name("ssl-context")
+        .displayName("SSL Context")
+        .addValidator(Validator.VALID)
+        .identifiesControllerService(SSLContextService.class)
+        .required(false)
+        .build();
+
+    private static final List<PropertyDescriptor> PROPERTIES = 
Collections.unmodifiableList(Arrays.asList(
+        AUTHORIZATION_SERVER_URL,
+        GRANT_TYPE,
+        USERNAME,
+        PASSWORD,
+        CLIENT_ID,
+        CLIENT_SECRET,
+        SSL_CONTEXT
+    ));
+
+    public static final ObjectMapper ACCESS_DETAILS_MAPPER = new ObjectMapper()
+        .configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false)
+        .setPropertyNamingStrategy(PropertyNamingStrategies.SNAKE_CASE);
+
+    private volatile String authorizationServerUrl;
+    private volatile OkHttpClient httpClient;
+
+    private volatile String grantType;
+    private volatile String username;
+    private volatile String password;
+    private volatile String clientId;
+    private volatile String clientSecret;
+
+    private volatile AccessToken accessDetails;
+
+    @Override
+    public List<PropertyDescriptor> getSupportedPropertyDescriptors() {
+        return PROPERTIES;
+    }
+
+    @OnEnabled
+    public void onEnabled(ConfigurationContext context) {
+        authorizationServerUrl = 
context.getProperty(AUTHORIZATION_SERVER_URL).evaluateAttributeExpressions().getValue();
+
+        httpClient = createHttpClient(context);
+
+        grantType = context.getProperty(GRANT_TYPE).getValue();
+        username = 
context.getProperty(USERNAME).evaluateAttributeExpressions().getValue();
+        password = context.getProperty(PASSWORD).getValue();
+        clientId = 
context.getProperty(CLIENT_ID).evaluateAttributeExpressions().getValue();
+        clientSecret = context.getProperty(CLIENT_SECRET).getValue();
+    }
+
+    @OnDisabled
+    public void onDisabled() {
+        accessDetails = null;
+    }
+
+    @Override
+    protected Collection<ValidationResult> customValidate(ValidationContext 
validationContext) {
+        final List<ValidationResult> validationResults = new 
ArrayList<>(super.customValidate(validationContext));
+
+        if (
+            
validationContext.getProperty(GRANT_TYPE).getValue().equals(CLIENT_CREDENTIALS_GRANT_TYPE.getValue())
+                && !validationContext.getProperty(CLIENT_ID).isSet()
+        ) {
+            validationResults.add(new 
ValidationResult.Builder().subject(CLIENT_ID.getDisplayName())
+                .valid(false)
+                .explanation(String.format(
+                    "When '%s' is set to '%s', '%s' is required",
+                    GRANT_TYPE.getDisplayName(),
+                    CLIENT_CREDENTIALS_GRANT_TYPE.getDisplayName(),
+                    CLIENT_ID.getDisplayName())
+                )
+                .build());
+        }
+
+        return validationResults;
+    }
+
+    protected OkHttpClient createHttpClient(ConfigurationContext context) {
+        OkHttpClient.Builder clientBuilder = new OkHttpClient.Builder();
+
+        SSLContextService sslService = 
context.getProperty(SSL_CONTEXT).asControllerService(SSLContextService.class);
+        if (sslService != null) {
+            final X509TrustManager trustManager = 
sslService.createTrustManager();
+            SSLContext sslContext = sslService.createContext();
+            clientBuilder.sslSocketFactory(sslContext.getSocketFactory(), 
trustManager);
+        }
+
+        return clientBuilder.build();
+    }
+
+    @Override
+    public AccessToken getAccessDetails() {
+        if (this.accessDetails == null) {
+            acquireAccessDetails();
+        } else if (this.accessDetails.isExpired()) {
+            if (this.accessDetails.getRefreshToken() == null) {
+                acquireAccessDetails();
+            } else {
+                try {
+                    refreshAccessDetails();
+                } catch (Exception e) {
+                    getLogger().info("Couldn't refresh access token.", e);
+                    acquireAccessDetails();
+                }
+            }
+        }
+
+        return accessDetails;
+    }
+
+    private void acquireAccessDetails() {
+        getLogger().debug("Getting a new access token.");
+
+        FormBody.Builder acquireTokenBuilder = new FormBody.Builder();
+
+        if 
(grantType.equals(RESOURCE_OWNER_PASSWORD_CREDENTIALS_GRANT_TYPE.getValue())) {
+            acquireTokenBuilder.add("grant_type", "password")
+                .add("username", username)
+                .add("password", password);
+        } else if (grantType.equals(CLIENT_CREDENTIALS_GRANT_TYPE.getValue())) 
{
+            acquireTokenBuilder.add("grant_type", "client_credentials");
+        }
+
+        if (clientId != null) {
+            acquireTokenBuilder.add("client_id", clientId);
+            acquireTokenBuilder.add("client_secret", clientSecret);
+        }
+
+        RequestBody acquireTokenRequestBody = acquireTokenBuilder.build();
+
+        Request acquireTokenRequest = new Request.Builder()
+            .url(authorizationServerUrl)
+            .post(acquireTokenRequestBody)
+            .build();
+
+        this.accessDetails = getAccessDetails(acquireTokenRequest);
+    }
+
+    private void refreshAccessDetails() {
+        getLogger().debug("Refreshing access token.");
+
+        FormBody.Builder refreshTokenBuilder = new FormBody.Builder()
+            .add("grant_type", "refresh_token")
+            .add("refresh_token", this.accessDetails.getRefreshToken());
+
+        if (clientId != null) {
+            refreshTokenBuilder.add("client_id", clientId);
