dsmiley commented on code in PR #1935:
URL: https://github.com/apache/solr/pull/1935#discussion_r1340591638


##########
solr/core/src/java/org/apache/solr/util/EnvUtils.java:
##########
@@ -0,0 +1,279 @@
+/*
+ * 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.solr.util;
+
+import java.io.IOException;
+import java.io.InputStream;
+import java.io.InputStreamReader;
+import java.nio.charset.StandardCharsets;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Locale;
+import java.util.Map;
+import java.util.Objects;
+import java.util.Properties;
+import java.util.SortedMap;
+import java.util.TreeMap;
+import java.util.stream.Collectors;
+import org.apache.solr.common.SolrException;
+import org.apache.solr.common.util.StrUtils;
+import org.apache.solr.common.util.Utils;
+
+/**
+ * This class is a unified provider of environment variables and system 
properties. It exposes a
+ * mutable copy of the environment variables. It also converts 'SOLR_FOO' 
variables to system
+ * properties 'solr.foo' and provide various convenience accessors for them.
+ */
+public class EnvUtils {
+  private static final SortedMap<String, String> ENV = new 
TreeMap<>(System.getenv());
+  private static final Map<String, String> CUSTOM_MAPPINGS = new HashMap<>();
+  private static boolean initialized = false;
+
+  static {
+    try {
+      Properties props = new Properties();
+      try (InputStream stream =
+          
EnvUtils.class.getClassLoader().getResourceAsStream("EnvToSyspropMappings.properties"))
 {
+        props.load(new InputStreamReader(Objects.requireNonNull(stream), 
StandardCharsets.UTF_8));
+        for (String key : props.stringPropertyNames()) {
+          CUSTOM_MAPPINGS.put(key, props.getProperty(key));
+        }
+        init(false);
+      }
+    } catch (IOException e) {
+      throw new SolrException(
+          SolrException.ErrorCode.INVALID_STATE, "Failed loading 
env.var->properties mapping", e);
+    }
+  }
+
+  /**
+   * Get Solr's mutable copy of all environment variables.
+   *
+   * @return sorted map of environment variables
+   */
+  public static SortedMap<String, String> getEnvs() {
+    return ENV;
+  }
+
+  /** Get a single environment variable as string */
+  public static String getEnv(String key) {
+    return ENV.get(key);
+  }
+
+  /** Get a single environment variable as string, or default */
+  public static String getEnv(String key, String defaultValue) {
+    return ENV.getOrDefault(key, defaultValue);
+  }
+
+  /** Get an environment variable as long */
+  public static long getEnvAsLong(String key) {
+    return Long.parseLong(ENV.get(key));
+  }
+
+  /** Get an environment variable as long, or default value */
+  public static long getEnvAsLong(String key, long defaultValue) {
+    String value = ENV.get(key);
+    if (value == null) {
+      return defaultValue;
+    }
+    return Long.parseLong(value);
+  }
+
+  /** Get an env var as boolean */
+  public static boolean getEnvAsBool(String key) {
+    return StrUtils.parseBool(ENV.get(key));
+  }
+
+  /** Get an env var as boolean, or default value */
+  public static boolean getEnvAsBool(String key, boolean defaultValue) {
+    String value = ENV.get(key);
+    if (value == null) {
+      return defaultValue;
+    }
+    return StrUtils.parseBool(value);
+  }
+
+  /** Get comma separated strings from env as List */
+  public static List<String> getEnvAsList(String key) {
+    return getEnv(key) != null ? stringValueToList(getEnv(key)) : null;
+  }
+
+  /** Get comma separated strings from env as List */
+  public static List<String> getEnvAsList(String key, List<String> 
defaultValue) {
+    return ENV.get(key) != null ? getEnvAsList(key) : defaultValue;
+  }
+
+  /** Set an environment variable */
+  public static void setEnv(String key, String value) {
+    ENV.put(key, value);
+  }
+
+  /** Set all environment variables */
+  public static synchronized void setEnvs(Map<String, String> env) {
+    ENV.clear();
+    ENV.putAll(env);
+  }
+
+  /** Get all Solr system properties as a sorted map */
+  public static SortedMap<String, String> getProps() {
+    ensureInitialized();
+    SortedMap<String, String> props = new TreeMap<>();
+    for (String key : System.getProperties().stringPropertyNames()) {

Review Comment:
   as Properties implements Map; can't we enumerate entrySet entries?  This is 
preferred in general when copying Maps (or Map subclasses) to calling "get" on 
each key that you separately enumerate.



