jdaugherty commented on code in PR #16414:
URL: https://github.com/apache/grails-core/pull/16414#discussion_r4245945911


##########
grails-converters/src/main/groovy/org/grails/web/converters/configuration/ConvertersConfigurationInitializer.java:
##########
@@ -111,26 +132,75 @@ private void initJSONConfiguration() {
 
         List<ObjectMarshaller<JSON>> marshallers = new ArrayList<>();
         marshallers.addAll(getPreviouslyConfiguredMarshallers(JSON.class));
-        marshallers.add(new 
org.grails.web.converters.marshaller.json.ArrayMarshaller());
-        marshallers.add(new 
org.grails.web.converters.marshaller.json.ByteArrayMarshaller());
-        marshallers.add(new 
org.grails.web.converters.marshaller.json.CollectionMarshaller());
-        marshallers.add(new 
org.grails.web.converters.marshaller.json.MapMarshaller());
-        marshallers.add(new 
org.grails.web.converters.marshaller.json.SimpleEnumMarshaller());
 
         Config grailsConfig = getGrailsConfig();
+        ProxyHandler proxyHandler = getProxyHandler();
+        boolean legacy = 
grailsConfig.getProperty(SETTING_CONVERTERS_JSON_LEGACY, Boolean.class, false);

Review Comment:
   I'm still asking for the inversion, and I'm -1 until the default changes.
   
   The Grails 8 precedent doesn't carry here. 9.0 is being accelerated, with 
few milestones, so the new output gets little pre-release exposure before it 
ships, and this change is not being backported to 8.1.x, so there is no release 
in which an application can opt into the mapper-backed output before it becomes 
the default. The first release that has the new output is also the one that 
turns it on for everyone. For a part of the framework this central, where the 
text goes to clients outside the application, that is changing a default, not 
offering a migration.
   
   So for 9.x: default to the Grails 8 marshallers and layout, let an 
application opt into the mapper with the setting, and flip the default in 10. 
The switch being complete and undeprecated is good, and the inversion doesn't 
lose any of that work.
   



##########
grails-converters/src/main/groovy/org/grails/plugins/converters/ConvertersGrailsPlugin.groovy:
##########
@@ -72,6 +79,20 @@ class ConvertersGrailsPlugin extends Plugin {
                 }
             }
 
+            // Spring Boot registers every JacksonModule bean with the 
application's JsonMapper
+            if 
(environment.getProperty(ConvertersConfigurationInitializer.SETTING_CONVERTERS_JSON_DOMAIN_JACKSON_ENABLED,
 Boolean, true) &&

Review Comment:
   Verified at 27a86d86b8: the module is registered only when 
`grails.converters.json.domain.jackson.enabled` is `true`, the default is 
`false` in the plugin, the metadata and both guides, and 
`ConvertersGrailsPluginSpec` covers the default (Jackson's bean view), the 
opt-in, and the opt-in together with the legacy switch.
   



##########
grails-converters/src/main/groovy/org/grails/web/converters/configuration/ConvertersConfigurationInitializer.java:
##########
@@ -65,6 +67,23 @@ public class ConvertersConfigurationInitializer implements 
ApplicationContextAwa
 
     public static final String SETTING_CONVERTERS_JSON_DATE = 
"grails.converters.json.date";
     public static final String SETTING_CONVERTERS_JSON_DEFAULT_DEEP = 
"grails.converters.json.default.deep";
+    /**
+     * Whether domain classes are registered with the application's {@code 
JsonMapper}, so that it renders them as the
+     * JSON converter does. Defaults to {@code true}.
+     *
+     * @since 9.0
+     */
+    public static final String SETTING_CONVERTERS_JSON_DOMAIN_JACKSON_ENABLED 
= "grails.converters.json.domain.jackson.enabled";
+    /**
+     * Whether the JSON converter renders values as Grails 8 did, with the 
marshallers of Grails 8 rather than the
+     * application's {@code JsonMapper}, and the application's {@code 
JsonMapper} renders domain classes as Jackson beans.
+     * Defaults to {@code false}.
+     *
+     * @since 9.0
+     * @deprecated a transition to the rendering of Grails 9, to be removed in 
Grails 10
+     */
+    @Deprecated(since = "9.0", forRemoval = true)
+    public static final String SETTING_CONVERTERS_JSON_LEGACY = 
"grails.converters.json.legacy";

Review Comment:
   Verified at 27a86d86b8: both keys are in 
`additional-spring-configuration-metadata.json` with `false` defaults, matching 
the plugin, the initializer and the guide.
   