+            refreshTokenBuilder.add("client_secret", clientSecret);
+        }
+
+        RequestBody refreshTokenRequestBody = refreshTokenBuilder.build();
+
+        Request refreshRequest = new Request.Builder()
+            .url(authorizationServerUrl)
+            .post(refreshTokenRequestBody)
+            .build();
+
+        this.accessDetails = getAccessDetails(refreshRequest);
+    }
+
+    private AccessToken getAccessDetails(Request newRequest) {
+        try {
+            Response response = httpClient.newCall(newRequest).execute();
+            String responseBody = response.body().string();
+            if (response.code() != 200) {
+                getLogger().error(String.format("Bad response from the server 
during oauth2 request:\n%s", responseBody));
+                throw new ProcessException(String.format("Got HTTP %d during 
oauth2 request.", response.code()));

Review comment:
       The newline character is platform-specific, recommend the following 
adjustments:
   ```suggestion
                   getLogger().error("OAuth2 Request Failed [HTTP {}] {}", 
response.code(), responseBody);
                   throw new ProcessException(String.format("OAuth2 Request 
Failed [HTTP %d]", response.code()));
   ```

##########
File path: 
nifi-nar-bundles/nifi-standard-services/nifi-oauth2-provider-bundle/nifi-oauth2-provider-service/src/main/java/org/apache/nifi/oauth2/StandardOauth2AccessTokenProvider.java
##########
@@ -0,0 +1,300 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.nifi.oauth2;
+
+import com.fasterxml.jackson.databind.DeserializationFeature;
+import com.fasterxml.jackson.databind.ObjectMapper;
+import com.fasterxml.jackson.databind.PropertyNamingStrategies;
+import okhttp3.FormBody;
+import okhttp3.OkHttpClient;
+import okhttp3.Request;
+import okhttp3.RequestBody;
+import okhttp3.Response;
+import org.apache.nifi.annotation.documentation.CapabilityDescription;
+import org.apache.nifi.annotation.documentation.Tags;
+import org.apache.nifi.annotation.lifecycle.OnDisabled;
+import org.apache.nifi.annotation.lifecycle.OnEnabled;
+import org.apache.nifi.components.AllowableValue;
+import org.apache.nifi.components.PropertyDescriptor;
+import org.apache.nifi.components.ValidationContext;
+import org.apache.nifi.components.ValidationResult;
+import org.apache.nifi.components.Validator;
+import org.apache.nifi.controller.AbstractControllerService;
+import org.apache.nifi.controller.ConfigurationContext;
+import org.apache.nifi.expression.ExpressionLanguageScope;
+import org.apache.nifi.processor.exception.ProcessException;
+import org.apache.nifi.processor.util.StandardValidators;
+import org.apache.nifi.ssl.SSLContextService;
+
+import javax.net.ssl.SSLContext;
+import javax.net.ssl.X509TrustManager;
+import java.io.IOException;
+import java.io.UncheckedIOException;
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.Collection;
+import java.util.Collections;
+import java.util.List;
+
+@Tags({"oauth2", "provider", "authorization", "access token", "http"})
+@CapabilityDescription("Provides OAuth 2.0 access tokens that can be used as 
Bearer authorization header in HTTP requests." +
+    " Uses Resource Owner Password Credentials Grant.")
+public class StandardOauth2AccessTokenProvider extends 
AbstractControllerService implements OAuth2AccessTokenProvider {
+    public static final PropertyDescriptor AUTHORIZATION_SERVER_URL = new 
PropertyDescriptor.Builder()
+        .name("authorization-server-url")
+        .displayName("Authorization Server URL")
+        .description("The URL of the authorization server that issues access 
tokens.")
+        .required(true)
+        .addValidator(StandardValidators.URL_VALIDATOR)
+        .expressionLanguageSupported(ExpressionLanguageScope.VARIABLE_REGISTRY)
+        .build();
+
+    public static AllowableValue 
RESOURCE_OWNER_PASSWORD_CREDENTIALS_GRANT_TYPE = new AllowableValue(
+        "password",
+        "User Password",
+        "Resource Owner Password Credentials Grant. Used to access resources 
available to users. Requires username and password and usually Client ID and 
Client Secret"
+    );
+    public static AllowableValue CLIENT_CREDENTIALS_GRANT_TYPE = new 
AllowableValue(
+        "client_credentials",
+        "Client Credentials",
+        "Client Credentials Grant. Used to access resources available to 
clients. Requires Client ID and Client Secret"
+    );
+
+    public static final PropertyDescriptor GRANT_TYPE = new 
PropertyDescriptor.Builder()
+        .name("grant-type")
+        .displayName("Grant Type")
+        .description("The OAuth2 Grant Type to be used when acquiring an 
access token.")
+        .required(true)
+        .allowableValues(RESOURCE_OWNER_PASSWORD_CREDENTIALS_GRANT_TYPE, 
CLIENT_CREDENTIALS_GRANT_TYPE)
+        
.defaultValue(RESOURCE_OWNER_PASSWORD_CREDENTIALS_GRANT_TYPE.getValue())
+        .build();
+
+    public static final PropertyDescriptor USERNAME = new 
PropertyDescriptor.Builder()
+        .name("service-user-name")
+        .displayName("Username")
+        .description("Username on the service that is being accessed.")
+        .dependsOn(GRANT_TYPE, RESOURCE_OWNER_PASSWORD_CREDENTIALS_GRANT_TYPE)
+        .required(true)
+        .addValidator(StandardValidators.NON_BLANK_VALIDATOR)
+        .expressionLanguageSupported(ExpressionLanguageScope.VARIABLE_REGISTRY)
+        .build();
+
+    public static final PropertyDescriptor PASSWORD = new 
PropertyDescriptor.Builder()
+        .name("service-password")
+        .displayName("Password")
+        .description("Password for the username on the service that is being 
accessed.")