##########
solr/core/src/java/org/apache/solr/util/EnvUtils.java:
##########
@@ -0,0 +1,279 @@
+/*
+ * 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.solr.util;
+
+import java.io.IOException;
+import java.io.InputStream;
+import java.io.InputStreamReader;
+import java.nio.charset.StandardCharsets;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Locale;
+import java.util.Map;
+import java.util.Objects;
+import java.util.Properties;
+import java.util.SortedMap;
+import java.util.TreeMap;
+import java.util.stream.Collectors;
+import org.apache.solr.common.SolrException;
+import org.apache.solr.common.util.StrUtils;
+import org.apache.solr.common.util.Utils;
+
+/**
+ * This class is a unified provider of environment variables and system 
properties. It exposes a
+ * mutable copy of the environment variables. It also converts 'SOLR_FOO' 
variables to system
+ * properties 'solr.foo' and provide various convenience accessors for them.
+ */
+public class EnvUtils {
+  private static final SortedMap<String, String> ENV = new 
TreeMap<>(System.getenv());
+  private static final Map<String, String> CUSTOM_MAPPINGS = new HashMap<>();
+  private static boolean initialized = false;
+
+  static {
+    try {
+      Properties props = new Properties();
+      try (InputStream stream =
+          
EnvUtils.class.getClassLoader().getResourceAsStream("EnvToSyspropMappings.properties"))
 {
+        props.load(new InputStreamReader(Objects.requireNonNull(stream), 
StandardCharsets.UTF_8));
+        for (String key : props.stringPropertyNames()) {
+          CUSTOM_MAPPINGS.put(key, props.getProperty(key));
+        }
+        init(false);
+      }
+    } catch (IOException e) {
+      throw new SolrException(
+          SolrException.ErrorCode.INVALID_STATE, "Failed loading 
env.var->properties mapping", e);
+    }
+  }
+
+  /**
+   * Get Solr's mutable copy of all environment variables.
+   *
+   * @return sorted map of environment variables
+   */
+  public static SortedMap<String, String> getEnvs() {
+    return ENV;
+  }
+
+  /** Get a single environment variable as string */
+  public static String getEnv(String key) {
+    return ENV.get(key);
+  }
+
+  /** Get a single environment variable as string, or default */
+  public static String getEnv(String key, String defaultValue) {
+    return ENV.getOrDefault(key, defaultValue);
+  }
+
+  /** Get an environment variable as long */
+  public static long getEnvAsLong(String key) {
+    return Long.parseLong(ENV.get(key));
+  }
+
+  /** Get an environment variable as long, or default value */
+  public static long getEnvAsLong(String key, long defaultValue) {
+    String value = ENV.get(key);
+    if (value == null) {
+      return defaultValue;
+    }
+    return Long.parseLong(value);
+  }
+
+  /** Get an env var as boolean */
+  public static boolean getEnvAsBool(String key) {
+    return StrUtils.parseBool(ENV.get(key));
+  }
+
+  /** Get an env var as boolean, or default value */
+  public static boolean getEnvAsBool(String key, boolean defaultValue) {
+    String value = ENV.get(key);
+    if (value == null) {
+      return defaultValue;
+    }
+    return StrUtils.parseBool(value);
+  }
+
+  /** Get comma separated strings from env as List */
+  public static List<String> getEnvAsList(String key) {
+    return getEnv(key) != null ? stringValueToList(getEnv(key)) : null;
+  }
+
+  /** Get comma separated strings from env as List */
+  public static List<String> getEnvAsList(String key, List<String> 
defaultValue) {
+    return ENV.get(key) != null ? getEnvAsList(key) : defaultValue;
+  }
+
+  /** Set an environment variable */
+  public static void setEnv(String key, String value) {
+    ENV.put(key, value);
+  }
+
+  /** Set all environment variables */
+  public static synchronized void setEnvs(Map<String, String> env) {
+    ENV.clear();
+    ENV.putAll(env);
+  }
+
+  /** Get all Solr system properties as a sorted map */
+  public static SortedMap<String, String> getProps() {
+    ensureInitialized();
+    SortedMap<String, String> props = new TreeMap<>();
+    for (String key : System.getProperties().stringPropertyNames()) {
+      props.put(key, System.getProperty(key));
+    }
+    return props;
+  }
+
+  /** Get a property as string */
+  public static String getProp(String key) {
+    return getProp(key, null);
+  }
+
+  /**
+   * Get a property as string with a fallback value. All other getProp* 
methods use this.
+   *
+   * @param key property key, which treats 'camelCase' the same as 'camel.case'
+   */
+  public static String getProp(String key, String defaultValue) {
+    ensureInitialized(); // Avoid race condition with init()
+    String value = getPropWithCamelCaseFallback(key);
+    return value != null ? value : defaultValue;
+  }
+
+  /**
+   * Get a property from given key or an alias key converted from CamelCase to 
dot separated.
+   *
+   * @return property value or value of dot-separated alias key or null if not 
found
+   */
+  private static String getPropWithCamelCaseFallback(String key) {
+    String value = System.getProperty(key);
+    if (value != null) {
+      return value;
+    } else {
+      // Figure out if string is CamelCase and convert to dot separated
+      String altKey = camelCaseToDotSeparated(key);
+      return System.getProperty(altKey);
+    }
+  }
+
+  private static String camelCaseToDotSeparated(String key) {
+    return String.join(".", key.split("(?=[A-Z])")).replace("..", 
".").toLowerCase(Locale.ROOT);
+  }
+
+  /** Get property as integer */
+  public static Long getPropAsLong(String key) {
+    return getPropAsLong(key, null);
+  }
+
+  /** Get property as long, or default value */
+  public static Long getPropAsLong(String key, Long defaultValue) {
+    String value = getProp(key);
+    if (value == null) {
+      return defaultValue;
+    }
+    return Long.parseLong(value);
+  }
+
+  /** Get property as boolean */
+  public static Boolean getPropAsBool(String key) {
+    return getPropAsBool(key, null);
+  }
+
+  /** Get property as boolean, or default value */
+  public static Boolean getPropAsBool(String key, Boolean defaultValue) {
+    String value = getProp(key);
+    if (value == null) {
+      return defaultValue;
+    }
+    return StrUtils.parseBool(value);
+  }
+
+  /**
+   * Get comma separated strings from sysprop as List
+   *
+   * @return list of strings, or null if not found
+   */
+  public static List<String> getPropAsList(String key) {
+    return getPropAsList(key, null);
+  }
+
+  /**
+   * Get comma separated strings from sysprop as List, or default value
+   *
+   * @return list of strings, or provided default if not found
+   */
+  public static List<String> getPropAsList(String key, List<String> 
defaultValue) {
+    return getProp(key) != null ? stringValueToList(getProp(key)) : 
defaultValue;
+  }
+
+  /** Set a system property. Shim to {@link System#setProperty(String, 
String)} */
+  public static void setProp(String key, String value) {
+    System.setProperty(key, value);
+    System.setProperty(camelCaseToDotSeparated(key), value);
+  }
+
+  /**
+   * Re-reads environment variables and updates the internal map.
+   *
+   * @param overwrite if true, overwrite existing system properties with 
environment variables
+   */
+  public static synchronized void init(boolean overwrite) {

Review Comment:
   Why would this be called?  If nobody should call it except deep in Solr one 
place (probably true) then say so.