##########
grails-doc/src/en/guide/upgrading/upgrading90x.adoc:
##########
@@ -0,0 +1,189 @@
+////
+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
+
+https://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.
+////
+
+=== Upgrade Instructions for Grails and Related Dependencies
+
+This guide outlines the changes to review when you upgrade a Grails project 
from Grails 8 to Grails 9.
+
+==== 1. `grails.converters.JSON` Writes JSON With Spring Boot's JsonMapper
+
+`render ... as JSON`, `respond` for a JSON request and every other use of 
`grails.converters.JSON` now write JSON through
+the application's Jackson `JsonMapper`, the one Spring Boot auto-configures. 
The mapper writes the JSON text (numbers and
+pretty printing), and writes every single value that it has a dedicated 
serializer for: dates and times, `Month`,
+`UUID`, `Locale`, `byte[]`, types with a `@JsonValue`, and the types of any 
Jackson module registered with the
+application. Those values render as Spring Boot renders them, and 
`spring.jackson.*` properties apply to them. For
+example, `spring.jackson.time-zone` writes `Date` and `Calendar` values in 
that time zone, and converts `OffsetDateTime`
+and `ZonedDateTime` values to it. When the application context has no 
`JsonMapper`, or has several and none is primary,
+as in a unit test, a default `JsonMapper` is used. To keep rendering JSON as 
Grails 8 did while you migrate, see
+<<renderingJsonAsGrails8,Rendering JSON As Grails 8 Did>>.
+
+Parsing JSON does not change. `JSON.parse`, `request.JSON` and `new 
JSONObject(String)` parse with Grails' own
+`JSONTokener`, as before: they accept the same syntax beyond strict JSON 
(single quotes, comments, unquoted values,
+uppercase literals, trailing commas, `new Date(...)`), return the same number 
types (`Integer`, `Long`, `BigInteger`,
+`Double` or `BigDecimal`, depending on the number) and the same `JSONObject` 
and `JSONArray` containers, which implement
+`Map` and `List`.
+
+Grails marshallers still render domain classes, beans, enums, collections, 
maps and arrays, and now records (as an
+object of their components) and `Optional` (as its value), so the values they 
contain are rendered as any other value
+is. A marshaller that an application registers, with 
`JSON.registerObjectMarshaller(...)` or an
+`ObjectMarshallerRegisterer`, still takes precedence over the mapper for the 
types it supports.

Review Comment:
   Checked #16237 at 5ce132c752 against this: `respond()` stays on the 
converter in 9 and `grails.web.rendering.json.spring` flips in 10, the 
marshaller trigger is gone, and `registerObjectMarshaller` is not deprecated 
there either, so the two layers line up as you describe. Its guide still says 
`render value as JSON` "uses the legacy converter" and calls the converter 
"legacy" throughout; after this PR that converter writes through the mapper, so 
that wording needs the update you mention in whichever merges second.
   



##########
grails-converters/src/main/groovy/org/grails/web/converters/marshaller/json/DomainClassMarshaller.java:
##########
@@ -120,154 +99,18 @@ public void setIncludeVersion(boolean includeVersion) {
 
     public boolean supports(Object object) {
         String name = 
ConverterUtil.trimProxySuffix(object.getClass().getName());
-        return application.isArtefactOfType(DomainClassArtefactHandler.TYPE, 
name);
+        return application != null && 
application.isArtefactOfType(DomainClassArtefactHandler.TYPE, name);
     }
 