+        .dependsOn(GRANT_TYPE, RESOURCE_OWNER_PASSWORD_CREDENTIALS_GRANT_TYPE)
+        .required(true)
+        .sensitive(true)
+        .addValidator(StandardValidators.NON_BLANK_VALIDATOR)
+        .build();
+
+    public static final PropertyDescriptor CLIENT_ID = new 
PropertyDescriptor.Builder()
+        .name("client-id")
+        .displayName("Client ID")
+        .required(false)
+        .addValidator(StandardValidators.NON_BLANK_VALIDATOR)
+        .expressionLanguageSupported(ExpressionLanguageScope.VARIABLE_REGISTRY)
+        .build();
+
+    public static final PropertyDescriptor CLIENT_SECRET = new 
PropertyDescriptor.Builder()
+        .name("client-secret")
+        .displayName("Client secret")
+        .dependsOn(CLIENT_ID)
+        .required(true)
+        .sensitive(true)
+        .addValidator(StandardValidators.NON_BLANK_VALIDATOR)
+        .build();
+
+    public static final PropertyDescriptor SSL_CONTEXT = new 
PropertyDescriptor.Builder()
+        .name("ssl-context")
+        .displayName("SSL Context")
+        .addValidator(Validator.VALID)
+        .identifiesControllerService(SSLContextService.class)
+        .required(false)
+        .build();
+
+    private static final List<PropertyDescriptor> PROPERTIES = 
Collections.unmodifiableList(Arrays.asList(
+        AUTHORIZATION_SERVER_URL,
+        GRANT_TYPE,
+        USERNAME,
+        PASSWORD,
+        CLIENT_ID,
+        CLIENT_SECRET,
+        SSL_CONTEXT
+    ));
+
+    public static final ObjectMapper ACCESS_DETAILS_MAPPER = new ObjectMapper()
+        .configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false)
+        .setPropertyNamingStrategy(PropertyNamingStrategies.SNAKE_CASE);
+
+    private volatile String authorizationServerUrl;
+    private volatile OkHttpClient httpClient;
+
+    private volatile String grantType;
+    private volatile String username;
+    private volatile String password;
+    private volatile String clientId;
+    private volatile String clientSecret;
+
+    private volatile AccessToken accessDetails;
+
+    @Override
+    public List<PropertyDescriptor> getSupportedPropertyDescriptors() {
+        return PROPERTIES;
+    }
+
+    @OnEnabled
+    public void onEnabled(ConfigurationContext context) {
+        authorizationServerUrl = 
context.getProperty(AUTHORIZATION_SERVER_URL).evaluateAttributeExpressions().getValue();
+
+        httpClient = createHttpClient(context);
+
+        grantType = context.getProperty(GRANT_TYPE).getValue();
+        username = 
context.getProperty(USERNAME).evaluateAttributeExpressions().getValue();
+        password = context.getProperty(PASSWORD).getValue();
+        clientId = 
context.getProperty(CLIENT_ID).evaluateAttributeExpressions().getValue();
+        clientSecret = context.getProperty(CLIENT_SECRET).getValue();
+    }
+
+    @OnDisabled
+    public void onDisabled() {
+        accessDetails = null;
+    }
+
+    @Override
+    protected Collection<ValidationResult> customValidate(ValidationContext 
validationContext) {
+        final List<ValidationResult> validationResults = new 
ArrayList<>(super.customValidate(validationContext));
+
+        if (
+            
validationContext.getProperty(GRANT_TYPE).getValue().equals(CLIENT_CREDENTIALS_GRANT_TYPE.getValue())
+                && !validationContext.getProperty(CLIENT_ID).isSet()
+        ) {
+            validationResults.add(new 
ValidationResult.Builder().subject(CLIENT_ID.getDisplayName())
+                .valid(false)
+                .explanation(String.format(
+                    "When '%s' is set to '%s', '%s' is required",
+                    GRANT_TYPE.getDisplayName(),
+                    CLIENT_CREDENTIALS_GRANT_TYPE.getDisplayName(),
+                    CLIENT_ID.getDisplayName())
+                )
+                .build());
+        }
+
+        return validationResults;
+    }
+
+    protected OkHttpClient createHttpClient(ConfigurationContext context) {
+        OkHttpClient.Builder clientBuilder = new OkHttpClient.Builder();
+
+        SSLContextService sslService = 
context.getProperty(SSL_CONTEXT).asControllerService(SSLContextService.class);
+        if (sslService != null) {
+            final X509TrustManager trustManager = 
sslService.createTrustManager();
+            SSLContext sslContext = sslService.createContext();
+            clientBuilder.sslSocketFactory(sslContext.getSocketFactory(), 
trustManager);
+        }
+
+        return clientBuilder.build();
+    }
+
+    @Override
+    public AccessToken getAccessDetails() {
+        if (this.accessDetails == null) {
+            acquireAccessDetails();
+        } else if (this.accessDetails.isExpired()) {
+            if (this.accessDetails.getRefreshToken() == null) {
+                acquireAccessDetails();
+            } else {
+                try {
+                    refreshAccessDetails();
+                } catch (Exception e) {
+                    getLogger().info("Couldn't refresh access token.", e);
+                    acquireAccessDetails();
+                }
+            }
+        }
+
+        return accessDetails;
+    }
+
+    private void acquireAccessDetails() {

Review comment:
       For clarity, what do you think about renaming this to 
`getNewAccessDetails()`?

##########
File path: 
nifi-nar-bundles/nifi-standard-services/nifi-oauth2-provider-bundle/nifi-oauth2-provider-service/src/main/java/org/apache/nifi/oauth2/StandardOauth2AccessTokenProvider.java
##########
@@ -0,0 +1,300 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.nifi.oauth2;
+
+import com.fasterxml.jackson.databind.DeserializationFeature;
+import com.fasterxml.jackson.databind.ObjectMapper;
+import com.fasterxml.jackson.databind.PropertyNamingStrategies;
+import okhttp3.FormBody;
+import okhttp3.OkHttpClient;
+import okhttp3.Request;
+import okhttp3.RequestBody;
+import okhttp3.Response;
+import org.apache.nifi.annotation.documentation.CapabilityDescription;
+import org.apache.nifi.annotation.documentation.Tags;
+import org.apache.nifi.annotation.lifecycle.OnDisabled;
+import org.apache.nifi.annotation.lifecycle.OnEnabled;
+import org.apache.nifi.components.AllowableValue;
+import org.apache.nifi.components.PropertyDescriptor;
+import org.apache.nifi.components.ValidationContext;
+import org.apache.nifi.components.ValidationResult;
+import org.apache.nifi.components.Validator;
+import org.apache.nifi.controller.AbstractControllerService;
+import org.apache.nifi.controller.ConfigurationContext;
+import org.apache.nifi.expression.ExpressionLanguageScope;
+import org.apache.nifi.processor.exception.ProcessException;
+import org.apache.nifi.processor.util.StandardValidators;
+import org.apache.nifi.ssl.SSLContextService;
+
+import javax.net.ssl.SSLContext;
+import javax.net.ssl.X509TrustManager;
+import java.io.IOException;
+import java.io.UncheckedIOException;
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.Collection;
+import java.util.Collections;
+import java.util.List;
+
+@Tags({"oauth2", "provider", "authorization", "access token", "http"})
+@CapabilityDescription("Provides OAuth 2.0 access tokens that can be used as 
Bearer authorization header in HTTP requests." +
+    " Uses Resource Owner Password Credentials Grant.")