##########
solr/core/src/java/org/apache/solr/util/EnvUtils.java:
##########
@@ -0,0 +1,279 @@
+/*
+ * 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.solr.util;
+
+import java.io.IOException;
+import java.io.InputStream;
+import java.io.InputStreamReader;
+import java.nio.charset.StandardCharsets;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Locale;
+import java.util.Map;
+import java.util.Objects;
+import java.util.Properties;
+import java.util.SortedMap;
+import java.util.TreeMap;
+import java.util.stream.Collectors;
+import org.apache.solr.common.SolrException;
+import org.apache.solr.common.util.StrUtils;
+import org.apache.solr.common.util.Utils;
+
+/**
+ * This class is a unified provider of environment variables and system 
properties. It exposes a
+ * mutable copy of the environment variables. It also converts 'SOLR_FOO' 
variables to system
+ * properties 'solr.foo' and provide various convenience accessors for them.
+ */
+public class EnvUtils {
+  private static final SortedMap<String, String> ENV = new 
TreeMap<>(System.getenv());
+  private static final Map<String, String> CUSTOM_MAPPINGS = new HashMap<>();
+  private static boolean initialized = false;
+
+  static {
+    try {
+      Properties props = new Properties();
+      try (InputStream stream =
+          
EnvUtils.class.getClassLoader().getResourceAsStream("EnvToSyspropMappings.properties"))
 {
+        props.load(new InputStreamReader(Objects.requireNonNull(stream), 
StandardCharsets.UTF_8));
+        for (String key : props.stringPropertyNames()) {
+          CUSTOM_MAPPINGS.put(key, props.getProperty(key));
+        }
+        init(false);
+      }
+    } catch (IOException e) {
+      throw new SolrException(
+          SolrException.ErrorCode.INVALID_STATE, "Failed loading 
env.var->properties mapping", e);
+    }
+  }
+
+  /**
+   * Get Solr's mutable copy of all environment variables.
+   *
+   * @return sorted map of environment variables
+   */
+  public static SortedMap<String, String> getEnvs() {
+    return ENV;
+  }
+
+  /** Get a single environment variable as string */
+  public static String getEnv(String key) {
+    return ENV.get(key);
+  }
+
+  /** Get a single environment variable as string, or default */
+  public static String getEnv(String key, String defaultValue) {
+    return ENV.getOrDefault(key, defaultValue);
+  }
+
+  /** Get an environment variable as long */
+  public static long getEnvAsLong(String key) {
+    return Long.parseLong(ENV.get(key));
+  }
+
+  /** Get an environment variable as long, or default value */
+  public static long getEnvAsLong(String key, long defaultValue) {
+    String value = ENV.get(key);
+    if (value == null) {
+      return defaultValue;
+    }
+    return Long.parseLong(value);
+  }
+
+  /** Get an env var as boolean */
+  public static boolean getEnvAsBool(String key) {
+    return StrUtils.parseBool(ENV.get(key));
+  }
+
+  /** Get an env var as boolean, or default value */
+  public static boolean getEnvAsBool(String key, boolean defaultValue) {
+    String value = ENV.get(key);
+    if (value == null) {
+      return defaultValue;
+    }
+    return StrUtils.parseBool(value);
+  }
+
+  /** Get comma separated strings from env as List */
+  public static List<String> getEnvAsList(String key) {
+    return getEnv(key) != null ? stringValueToList(getEnv(key)) : null;
+  }
+
+  /** Get comma separated strings from env as List */
+  public static List<String> getEnvAsList(String key, List<String> 
defaultValue) {
+    return ENV.get(key) != null ? getEnvAsList(key) : defaultValue;
+  }
+
+  /** Set an environment variable */
+  public static void setEnv(String key, String value) {
+    ENV.put(key, value);
+  }
+
+  /** Set all environment variables */
+  public static synchronized void setEnvs(Map<String, String> env) {
+    ENV.clear();
+    ENV.putAll(env);
+  }
+
+  /** Get all Solr system properties as a sorted map */
+  public static SortedMap<String, String> getProps() {

Review Comment:
   I assume sorting is for making the results pretty if printed.  But can we 
assume the caller is going to print them?  Not sure why we need this a tall 
actually.