+public class StandardOauth2AccessTokenProvider extends 
AbstractControllerService implements OAuth2AccessTokenProvider {
+    public static final PropertyDescriptor AUTHORIZATION_SERVER_URL = new 
PropertyDescriptor.Builder()
+        .name("authorization-server-url")
+        .displayName("Authorization Server URL")
+        .description("The URL of the authorization server that issues access 
tokens.")
+        .required(true)
+        .addValidator(StandardValidators.URL_VALIDATOR)
+        .expressionLanguageSupported(ExpressionLanguageScope.VARIABLE_REGISTRY)
+        .build();
+
+    public static AllowableValue 
RESOURCE_OWNER_PASSWORD_CREDENTIALS_GRANT_TYPE = new AllowableValue(
+        "password",
+        "User Password",
+        "Resource Owner Password Credentials Grant. Used to access resources 
available to users. Requires username and password and usually Client ID and 
Client Secret"
+    );
+    public static AllowableValue CLIENT_CREDENTIALS_GRANT_TYPE = new 
AllowableValue(
+        "client_credentials",
+        "Client Credentials",
+        "Client Credentials Grant. Used to access resources available to 
clients. Requires Client ID and Client Secret"
+    );
+
+    public static final PropertyDescriptor GRANT_TYPE = new 
PropertyDescriptor.Builder()
+        .name("grant-type")
+        .displayName("Grant Type")
+        .description("The OAuth2 Grant Type to be used when acquiring an 
access token.")
+        .required(true)
+        .allowableValues(RESOURCE_OWNER_PASSWORD_CREDENTIALS_GRANT_TYPE, 
CLIENT_CREDENTIALS_GRANT_TYPE)
+        
.defaultValue(RESOURCE_OWNER_PASSWORD_CREDENTIALS_GRANT_TYPE.getValue())
+        .build();
+
+    public static final PropertyDescriptor USERNAME = new 
PropertyDescriptor.Builder()
+        .name("service-user-name")
+        .displayName("Username")
+        .description("Username on the service that is being accessed.")
+        .dependsOn(GRANT_TYPE, RESOURCE_OWNER_PASSWORD_CREDENTIALS_GRANT_TYPE)
+        .required(true)
+        .addValidator(StandardValidators.NON_BLANK_VALIDATOR)
+        .expressionLanguageSupported(ExpressionLanguageScope.VARIABLE_REGISTRY)
+        .build();
+
+    public static final PropertyDescriptor PASSWORD = new 
PropertyDescriptor.Builder()
+        .name("service-password")
+        .displayName("Password")
+        .description("Password for the username on the service that is being 
accessed.")
+        .dependsOn(GRANT_TYPE, RESOURCE_OWNER_PASSWORD_CREDENTIALS_GRANT_TYPE)
+        .required(true)
+        .sensitive(true)
+        .addValidator(StandardValidators.NON_BLANK_VALIDATOR)
+        .build();
+
+    public static final PropertyDescriptor CLIENT_ID = new 
PropertyDescriptor.Builder()
+        .name("client-id")
+        .displayName("Client ID")
+        .required(false)
+        .addValidator(StandardValidators.NON_BLANK_VALIDATOR)
+        .expressionLanguageSupported(ExpressionLanguageScope.VARIABLE_REGISTRY)
+        .build();
+
+    public static final PropertyDescriptor CLIENT_SECRET = new 
PropertyDescriptor.Builder()
+        .name("client-secret")
+        .displayName("Client secret")
+        .dependsOn(CLIENT_ID)
+        .required(true)
+        .sensitive(true)
+        .addValidator(StandardValidators.NON_BLANK_VALIDATOR)
+        .build();
+
+    public static final PropertyDescriptor SSL_CONTEXT = new 
PropertyDescriptor.Builder()
+        .name("ssl-context")
+        .displayName("SSL Context")
+        .addValidator(Validator.VALID)
+        .identifiesControllerService(SSLContextService.class)
+        .required(false)
+        .build();
+
+    private static final List<PropertyDescriptor> PROPERTIES = 
Collections.unmodifiableList(Arrays.asList(
+        AUTHORIZATION_SERVER_URL,
+        GRANT_TYPE,
+        USERNAME,
+        PASSWORD,
+        CLIENT_ID,
+        CLIENT_SECRET,
+        SSL_CONTEXT
+    ));
+
+    public static final ObjectMapper ACCESS_DETAILS_MAPPER = new ObjectMapper()
+        .configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false)
+        .setPropertyNamingStrategy(PropertyNamingStrategies.SNAKE_CASE);
+
+    private volatile String authorizationServerUrl;
+    private volatile OkHttpClient httpClient;
+
+    private volatile String grantType;
+    private volatile String username;
+    private volatile String password;
+    private volatile String clientId;
+    private volatile String clientSecret;
+
+    private volatile AccessToken accessDetails;
+
+    @Override
+    public List<PropertyDescriptor> getSupportedPropertyDescriptors() {
+        return PROPERTIES;
+    }
+
+    @OnEnabled
+    public void onEnabled(ConfigurationContext context) {
+        authorizationServerUrl = 
context.getProperty(AUTHORIZATION_SERVER_URL).evaluateAttributeExpressions().getValue();
+
+        httpClient = createHttpClient(context);
+
+        grantType = context.getProperty(GRANT_TYPE).getValue();
+        username = 
context.getProperty(USERNAME).evaluateAttributeExpressions().getValue();
+        password = context.getProperty(PASSWORD).getValue();
+        clientId = 
context.getProperty(CLIENT_ID).evaluateAttributeExpressions().getValue();
+        clientSecret = context.getProperty(CLIENT_SECRET).getValue();
+    }
+
+    @OnDisabled
+    public void onDisabled() {
+        accessDetails = null;
+    }
+
+    @Override
+    protected Collection<ValidationResult> customValidate(ValidationContext 
validationContext) {
+        final List<ValidationResult> validationResults = new 
ArrayList<>(super.customValidate(validationContext));
+
+        if (
+            
validationContext.getProperty(GRANT_TYPE).getValue().equals(CLIENT_CREDENTIALS_GRANT_TYPE.getValue())
+                && !validationContext.getProperty(CLIENT_ID).isSet()
+        ) {
+            validationResults.add(new 
ValidationResult.Builder().subject(CLIENT_ID.getDisplayName())
+                .valid(false)
+                .explanation(String.format(
+                    "When '%s' is set to '%s', '%s' is required",
+                    GRANT_TYPE.getDisplayName(),
+                    CLIENT_CREDENTIALS_GRANT_TYPE.getDisplayName(),
+                    CLIENT_ID.getDisplayName())
+                )
+                .build());
+        }
+
+        return validationResults;
+    }
+
+    protected OkHttpClient createHttpClient(ConfigurationContext context) {
+        OkHttpClient.Builder clientBuilder = new OkHttpClient.Builder();
+
+        SSLContextService sslService = 
context.getProperty(SSL_CONTEXT).asControllerService(SSLContextService.class);
+        if (sslService != null) {
+            final X509TrustManager trustManager = 
sslService.createTrustManager();
+            SSLContext sslContext = sslService.createContext();
+            clientBuilder.sslSocketFactory(sslContext.getSocketFactory(), 
trustManager);
+        }
+
+        return clientBuilder.build();
+    }
+
+    @Override
+    public AccessToken getAccessDetails() {
+        if (this.accessDetails == null) {
+            acquireAccessDetails();
+        } else if (this.accessDetails.isExpired()) {
+            if (this.accessDetails.getRefreshToken() == null) {
+                acquireAccessDetails();
+            } else {
+                try {
+                    refreshAccessDetails();
+                } catch (Exception e) {
+                    getLogger().info("Couldn't refresh access token.", e);
+                    acquireAccessDetails();
+                }
+            }
+        }
+
+        return accessDetails;
+    }
+
+    private void acquireAccessDetails() {
+        getLogger().debug("Getting a new access token.");
+
+        FormBody.Builder acquireTokenBuilder = new FormBody.Builder();
+
+        if 
(grantType.equals(RESOURCE_OWNER_PASSWORD_CREDENTIALS_GRANT_TYPE.getValue())) {
+            acquireTokenBuilder.add("grant_type", "password")

Review comment:
       It would be helpful to define static strings for reused parameter names 
such as `String GRANT_TYPE = "grant_type"`.

##########
File path: 
nifi-nar-bundles/nifi-standard-services/nifi-oauth2-provider-bundle/nifi-oauth2-provider-service/src/main/java/org/apache/nifi/oauth2/StandardOauth2AccessTokenProvider.java
##########
@@ -0,0 +1,300 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.nifi.oauth2;
+
+import com.fasterxml.jackson.databind.DeserializationFeature;
+import com.fasterxml.jackson.databind.ObjectMapper;
+import com.fasterxml.jackson.databind.PropertyNamingStrategies;
+import okhttp3.FormBody;
+import okhttp3.OkHttpClient;
+import okhttp3.Request;
+import okhttp3.RequestBody;
+import okhttp3.Response;
+import org.apache.nifi.annotation.documentation.CapabilityDescription;
+import org.apache.nifi.annotation.documentation.Tags;
+import org.apache.nifi.annotation.lifecycle.OnDisabled;
+import org.apache.nifi.annotation.lifecycle.OnEnabled;
+import org.apache.nifi.components.AllowableValue;
+import org.apache.nifi.components.PropertyDescriptor;
+import org.apache.nifi.components.ValidationContext;
+import org.apache.nifi.components.ValidationResult;
+import org.apache.nifi.components.Validator;
+import org.apache.nifi.controller.AbstractControllerService;
+import org.apache.nifi.controller.ConfigurationContext;
+import org.apache.nifi.expression.ExpressionLanguageScope;
+import org.apache.nifi.processor.exception.ProcessException;
+import org.apache.nifi.processor.util.StandardValidators;
+import org.apache.nifi.ssl.SSLContextService;
+
+import javax.net.ssl.SSLContext;
+import javax.net.ssl.X509TrustManager;
+import java.io.IOException;
+import java.io.UncheckedIOException;
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.Collection;
+import java.util.Collections;
+import java.util.List;
+
+@Tags({"oauth2", "provider", "authorization", "access token", "http"})
+@CapabilityDescription("Provides OAuth 2.0 access tokens that can be used as 
Bearer authorization header in HTTP requests." +
+    " Uses Resource Owner Password Credentials Grant.")
+public class StandardOauth2AccessTokenProvider extends 
AbstractControllerService implements OAuth2AccessTokenProvider {
+    public static final PropertyDescriptor AUTHORIZATION_SERVER_URL = new 
PropertyDescriptor.Builder()
+        .name("authorization-server-url")
+        .displayName("Authorization Server URL")
+        .description("The URL of the authorization server that issues access 
tokens.")
+        .required(true)
+        .addValidator(StandardValidators.URL_VALIDATOR)
+        .expressionLanguageSupported(ExpressionLanguageScope.VARIABLE_REGISTRY)
+        .build();
+
+    public static AllowableValue 
RESOURCE_OWNER_PASSWORD_CREDENTIALS_GRANT_TYPE = new AllowableValue(
+        "password",
+        "User Password",
+        "Resource Owner Password Credentials Grant. Used to access resources 
available to users. Requires username and password and usually Client ID and 
Client Secret"
+    );
+    public static AllowableValue CLIENT_CREDENTIALS_GRANT_TYPE = new 
AllowableValue(
+        "client_credentials",
+        "Client Credentials",
+        "Client Credentials Grant. Used to access resources available to 
clients. Requires Client ID and Client Secret"
+    );
+
+    public static final PropertyDescriptor GRANT_TYPE = new 
PropertyDescriptor.Builder()
+        .name("grant-type")
+        .displayName("Grant Type")
+        .description("The OAuth2 Grant Type to be used when acquiring an 
access token.")
+        .required(true)
+        .allowableValues(RESOURCE_OWNER_PASSWORD_CREDENTIALS_GRANT_TYPE, 
CLIENT_CREDENTIALS_GRANT_TYPE)
+        
.defaultValue(RESOURCE_OWNER_PASSWORD_CREDENTIALS_GRANT_TYPE.getValue())
+        .build();
+
+    public static final PropertyDescriptor USERNAME = new 
PropertyDescriptor.Builder()
+        .name("service-user-name")
+        .displayName("Username")
+        .description("Username on the service that is being accessed.")