##########
solr/core/src/java/org/apache/solr/util/EnvUtils.java:
##########
@@ -0,0 +1,279 @@
+/*
+ * 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.solr.util;
+
+import java.io.IOException;
+import java.io.InputStream;
+import java.io.InputStreamReader;
+import java.nio.charset.StandardCharsets;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Locale;
+import java.util.Map;
+import java.util.Objects;
+import java.util.Properties;
+import java.util.SortedMap;
+import java.util.TreeMap;
+import java.util.stream.Collectors;
+import org.apache.solr.common.SolrException;
+import org.apache.solr.common.util.StrUtils;
+import org.apache.solr.common.util.Utils;
+
+/**
+ * This class is a unified provider of environment variables and system 
properties. It exposes a
+ * mutable copy of the environment variables. It also converts 'SOLR_FOO' 
variables to system
+ * properties 'solr.foo' and provide various convenience accessors for them.
+ */
+public class EnvUtils {
+  private static final SortedMap<String, String> ENV = new 
TreeMap<>(System.getenv());
+  private static final Map<String, String> CUSTOM_MAPPINGS = new HashMap<>();
+  private static boolean initialized = false;
+
+  static {
+    try {
+      Properties props = new Properties();
+      try (InputStream stream =
+          
EnvUtils.class.getClassLoader().getResourceAsStream("EnvToSyspropMappings.properties"))
 {
+        props.load(new InputStreamReader(Objects.requireNonNull(stream), 
StandardCharsets.UTF_8));
+        for (String key : props.stringPropertyNames()) {
+          CUSTOM_MAPPINGS.put(key, props.getProperty(key));
+        }
+        init(false);
+      }
+    } catch (IOException e) {
+      throw new SolrException(
+          SolrException.ErrorCode.INVALID_STATE, "Failed loading 
env.var->properties mapping", e);
+    }
+  }
+
+  /**
+   * Get Solr's mutable copy of all environment variables.
+   *
+   * @return sorted map of environment variables
+   */
+  public static SortedMap<String, String> getEnvs() {
+    return ENV;
+  }
+
+  /** Get a single environment variable as string */
+  public static String getEnv(String key) {
+    return ENV.get(key);
+  }
+
+  /** Get a single environment variable as string, or default */
+  public static String getEnv(String key, String defaultValue) {
+    return ENV.getOrDefault(key, defaultValue);
+  }
+
+  /** Get an environment variable as long */
+  public static long getEnvAsLong(String key) {
+    return Long.parseLong(ENV.get(key));
+  }
+
+  /** Get an environment variable as long, or default value */
+  public static long getEnvAsLong(String key, long defaultValue) {
+    String value = ENV.get(key);
+    if (value == null) {
+      return defaultValue;
+    }
+    return Long.parseLong(value);
+  }
+
+  /** Get an env var as boolean */
+  public static boolean getEnvAsBool(String key) {
+    return StrUtils.parseBool(ENV.get(key));
+  }
+
+  /** Get an env var as boolean, or default value */
+  public static boolean getEnvAsBool(String key, boolean defaultValue) {
+    String value = ENV.get(key);
+    if (value == null) {
+      return defaultValue;
+    }
+    return StrUtils.parseBool(value);
+  }
+
+  /** Get comma separated strings from env as List */
+  public static List<String> getEnvAsList(String key) {
+    return getEnv(key) != null ? stringValueToList(getEnv(key)) : null;
+  }
+
+  /** Get comma separated strings from env as List */
+  public static List<String> getEnvAsList(String key, List<String> 
defaultValue) {
+    return ENV.get(key) != null ? getEnvAsList(key) : defaultValue;
+  }
+
+  /** Set an environment variable */
+  public static void setEnv(String key, String value) {
+    ENV.put(key, value);
+  }
+
+  /** Set all environment variables */
+  public static synchronized void setEnvs(Map<String, String> env) {
+    ENV.clear();
+    ENV.putAll(env);
+  }
+
+  /** Get all Solr system properties as a sorted map */
+  public static SortedMap<String, String> getProps() {
+    ensureInitialized();
+    SortedMap<String, String> props = new TreeMap<>();
+    for (String key : System.getProperties().stringPropertyNames()) {
+      props.put(key, System.getProperty(key));
+    }
+    return props;
+  }
+
+  /** Get a property as string */
+  public static String getProp(String key) {
+    return getProp(key, null);
+  }
+
+  /**
+   * Get a property as string with a fallback value. All other getProp* 
methods use this.
+   *
+   * @param key property key, which treats 'camelCase' the same as 'camel.case'
+   */
+  public static String getProp(String key, String defaultValue) {
+    ensureInitialized(); // Avoid race condition with init()
+    String value = getPropWithCamelCaseFallback(key);
+    return value != null ? value : defaultValue;
+  }
+
+  /**
+   * Get a property from given key or an alias key converted from CamelCase to 
dot separated.
+   *
+   * @return property value or value of dot-separated alias key or null if not 
found
+   */
+  private static String getPropWithCamelCaseFallback(String key) {

Review Comment:
   We should put cached answers in a ConcurrentHashMap so that string 
manipulations needn't be re-parsed/resolved on every fetch (calling 
camelCaseToDotSeparated).  The query path might reasonably want to call getProp 
and it'd be annoying to see that for obscure features.



##########
solr/core/src/java/org/apache/solr/util/EnvUtils.java:
##########
@@ -0,0 +1,279 @@
+/*
+ * 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.solr.util;
+
+import java.io.IOException;
+import java.io.InputStream;
+import java.io.InputStreamReader;
+import java.nio.charset.StandardCharsets;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Locale;
+import java.util.Map;
+import java.util.Objects;
+import java.util.Properties;
+import java.util.SortedMap;
+import java.util.TreeMap;
+import java.util.stream.Collectors;
+import org.apache.solr.common.SolrException;
+import org.apache.solr.common.util.StrUtils;
+import org.apache.solr.common.util.Utils;
+
+/**
+ * This class is a unified provider of environment variables and system 
properties. It exposes a
+ * mutable copy of the environment variables. It also converts 'SOLR_FOO' 
variables to system
+ * properties 'solr.foo' and provide various convenience accessors for them.
+ */
+public class EnvUtils {
+  private static final SortedMap<String, String> ENV = new 
TreeMap<>(System.getenv());
+  private static final Map<String, String> CUSTOM_MAPPINGS = new HashMap<>();
+  private static boolean initialized = false;
+
+  static {
+    try {
+      Properties props = new Properties();
+      try (InputStream stream =
+          
EnvUtils.class.getClassLoader().getResourceAsStream("EnvToSyspropMappings.properties"))
 {
+        props.load(new InputStreamReader(Objects.requireNonNull(stream), 
StandardCharsets.UTF_8));
+        for (String key : props.stringPropertyNames()) {
+          CUSTOM_MAPPINGS.put(key, props.getProperty(key));
+        }
+        init(false);
+      }
+    } catch (IOException e) {
+      throw new SolrException(
+          SolrException.ErrorCode.INVALID_STATE, "Failed loading 
env.var->properties mapping", e);
+    }
+  }
+
+  /**
+   * Get Solr's mutable copy of all environment variables.
+   *
+   * @return sorted map of environment variables
+   */
+  public static SortedMap<String, String> getEnvs() {
+    return ENV;
+  }
+
+  /** Get a single environment variable as string */
+  public static String getEnv(String key) {
+    return ENV.get(key);
+  }
+
+  /** Get a single environment variable as string, or default */
+  public static String getEnv(String key, String defaultValue) {
+    return ENV.getOrDefault(key, defaultValue);
+  }
+
+  /** Get an environment variable as long */
+  public static long getEnvAsLong(String key) {
+    return Long.parseLong(ENV.get(key));
+  }
+
+  /** Get an environment variable as long, or default value */
+  public static long getEnvAsLong(String key, long defaultValue) {
+    String value = ENV.get(key);
+    if (value == null) {
+      return defaultValue;
+    }
+    return Long.parseLong(value);
+  }
+
+  /** Get an env var as boolean */
+  public static boolean getEnvAsBool(String key) {
+    return StrUtils.parseBool(ENV.get(key));
+  }
+
+  /** Get an env var as boolean, or default value */
+  public static boolean getEnvAsBool(String key, boolean defaultValue) {
+    String value = ENV.get(key);
+    if (value == null) {
+      return defaultValue;
+    }
+    return StrUtils.parseBool(value);
+  }
+
+  /** Get comma separated strings from env as List */
+  public static List<String> getEnvAsList(String key) {
+    return getEnv(key) != null ? stringValueToList(getEnv(key)) : null;
+  }
+
+  /** Get comma separated strings from env as List */
+  public static List<String> getEnvAsList(String key, List<String> 
defaultValue) {
+    return ENV.get(key) != null ? getEnvAsList(key) : defaultValue;
+  }
+
+  /** Set an environment variable */
+  public static void setEnv(String key, String value) {
+    ENV.put(key, value);
+  }
+
+  /** Set all environment variables */
+  public static synchronized void setEnvs(Map<String, String> env) {
+    ENV.clear();
+    ENV.putAll(env);
+  }
+
+  /** Get all Solr system properties as a sorted map */
+  public static SortedMap<String, String> getProps() {
+    ensureInitialized();
+    SortedMap<String, String> props = new TreeMap<>();
+    for (String key : System.getProperties().stringPropertyNames()) {
+      props.put(key, System.getProperty(key));
+    }
+    return props;
+  }
+
+  /** Get a property as string */
+  public static String getProp(String key) {
+    return getProp(key, null);
+  }
+
+  /**
+   * Get a property as string with a fallback value. All other getProp* 
methods use this.
+   *
+   * @param key property key, which treats 'camelCase' the same as 'camel.case'
+   */
+  public static String getProp(String key, String defaultValue) {
+    ensureInitialized(); // Avoid race condition with init()
+    String value = getPropWithCamelCaseFallback(key);