+        .dependsOn(GRANT_TYPE, RESOURCE_OWNER_PASSWORD_CREDENTIALS_GRANT_TYPE)
+        .required(true)
+        .addValidator(StandardValidators.NON_BLANK_VALIDATOR)
+        .expressionLanguageSupported(ExpressionLanguageScope.VARIABLE_REGISTRY)
+        .build();
+
+    public static final PropertyDescriptor PASSWORD = new 
PropertyDescriptor.Builder()
+        .name("service-password")
+        .displayName("Password")
+        .description("Password for the username on the service that is being 
accessed.")
+        .dependsOn(GRANT_TYPE, RESOURCE_OWNER_PASSWORD_CREDENTIALS_GRANT_TYPE)
+        .required(true)
+        .sensitive(true)
+        .addValidator(StandardValidators.NON_BLANK_VALIDATOR)
+        .build();
+
+    public static final PropertyDescriptor CLIENT_ID = new 
PropertyDescriptor.Builder()
+        .name("client-id")
+        .displayName("Client ID")
+        .required(false)
+        .addValidator(StandardValidators.NON_BLANK_VALIDATOR)
+        .expressionLanguageSupported(ExpressionLanguageScope.VARIABLE_REGISTRY)
+        .build();
+
+    public static final PropertyDescriptor CLIENT_SECRET = new 
PropertyDescriptor.Builder()
+        .name("client-secret")
+        .displayName("Client secret")
+        .dependsOn(CLIENT_ID)
+        .required(true)
+        .sensitive(true)
+        .addValidator(StandardValidators.NON_BLANK_VALIDATOR)
+        .build();
+
+    public static final PropertyDescriptor SSL_CONTEXT = new 
PropertyDescriptor.Builder()
+        .name("ssl-context")
+        .displayName("SSL Context")
+        .addValidator(Validator.VALID)
+        .identifiesControllerService(SSLContextService.class)
+        .required(false)
+        .build();
+
+    private static final List<PropertyDescriptor> PROPERTIES = 
Collections.unmodifiableList(Arrays.asList(
+        AUTHORIZATION_SERVER_URL,
+        GRANT_TYPE,
+        USERNAME,
+        PASSWORD,
+        CLIENT_ID,
+        CLIENT_SECRET,
+        SSL_CONTEXT
+    ));
+
+    public static final ObjectMapper ACCESS_DETAILS_MAPPER = new ObjectMapper()
+        .configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false)
+        .setPropertyNamingStrategy(PropertyNamingStrategies.SNAKE_CASE);
+
+    private volatile String authorizationServerUrl;
+    private volatile OkHttpClient httpClient;
+
+    private volatile String grantType;
+    private volatile String username;
+    private volatile String password;
+    private volatile String clientId;
+    private volatile String clientSecret;
+
+    private volatile AccessToken accessDetails;
+
+    @Override
+    public List<PropertyDescriptor> getSupportedPropertyDescriptors() {
+        return PROPERTIES;
+    }
+
+    @OnEnabled
+    public void onEnabled(ConfigurationContext context) {
+        authorizationServerUrl = 
context.getProperty(AUTHORIZATION_SERVER_URL).evaluateAttributeExpressions().getValue();
+
+        httpClient = createHttpClient(context);
+
+        grantType = context.getProperty(GRANT_TYPE).getValue();
+        username = 
context.getProperty(USERNAME).evaluateAttributeExpressions().getValue();
+        password = context.getProperty(PASSWORD).getValue();
+        clientId = 
context.getProperty(CLIENT_ID).evaluateAttributeExpressions().getValue();
+        clientSecret = context.getProperty(CLIENT_SECRET).getValue();
+    }
+
+    @OnDisabled
+    public void onDisabled() {
+        accessDetails = null;
+    }
+
+    @Override
+    protected Collection<ValidationResult> customValidate(ValidationContext 
validationContext) {
+        final List<ValidationResult> validationResults = new 
ArrayList<>(super.customValidate(validationContext));
+
+        if (
+            
validationContext.getProperty(GRANT_TYPE).getValue().equals(CLIENT_CREDENTIALS_GRANT_TYPE.getValue())
+                && !validationContext.getProperty(CLIENT_ID).isSet()
+        ) {
+            validationResults.add(new 
ValidationResult.Builder().subject(CLIENT_ID.getDisplayName())
+                .valid(false)
+                .explanation(String.format(
+                    "When '%s' is set to '%s', '%s' is required",
+                    GRANT_TYPE.getDisplayName(),
+                    CLIENT_CREDENTIALS_GRANT_TYPE.getDisplayName(),
+                    CLIENT_ID.getDisplayName())
+                )
+                .build());
+        }
+
+        return validationResults;
+    }
+
+    protected OkHttpClient createHttpClient(ConfigurationContext context) {
+        OkHttpClient.Builder clientBuilder = new OkHttpClient.Builder();
+
+        SSLContextService sslService = 
context.getProperty(SSL_CONTEXT).asControllerService(SSLContextService.class);
+        if (sslService != null) {
+            final X509TrustManager trustManager = 
sslService.createTrustManager();
+            SSLContext sslContext = sslService.createContext();
+            clientBuilder.sslSocketFactory(sslContext.getSocketFactory(), 
trustManager);
+        }
+
+        return clientBuilder.build();
+    }
+
+    @Override
+    public AccessToken getAccessDetails() {
+        if (this.accessDetails == null) {
+            acquireAccessDetails();
+        } else if (this.accessDetails.isExpired()) {
+            if (this.accessDetails.getRefreshToken() == null) {
+                acquireAccessDetails();
+            } else {
+                try {
+                    refreshAccessDetails();
+                } catch (Exception e) {
+                    getLogger().info("Couldn't refresh access token.", e);

Review comment:
       If this generalized exception handling and retry remains, should this be 
logged as warning?