+    return value != null ? value : defaultValue;
+  }
+
+  /**
+   * Get a property from given key or an alias key converted from CamelCase to 
dot separated.
+   *
+   * @return property value or value of dot-separated alias key or null if not 
found
+   */
+  private static String getPropWithCamelCaseFallback(String key) {
+    String value = System.getProperty(key);
+    if (value != null) {
+      return value;
+    } else {
+      // Figure out if string is CamelCase and convert to dot separated
+      String altKey = camelCaseToDotSeparated(key);
+      return System.getProperty(altKey);
+    }
+  }
+
+  private static String camelCaseToDotSeparated(String key) {
+    return String.join(".", key.split("(?=[A-Z])")).replace("..", 
".").toLowerCase(Locale.ROOT);
+  }
+
+  /** Get property as integer */
+  public static Long getPropAsLong(String key) {
+    return getPropAsLong(key, null);
+  }
+
+  /** Get property as long, or default value */
+  public static Long getPropAsLong(String key, Long defaultValue) {
+    String value = getProp(key);
+    if (value == null) {
+      return defaultValue;
+    }
+    return Long.parseLong(value);
+  }
+
+  /** Get property as boolean */
+  public static Boolean getPropAsBool(String key) {
+    return getPropAsBool(key, null);
+  }
+
+  /** Get property as boolean, or default value */
+  public static Boolean getPropAsBool(String key, Boolean defaultValue) {
+    String value = getProp(key);
+    if (value == null) {
+      return defaultValue;
+    }
+    return StrUtils.parseBool(value);
+  }
+
+  /**
+   * Get comma separated strings from sysprop as List
+   *
+   * @return list of strings, or null if not found
+   */
+  public static List<String> getPropAsList(String key) {
+    return getPropAsList(key, null);
+  }
+
+  /**
+   * Get comma separated strings from sysprop as List, or default value
+   *
+   * @return list of strings, or provided default if not found
+   */
+  public static List<String> getPropAsList(String key, List<String> 
defaultValue) {
+    return getProp(key) != null ? stringValueToList(getProp(key)) : 
defaultValue;
+  }
+
+  /** Set a system property. Shim to {@link System#setProperty(String, 
String)} */
+  public static void setProp(String key, String value) {
+    System.setProperty(key, value);
+    System.setProperty(camelCaseToDotSeparated(key), value);
+  }
+
+  /**
+   * Re-reads environment variables and updates the internal map.
+   *
+   * @param overwrite if true, overwrite existing system properties with 
environment variables
+   */
+  public static synchronized void init(boolean overwrite) {
+    // Convert eligible environment variables to system properties
+    for (String key : ENV.keySet().toArray(String[]::new)) {
+      if (key.startsWith("SOLR_") || CUSTOM_MAPPINGS.containsKey(key)) {
+        String sysPropKey = envNameToSyspropName(key);
+        // Existing system properties take precedence
+        if (!sysPropKey.isBlank() && (overwrite || getProp(sysPropKey) == 
null)) {
+          setProp(sysPropKey, ENV.get(key));
+        }
+      }
+    }
+    initialized = true;
+  }
+
+  protected static String envNameToSyspropName(String envName) {
+    return CUSTOM_MAPPINGS.containsKey(envName)
+        ? CUSTOM_MAPPINGS.get(envName)
+        : envName.toLowerCase(Locale.ROOT).replace("_", ".");
+  }
+
+  @SuppressWarnings("unchecked")
+  private static List<String> stringValueToList(String string) {
+    if (string.startsWith("[") && string.endsWith("]")) {
+      // Convert a JSON string to a List<String> using Noggit parser
+      return (List<String>) Utils.fromJSONString(string);
+    } else {
+      return StrUtils.splitSmart(string, ",", true).stream()

Review Comment:
   maybe comment to say this decodes backslash encoding



##########
solr/core/src/java/org/apache/solr/util/ModuleUtils.java:
##########
@@ -56,17 +56,7 @@ public static Path getModuleLibPath(Path solrInstallDirPath, 
String moduleName)
    * @return set of raw volume names from sysprop and/or env.var
    */
   static Set<String> resolveFromSyspropOrEnv() {
-    // Fall back to sysprop and env.var if nothing configured through solr.xml
-    Set<String> mods = new HashSet<>();
-    String modulesFromProps = System.getProperty("solr.modules");
-    if (StrUtils.isNotNullOrEmpty(modulesFromProps)) {
-      mods.addAll(StrUtils.splitSmart(modulesFromProps, ',', true));
-    }
-    String modulesFromEnv = System.getenv("SOLR_MODULES");
-    if (StrUtils.isNotNullOrEmpty(modulesFromEnv)) {
-      mods.addAll(StrUtils.splitSmart(modulesFromEnv, ',', true));
-    }
-    return mods.stream().map(String::trim).collect(Collectors.toSet());
+    return new HashSet<>(EnvUtils.getPropAsList("solr.modules", 
Collections.emptyList()));

Review Comment:
   `Set.copyOf` since we probably don't need mutable result



##########
solr/core/src/test/org/apache/solr/util/EnvUtilsTest.java:
##########
@@ -0,0 +1,117 @@
+/*
+ * 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.solr.util;
+
+import java.util.List;
+import java.util.Map;
+import org.apache.solr.SolrTestCase;
+import org.junit.BeforeClass;
+import org.junit.Test;
+
+public class EnvUtilsTest extends SolrTestCase {
+  @BeforeClass
+  public static void beforeClass() throws Exception {
+    // Make a map of some common Solr environment variables for testing, and 
initialize EnvUtils
+    EnvUtils.setEnvs(

Review Comment:
   probably also test backslash encoding of newline, say, since you called 
splitSmart with "true" for decode.



##########
solr/core/src/java/org/apache/solr/util/EnvUtils.java:
##########
@@ -0,0 +1,279 @@
+/*
+ * 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.solr.util;
+
+import java.io.IOException;
+import java.io.InputStream;
+import java.io.InputStreamReader;
+import java.nio.charset.StandardCharsets;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Locale;
+import java.util.Map;
+import java.util.Objects;
+import java.util.Properties;
+import java.util.SortedMap;
+import java.util.TreeMap;
+import java.util.stream.Collectors;
+import org.apache.solr.common.SolrException;
+import org.apache.solr.common.util.StrUtils;
+import org.apache.solr.common.util.Utils;
+
+/**
+ * This class is a unified provider of environment variables and system 
properties. It exposes a
+ * mutable copy of the environment variables. It also converts 'SOLR_FOO' 
variables to system
+ * properties 'solr.foo' and provide various convenience accessors for them.
+ */
+public class EnvUtils {
+  private static final SortedMap<String, String> ENV = new 
TreeMap<>(System.getenv());
+  private static final Map<String, String> CUSTOM_MAPPINGS = new HashMap<>();
+  private static boolean initialized = false;
+
+  static {
+    try {
+      Properties props = new Properties();
+      try (InputStream stream =
+          
EnvUtils.class.getClassLoader().getResourceAsStream("EnvToSyspropMappings.properties"))
 {
+        props.load(new InputStreamReader(Objects.requireNonNull(stream), 
StandardCharsets.UTF_8));
+        for (String key : props.stringPropertyNames()) {
+          CUSTOM_MAPPINGS.put(key, props.getProperty(key));
+        }
+        init(false);
+      }
+    } catch (IOException e) {
+      throw new SolrException(
+          SolrException.ErrorCode.INVALID_STATE, "Failed loading 
env.var->properties mapping", e);
+    }
+  }
+
+  /**
+   * Get Solr's mutable copy of all environment variables.
+   *
+   * @return sorted map of environment variables
+   */
+  public static SortedMap<String, String> getEnvs() {
+    return ENV;
+  }
+
+  /** Get a single environment variable as string */
+  public static String getEnv(String key) {
+    return ENV.get(key);
+  }
+
+  /** Get a single environment variable as string, or default */
+  public static String getEnv(String key, String defaultValue) {
+    return ENV.getOrDefault(key, defaultValue);
+  }
+
+  /** Get an environment variable as long */
+  public static long getEnvAsLong(String key) {
+    return Long.parseLong(ENV.get(key));
+  }
+
+  /** Get an environment variable as long, or default value */
+  public static long getEnvAsLong(String key, long defaultValue) {
+    String value = ENV.get(key);
+    if (value == null) {
+      return defaultValue;
+    }
+    return Long.parseLong(value);
+  }
+
+  /** Get an env var as boolean */
+  public static boolean getEnvAsBool(String key) {
+    return StrUtils.parseBool(ENV.get(key));
+  }
+
+  /** Get an env var as boolean, or default value */
+  public static boolean getEnvAsBool(String key, boolean defaultValue) {
+    String value = ENV.get(key);
+    if (value == null) {
+      return defaultValue;
+    }
+    return StrUtils.parseBool(value);
+  }
+
+  /** Get comma separated strings from env as List */
+  public static List<String> getEnvAsList(String key) {
+    return getEnv(key) != null ? stringValueToList(getEnv(key)) : null;
+  }
+
+  /** Get comma separated strings from env as List */
+  public static List<String> getEnvAsList(String key, List<String> 
defaultValue) {
+    return ENV.get(key) != null ? getEnvAsList(key) : defaultValue;
+  }
+
+  /** Set an environment variable */
+  public static void setEnv(String key, String value) {
+    ENV.put(key, value);
+  }
+
+  /** Set all environment variables */
+  public static synchronized void setEnvs(Map<String, String> env) {
+    ENV.clear();
+    ENV.putAll(env);
+  }
+
+  /** Get all Solr system properties as a sorted map */
+  public static SortedMap<String, String> getProps() {
+    ensureInitialized();
+    SortedMap<String, String> props = new TreeMap<>();
+    for (String key : System.getProperties().stringPropertyNames()) {
+      props.put(key, System.getProperty(key));
+    }
+    return props;
+  }
+
+  /** Get a property as string */
+  public static String getProp(String key) {
+    return getProp(key, null);
+  }
+
+  /**
+   * Get a property as string with a fallback value. All other getProp* 
methods use this.
+   *
+   * @param key property key, which treats 'camelCase' the same as 'camel.case'
+   */
+  public static String getProp(String key, String defaultValue) {
+    ensureInitialized(); // Avoid race condition with init()
+    String value = getPropWithCamelCaseFallback(key);
+    return value != null ? value : defaultValue;
+  }
+
+  /**
+   * Get a property from given key or an alias key converted from CamelCase to 
dot separated.
+   *
+   * @return property value or value of dot-separated alias key or null if not 
found
+   */
+  private static String getPropWithCamelCaseFallback(String key) {
+    String value = System.getProperty(key);
+    if (value != null) {
+      return value;
+    } else {
+      // Figure out if string is CamelCase and convert to dot separated
+      String altKey = camelCaseToDotSeparated(key);
+      return System.getProperty(altKey);
+    }
+  }
+
+  private static String camelCaseToDotSeparated(String key) {
+    return String.join(".", key.split("(?=[A-Z])")).replace("..", 
".").toLowerCase(Locale.ROOT);
+  }
+
+  /** Get property as integer */
+  public static Long getPropAsLong(String key) {
+    return getPropAsLong(key, null);
+  }
+
+  /** Get property as long, or default value */
+  public static Long getPropAsLong(String key, Long defaultValue) {
+    String value = getProp(key);
+    if (value == null) {
+      return defaultValue;
+    }
+    return Long.parseLong(value);
+  }
+
+  /** Get property as boolean */
+  public static Boolean getPropAsBool(String key) {
+    return getPropAsBool(key, null);
+  }
+
+  /** Get property as boolean, or default value */
+  public static Boolean getPropAsBool(String key, Boolean defaultValue) {
+    String value = getProp(key);
+    if (value == null) {
+      return defaultValue;
+    }
+    return StrUtils.parseBool(value);
+  }
+
+  /**
+   * Get comma separated strings from sysprop as List
+   *
+   * @return list of strings, or null if not found
+   */
+  public static List<String> getPropAsList(String key) {
+    return getPropAsList(key, null);
+  }
+
+  /**
+   * Get comma separated strings from sysprop as List, or default value
+   *
+   * @return list of strings, or provided default if not found
+   */
+  public static List<String> getPropAsList(String key, List<String> 
defaultValue) {
+    return getProp(key) != null ? stringValueToList(getProp(key)) : 
defaultValue;
+  }
+
+  /** Set a system property. Shim to {@link System#setProperty(String, 
String)} */
+  public static void setProp(String key, String value) {
+    System.setProperty(key, value);
+    System.setProperty(camelCaseToDotSeparated(key), value);
+  }
+
+  /**
+   * Re-reads environment variables and updates the internal map.
+   *
+   * @param overwrite if true, overwrite existing system properties with 
environment variables
+   */
+  public static synchronized void init(boolean overwrite) {
+    // Convert eligible environment variables to system properties
+    for (String key : ENV.keySet().toArray(String[]::new)) {
+      if (key.startsWith("SOLR_") || CUSTOM_MAPPINGS.containsKey(key)) {
+        String sysPropKey = envNameToSyspropName(key);
+        // Existing system properties take precedence
+        if (!sysPropKey.isBlank() && (overwrite || getProp(sysPropKey) == 
null)) {
+          setProp(sysPropKey, ENV.get(key));
+        }
+      }
+    }
+    initialized = true;
+  }
+
+  protected static String envNameToSyspropName(String envName) {
+    return CUSTOM_MAPPINGS.containsKey(envName)
+        ? CUSTOM_MAPPINGS.get(envName)
+        : envName.toLowerCase(Locale.ROOT).replace("_", ".");
+  }
+
+  @SuppressWarnings("unchecked")
+  private static List<String> stringValueToList(String string) {
+    if (string.startsWith("[") && string.endsWith("]")) {
+      // Convert a JSON string to a List<String> using Noggit parser
+      return (List<String>) Utils.fromJSONString(string);