   ```suggestion
                       getLogger().warn("Refresh Access Token Failed", e);
   ```

##########
File path: 
nifi-nar-bundles/nifi-standard-services/nifi-oauth2-provider-bundle/nifi-oauth2-provider-service/src/main/java/org/apache/nifi/oauth2/StandardOauth2AccessTokenProvider.java
##########
@@ -0,0 +1,300 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.nifi.oauth2;
+
+import com.fasterxml.jackson.databind.DeserializationFeature;
+import com.fasterxml.jackson.databind.ObjectMapper;
+import com.fasterxml.jackson.databind.PropertyNamingStrategies;
+import okhttp3.FormBody;
+import okhttp3.OkHttpClient;
+import okhttp3.Request;
+import okhttp3.RequestBody;
+import okhttp3.Response;
+import org.apache.nifi.annotation.documentation.CapabilityDescription;
+import org.apache.nifi.annotation.documentation.Tags;
+import org.apache.nifi.annotation.lifecycle.OnDisabled;
+import org.apache.nifi.annotation.lifecycle.OnEnabled;
+import org.apache.nifi.components.AllowableValue;
+import org.apache.nifi.components.PropertyDescriptor;
+import org.apache.nifi.components.ValidationContext;
+import org.apache.nifi.components.ValidationResult;
+import org.apache.nifi.components.Validator;
+import org.apache.nifi.controller.AbstractControllerService;
+import org.apache.nifi.controller.ConfigurationContext;
+import org.apache.nifi.expression.ExpressionLanguageScope;
+import org.apache.nifi.processor.exception.ProcessException;
+import org.apache.nifi.processor.util.StandardValidators;
+import org.apache.nifi.ssl.SSLContextService;
+
+import javax.net.ssl.SSLContext;
+import javax.net.ssl.X509TrustManager;
+import java.io.IOException;
+import java.io.UncheckedIOException;
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.Collection;
+import java.util.Collections;
+import java.util.List;
+
+@Tags({"oauth2", "provider", "authorization", "access token", "http"})
+@CapabilityDescription("Provides OAuth 2.0 access tokens that can be used as 
Bearer authorization header in HTTP requests." +
+    " Uses Resource Owner Password Credentials Grant.")
+public class StandardOauth2AccessTokenProvider extends 
AbstractControllerService implements OAuth2AccessTokenProvider {
+    public static final PropertyDescriptor AUTHORIZATION_SERVER_URL = new 
PropertyDescriptor.Builder()
+        .name("authorization-server-url")
+        .displayName("Authorization Server URL")
+        .description("The URL of the authorization server that issues access 
tokens.")
+        .required(true)
+        .addValidator(StandardValidators.URL_VALIDATOR)
+        .expressionLanguageSupported(ExpressionLanguageScope.VARIABLE_REGISTRY)
+        .build();
+
+    public static AllowableValue 
RESOURCE_OWNER_PASSWORD_CREDENTIALS_GRANT_TYPE = new AllowableValue(
+        "password",
+        "User Password",
+        "Resource Owner Password Credentials Grant. Used to access resources 
available to users. Requires username and password and usually Client ID and 
Client Secret"
+    );
+    public static AllowableValue CLIENT_CREDENTIALS_GRANT_TYPE = new 
AllowableValue(
+        "client_credentials",
+        "Client Credentials",
+        "Client Credentials Grant. Used to access resources available to 
clients. Requires Client ID and Client Secret"
+    );
+
+    public static final PropertyDescriptor GRANT_TYPE = new 
PropertyDescriptor.Builder()
+        .name("grant-type")
+        .displayName("Grant Type")
+        .description("The OAuth2 Grant Type to be used when acquiring an 
access token.")
+        .required(true)
+        .allowableValues(RESOURCE_OWNER_PASSWORD_CREDENTIALS_GRANT_TYPE, 
CLIENT_CREDENTIALS_GRANT_TYPE)
+        
.defaultValue(RESOURCE_OWNER_PASSWORD_CREDENTIALS_GRANT_TYPE.getValue())
+        .build();
+
+    public static final PropertyDescriptor USERNAME = new 
PropertyDescriptor.Builder()
+        .name("service-user-name")
+        .displayName("Username")
+        .description("Username on the service that is being accessed.")
+        .dependsOn(GRANT_TYPE, RESOURCE_OWNER_PASSWORD_CREDENTIALS_GRANT_TYPE)
+        .required(true)
+        .addValidator(StandardValidators.NON_BLANK_VALIDATOR)
+        .expressionLanguageSupported(ExpressionLanguageScope.VARIABLE_REGISTRY)
+        .build();
+
+    public static final PropertyDescriptor PASSWORD = new 
PropertyDescriptor.Builder()
+        .name("service-password")
+        .displayName("Password")
+        .description("Password for the username on the service that is being 
accessed.")
+        .dependsOn(GRANT_TYPE, RESOURCE_OWNER_PASSWORD_CREDENTIALS_GRANT_TYPE)
+        .required(true)
+        .sensitive(true)
+        .addValidator(StandardValidators.NON_BLANK_VALIDATOR)
+        .build();
+
+    public static final PropertyDescriptor CLIENT_ID = new 
PropertyDescriptor.Builder()
+        .name("client-id")
+        .displayName("Client ID")
+        .required(false)
+        .addValidator(StandardValidators.NON_BLANK_VALIDATOR)
+        .expressionLanguageSupported(ExpressionLanguageScope.VARIABLE_REGISTRY)
+        .build();
+
+    public static final PropertyDescriptor CLIENT_SECRET = new 
PropertyDescriptor.Builder()
+        .name("client-secret")
+        .displayName("Client secret")
+        .dependsOn(CLIENT_ID)
+        .required(true)
+        .sensitive(true)
+        .addValidator(StandardValidators.NON_BLANK_VALIDATOR)
+        .build();
+
+    public static final PropertyDescriptor SSL_CONTEXT = new 
PropertyDescriptor.Builder()
+        .name("ssl-context")
+        .displayName("SSL Context")
+        .addValidator(Validator.VALID)
+        .identifiesControllerService(SSLContextService.class)
+        .required(false)
+        .build();
+
+    private static final List<PropertyDescriptor> PROPERTIES = 
Collections.unmodifiableList(Arrays.asList(
+        AUTHORIZATION_SERVER_URL,
+        GRANT_TYPE,
+        USERNAME,
+        PASSWORD,
+        CLIENT_ID,
+        CLIENT_SECRET,
+        SSL_CONTEXT
+    ));
+
+    public static final ObjectMapper ACCESS_DETAILS_MAPPER = new ObjectMapper()
+        .configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false)
+        .setPropertyNamingStrategy(PropertyNamingStrategies.SNAKE_CASE);
+
+    private volatile String authorizationServerUrl;
+    private volatile OkHttpClient httpClient;
+
+    private volatile String grantType;
+    private volatile String username;