Review Comment:
   nice



##########
solr/core/src/java/org/apache/solr/util/EnvUtils.java:
##########
@@ -0,0 +1,279 @@
+/*
+ * 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.solr.util;
+
+import java.io.IOException;
+import java.io.InputStream;
+import java.io.InputStreamReader;
+import java.nio.charset.StandardCharsets;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Locale;
+import java.util.Map;
+import java.util.Objects;
+import java.util.Properties;
+import java.util.SortedMap;
+import java.util.TreeMap;
+import java.util.stream.Collectors;
+import org.apache.solr.common.SolrException;
+import org.apache.solr.common.util.StrUtils;
+import org.apache.solr.common.util.Utils;
+
+/**
+ * This class is a unified provider of environment variables and system 
properties. It exposes a
+ * mutable copy of the environment variables. It also converts 'SOLR_FOO' 
variables to system
+ * properties 'solr.foo' and provide various convenience accessors for them.
+ */
+public class EnvUtils {
+  private static final SortedMap<String, String> ENV = new 
TreeMap<>(System.getenv());
+  private static final Map<String, String> CUSTOM_MAPPINGS = new HashMap<>();
+  private static boolean initialized = false;
+
+  static {
+    try {
+      Properties props = new Properties();
+      try (InputStream stream =
+          
EnvUtils.class.getClassLoader().getResourceAsStream("EnvToSyspropMappings.properties"))
 {
+        props.load(new InputStreamReader(Objects.requireNonNull(stream), 
StandardCharsets.UTF_8));
+        for (String key : props.stringPropertyNames()) {
+          CUSTOM_MAPPINGS.put(key, props.getProperty(key));
+        }
+        init(false);
+      }
+    } catch (IOException e) {
+      throw new SolrException(
+          SolrException.ErrorCode.INVALID_STATE, "Failed loading 
env.var->properties mapping", e);
+    }
+  }
+
+  /**
+   * Get Solr's mutable copy of all environment variables.
+   *
+   * @return sorted map of environment variables
+   */
+  public static SortedMap<String, String> getEnvs() {
+    return ENV;
+  }
+
+  /** Get a single environment variable as string */
+  public static String getEnv(String key) {
+    return ENV.get(key);
+  }
+
+  /** Get a single environment variable as string, or default */
+  public static String getEnv(String key, String defaultValue) {
+    return ENV.getOrDefault(key, defaultValue);
+  }
+
+  /** Get an environment variable as long */
+  public static long getEnvAsLong(String key) {
+    return Long.parseLong(ENV.get(key));
+  }
+
+  /** Get an environment variable as long, or default value */
+  public static long getEnvAsLong(String key, long defaultValue) {
+    String value = ENV.get(key);
+    if (value == null) {
+      return defaultValue;
+    }
+    return Long.parseLong(value);
+  }
+
+  /** Get an env var as boolean */
+  public static boolean getEnvAsBool(String key) {
+    return StrUtils.parseBool(ENV.get(key));
+  }
+
+  /** Get an env var as boolean, or default value */
+  public static boolean getEnvAsBool(String key, boolean defaultValue) {
+    String value = ENV.get(key);
+    if (value == null) {
+      return defaultValue;
+    }
+    return StrUtils.parseBool(value);
+  }
+
+  /** Get comma separated strings from env as List */
+  public static List<String> getEnvAsList(String key) {
+    return getEnv(key) != null ? stringValueToList(getEnv(key)) : null;
+  }
+
+  /** Get comma separated strings from env as List */
+  public static List<String> getEnvAsList(String key, List<String> 
defaultValue) {
+    return ENV.get(key) != null ? getEnvAsList(key) : defaultValue;
+  }
+
+  /** Set an environment variable */
+  public static void setEnv(String key, String value) {
+    ENV.put(key, value);
+  }
+
+  /** Set all environment variables */
+  public static synchronized void setEnvs(Map<String, String> env) {
+    ENV.clear();
+    ENV.putAll(env);
+  }
+
+  /** Get all Solr system properties as a sorted map */
+  public static SortedMap<String, String> getProps() {
+    ensureInitialized();
+    SortedMap<String, String> props = new TreeMap<>();
+    for (String key : System.getProperties().stringPropertyNames()) {
+      props.put(key, System.getProperty(key));
+    }
+    return props;
+  }
+
+  /** Get a property as string */
+  public static String getProp(String key) {
+    return getProp(key, null);
+  }
+
+  /**
+   * Get a property as string with a fallback value. All other getProp* 
methods use this.
+   *
+   * @param key property key, which treats 'camelCase' the same as 'camel.case'
+   */
+  public static String getProp(String key, String defaultValue) {
+    ensureInitialized(); // Avoid race condition with init()
+    String value = getPropWithCamelCaseFallback(key);
+    return value != null ? value : defaultValue;
+  }
+
+  /**
+   * Get a property from given key or an alias key converted from CamelCase to 
dot separated.
+   *
+   * @return property value or value of dot-separated alias key or null if not 
found
+   */
+  private static String getPropWithCamelCaseFallback(String key) {
+    String value = System.getProperty(key);
+    if (value != null) {
+      return value;
+    } else {
+      // Figure out if string is CamelCase and convert to dot separated
+      String altKey = camelCaseToDotSeparated(key);
+      return System.getProperty(altKey);
+    }
+  }
+
+  private static String camelCaseToDotSeparated(String key) {
+    return String.join(".", key.split("(?=[A-Z])")).replace("..", 
".").toLowerCase(Locale.ROOT);
+  }
+
+  /** Get property as integer */
+  public static Long getPropAsLong(String key) {
+    return getPropAsLong(key, null);
+  }
+
+  /** Get property as long, or default value */
+  public static Long getPropAsLong(String key, Long defaultValue) {
+    String value = getProp(key);
+    if (value == null) {
+      return defaultValue;
+    }
+    return Long.parseLong(value);
+  }
+
+  /** Get property as boolean */
+  public static Boolean getPropAsBool(String key) {
+    return getPropAsBool(key, null);
+  }
+
+  /** Get property as boolean, or default value */
+  public static Boolean getPropAsBool(String key, Boolean defaultValue) {
+    String value = getProp(key);
+    if (value == null) {
+      return defaultValue;
+    }
+    return StrUtils.parseBool(value);
+  }
+
+  /**
+   * Get comma separated strings from sysprop as List
+   *
+   * @return list of strings, or null if not found
+   */
+  public static List<String> getPropAsList(String key) {
+    return getPropAsList(key, null);
+  }
+
+  /**
+   * Get comma separated strings from sysprop as List, or default value
+   *
+   * @return list of strings, or provided default if not found
+   */
+  public static List<String> getPropAsList(String key, List<String> 
defaultValue) {
+    return getProp(key) != null ? stringValueToList(getProp(key)) : 
defaultValue;
+  }
+
+  /** Set a system property. Shim to {@link System#setProperty(String, 
String)} */
+  public static void setProp(String key, String value) {
+    System.setProperty(key, value);
+    System.setProperty(camelCaseToDotSeparated(key), value);
+  }
+
+  /**
+   * Re-reads environment variables and updates the internal map.
+   *
+   * @param overwrite if true, overwrite existing system properties with 
environment variables
+   */
+  public static synchronized void init(boolean overwrite) {
+    // Convert eligible environment variables to system properties
+    for (String key : ENV.keySet().toArray(String[]::new)) {
+      if (key.startsWith("SOLR_") || CUSTOM_MAPPINGS.containsKey(key)) {
+        String sysPropKey = envNameToSyspropName(key);
+        // Existing system properties take precedence
+        if (!sysPropKey.isBlank() && (overwrite || getProp(sysPropKey) == 
null)) {
+          setProp(sysPropKey, ENV.get(key));
+        }
+      }
+    }
+    initialized = true;
+  }
+
+  protected static String envNameToSyspropName(String envName) {
+    return CUSTOM_MAPPINGS.containsKey(envName)
+        ? CUSTOM_MAPPINGS.get(envName)
+        : envName.toLowerCase(Locale.ROOT).replace("_", ".");
+  }
+
+  @SuppressWarnings("unchecked")
+  private static List<String> stringValueToList(String string) {
+    if (string.startsWith("[") && string.endsWith("]")) {
+      // Convert a JSON string to a List<String> using Noggit parser
+      return (List<String>) Utils.fromJSONString(string);
+    } else {
+      return StrUtils.splitSmart(string, ",", true).stream()
+          .map(String::trim)
+          .collect(Collectors.toList());
+    }
+  }
+
+  private static synchronized void ensureInitialized() {

Review Comment:
   whoah; why?



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