+    private volatile String password;
+    private volatile String clientId;
+    private volatile String clientSecret;
+
+    private volatile AccessToken accessDetails;
+
+    @Override
+    public List<PropertyDescriptor> getSupportedPropertyDescriptors() {
+        return PROPERTIES;
+    }
+
+    @OnEnabled
+    public void onEnabled(ConfigurationContext context) {
+        authorizationServerUrl = 
context.getProperty(AUTHORIZATION_SERVER_URL).evaluateAttributeExpressions().getValue();
+
+        httpClient = createHttpClient(context);
+
+        grantType = context.getProperty(GRANT_TYPE).getValue();
+        username = 
context.getProperty(USERNAME).evaluateAttributeExpressions().getValue();
+        password = context.getProperty(PASSWORD).getValue();
+        clientId = 
context.getProperty(CLIENT_ID).evaluateAttributeExpressions().getValue();
+        clientSecret = context.getProperty(CLIENT_SECRET).getValue();
+    }
+
+    @OnDisabled
+    public void onDisabled() {
+        accessDetails = null;
+    }
+
+    @Override
+    protected Collection<ValidationResult> customValidate(ValidationContext 
validationContext) {
+        final List<ValidationResult> validationResults = new 
ArrayList<>(super.customValidate(validationContext));
+
+        if (
+            
validationContext.getProperty(GRANT_TYPE).getValue().equals(CLIENT_CREDENTIALS_GRANT_TYPE.getValue())
+                && !validationContext.getProperty(CLIENT_ID).isSet()
+        ) {
+            validationResults.add(new 
ValidationResult.Builder().subject(CLIENT_ID.getDisplayName())
+                .valid(false)
+                .explanation(String.format(
+                    "When '%s' is set to '%s', '%s' is required",
+                    GRANT_TYPE.getDisplayName(),
+                    CLIENT_CREDENTIALS_GRANT_TYPE.getDisplayName(),
+                    CLIENT_ID.getDisplayName())
+                )
+                .build());
+        }
+
+        return validationResults;
+    }
+
+    protected OkHttpClient createHttpClient(ConfigurationContext context) {
+        OkHttpClient.Builder clientBuilder = new OkHttpClient.Builder();
+
+        SSLContextService sslService = 
context.getProperty(SSL_CONTEXT).asControllerService(SSLContextService.class);
+        if (sslService != null) {
+            final X509TrustManager trustManager = 
sslService.createTrustManager();
+            SSLContext sslContext = sslService.createContext();
+            clientBuilder.sslSocketFactory(sslContext.getSocketFactory(), 
trustManager);
+        }
+
+        return clientBuilder.build();
+    }
+
+    @Override
+    public AccessToken getAccessDetails() {
+        if (this.accessDetails == null) {
+            acquireAccessDetails();
+        } else if (this.accessDetails.isExpired()) {
+            if (this.accessDetails.getRefreshToken() == null) {
+                acquireAccessDetails();
+            } else {
+                try {
+                    refreshAccessDetails();
+                } catch (Exception e) {
+                    getLogger().info("Couldn't refresh access token.", e);
+                    acquireAccessDetails();
+                }
+            }
+        }
+
+        return accessDetails;
+    }
+
+    private void acquireAccessDetails() {
+        getLogger().debug("Getting a new access token.");
+
+        FormBody.Builder acquireTokenBuilder = new FormBody.Builder();
+
+        if 
(grantType.equals(RESOURCE_OWNER_PASSWORD_CREDENTIALS_GRANT_TYPE.getValue())) {
+            acquireTokenBuilder.add("grant_type", "password")
+                .add("username", username)
+                .add("password", password);
+        } else if (grantType.equals(CLIENT_CREDENTIALS_GRANT_TYPE.getValue())) 
{
+            acquireTokenBuilder.add("grant_type", "client_credentials");
+        }
+
+        if (clientId != null) {
+            acquireTokenBuilder.add("client_id", clientId);
+            acquireTokenBuilder.add("client_secret", clientSecret);
+        }
+
+        RequestBody acquireTokenRequestBody = acquireTokenBuilder.build();
+
+        Request acquireTokenRequest = new Request.Builder()
+            .url(authorizationServerUrl)
+            .post(acquireTokenRequestBody)
+            .build();
+
+        this.accessDetails = getAccessDetails(acquireTokenRequest);
+    }
+
+    private void refreshAccessDetails() {
+        getLogger().debug("Refreshing access token.");
+
+        FormBody.Builder refreshTokenBuilder = new FormBody.Builder()
+            .add("grant_type", "refresh_token")
+            .add("refresh_token", this.accessDetails.getRefreshToken());
+
+        if (clientId != null) {
+            refreshTokenBuilder.add("client_id", clientId);
+            refreshTokenBuilder.add("client_secret", clientSecret);
+        }
+
+        RequestBody refreshTokenRequestBody = refreshTokenBuilder.build();
+
+        Request refreshRequest = new Request.Builder()
+            .url(authorizationServerUrl)
+            .post(refreshTokenRequestBody)
+            .build();
+
+        this.accessDetails = getAccessDetails(refreshRequest);
+    }
+
+    private AccessToken getAccessDetails(Request newRequest) {
+        try {
+            Response response = httpClient.newCall(newRequest).execute();
+            String responseBody = response.body().string();
+            if (response.code() != 200) {
+                getLogger().error(String.format("Bad response from the server 
during oauth2 request:\n%s", responseBody));
+                throw new ProcessException(String.format("Got HTTP %d during 
oauth2 request.", response.code()));
+            }
+
+            AccessToken accessDetails = 
ACCESS_DETAILS_MAPPER.readValue(responseBody, AccessToken.class);
+
+            return accessDetails;
+        } catch (IOException e) {
+            throw new UncheckedIOException(e);

Review comment:
       Recommend including a message with this exception:
   ```suggestion
               throw new UncheckedIOException("OAuth2 Request Failed 
[Connection Problem]", e);
   ```




-- 
This is an automated message from the Apache Git Service.
To respond to the message, please log on to GitHub and use the
URL above to go to the specific comment.

To unsubscribe, e-mail: issues-unsubscr...@nifi.apache.org

For queries about this service, please contact Infrastructure at:
us...@infra.apache.org


Reply via email to