-    @SuppressWarnings({ "unchecked", "rawtypes" })
     public void marshalObject(Object value, JSON json) throws 
ConverterException {
-        JSONWriter writer = json.getWriter();
-        value = proxyHandler.unwrapIfProxy(value);
-        Class<?> clazz = value.getClass();
-
-        List<String> excludes = json.getExcludes(clazz);
-        List<String> includes = json.getIncludes(clazz);
-        IncludeExcludeSupport<String> includeExcludeSupport = new 
IncludeExcludeSupport<>();
-
-        BeanWrapper beanWrapper = new BeanWrapperImpl(value);
-
-        writer.object();
-
-        if (includeClass && shouldInclude(includeExcludeSupport, includes, 
excludes, value, "class")) {
-            writer.key("class").value(clazz.getName());
-        }
-
-        PersistentEntity domainClass = findDomainClass(value);
-
-        if (domainClass == null) {
-            throw new GrailsConfigurationException("Could not retrieve the 
respective entity for domain " + value.getClass().getName() + " in the mapping 
context API");
-        }
-
-        PersistentProperty id = domainClass.getIdentity();
-        if (id != null) {
-            //Composite keys dont return an identity. They also do not render 
in the JSON.
-            //If using Composite keys, it may be advisable to use a customer 
Marshaller.
-            if (shouldInclude(includeExcludeSupport, includes, excludes, 
value, id.getName())) {
-                Object idValue = extractValue(value, id);
-                if (idValue != null) {
-                    json.property(id.getName(), idValue);
-                }
-            }
-        }
-
-        if (shouldInclude(includeExcludeSupport, includes, excludes, value, 
GormProperties.VERSION) && isIncludeVersion()) {
-            PersistentProperty versionProperty = domainClass.getVersion();
-            Object version = extractValue(value, versionProperty);
-            if (version != null) {
-                json.property(GormProperties.VERSION, version);
-            }
-        }
-
-        List<PersistentProperty> properties = 
domainClass.getPersistentProperties();
-
-        for (PersistentProperty property : properties) {
-            if (property.equals(domainClass.getVersion())) {
-                continue;
-            }
-
-            if (!shouldInclude(includeExcludeSupport, includes, excludes, 
value, property.getName())) continue;
-
-            writer.key(property.getName());
-            if (!(property instanceof Association)) {
-                // Write non-relation property
-                Object val = beanWrapper.getPropertyValue(property.getName());
-                json.convertAnother(val);
-            }
-            else {
-                Object referenceObject = 
beanWrapper.getPropertyValue(property.getName());
-                if (isRenderDomainClassRelations()) {
-                    if (referenceObject == null) {
-                        writer.valueNull();
-                    }
-                    else {
-                        referenceObject = 
proxyHandler.unwrapIfProxy(referenceObject);
-                        if (referenceObject instanceof SortedMap) {
-                            referenceObject = new TreeMap((SortedMap) 
referenceObject);
-                        }
-                        else if (referenceObject instanceof SortedSet) {
-                            referenceObject = new TreeSet((SortedSet) 
referenceObject);
-                        }
-                        else if (referenceObject instanceof Set) {
-                            referenceObject = new LinkedHashSet((Set) 
referenceObject);
-                        }
-                        else if (referenceObject instanceof Map) {
-                            referenceObject = new LinkedHashMap((Map) 
referenceObject);
-                        }
-                        else if (referenceObject instanceof Collection) {
-                            referenceObject = new ArrayList((Collection) 
referenceObject);
-                        }
-                        json.convertAnother(referenceObject);
-                    }
-                }
-                else {
-                    if (referenceObject == null) {
-                        json.value(null);
-                    }
-                    else {
-
-                        PersistentEntity referencedDomainClass = 
((Association) property).getAssociatedEntity();
-
-                        // Embedded are now always fully rendered
-                        if (referencedDomainClass == null || ((Association) 
property).isEmbedded() || property.getType().isEnum()) {
-                            json.convertAnother(referenceObject);
-                        }
-                        else if ((property instanceof OneToOne) || (property 
instanceof ManyToOne) || ((Association) property).isEmbedded()) {
-                            asShortObject(referenceObject, json, 
referencedDomainClass.getIdentity(), referencedDomainClass);
-                        }
-                        else {
-                            PersistentProperty referencedIdProperty = 
referencedDomainClass.getIdentity();
-                            @SuppressWarnings("unused")
-                            String refPropertyName = ((Association) 
property).getReferencedPropertyName();
-                            if (referenceObject instanceof Collection) {
-                                Collection o = (Collection) referenceObject;
-                                writer.array();
-                                for (Object el : o) {
-                                    asShortObject(el, json, 
referencedIdProperty, referencedDomainClass);
-                                }
-                                writer.endArray();
-                            }
-                            else if (referenceObject instanceof Map) {
-                                Map<Object, Object> map = (Map<Object, 
Object>) referenceObject;
-                                for (Map.Entry<Object, Object> entry : 
map.entrySet()) {
-                                    String key = 
String.valueOf(entry.getKey());
-                                    Object o = entry.getValue();
-                                    writer.object();
-                                    writer.key(key);
-                                    asShortObject(o, json, 
referencedIdProperty, referencedDomainClass);
-                                    writer.endObject();
-                                }
-                            }
-                        }
-                    }
-                }
-            }
-        }
-        writer.endObject();
-    }
-
-    private PersistentEntity findDomainClass(Object value) {
-        for (DomainClassFetcher fetcher : domainClassFetchers) {
-            PersistentEntity domain = fetcher.findDomainClass(value);
-            if (domain != null) {
-                return domain;
-            }
-        }
-        return null;
-    }
-
-    private boolean shouldInclude(IncludeExcludeSupport<String> 
includeExcludeSupport, List<String> includes, List<String> excludes, Object 
object, String propertyName) {
-        return includeExcludeSupport.shouldInclude(includes, excludes, 
propertyName) && shouldInclude(object, propertyName);
+        Object object = proxyHandler.unwrapIfProxy(value);
+        JsonMapperValueMarshaller.write(DomainClassSerializer.value(object, 
new Rendering(json, object.getClass())), json);

Review Comment:
   Verified at 27a86d86b8: `marshalObjectAsGrails8` is the 8.0.x 
`marshalObject` body line for line, apart from the to-many `Map` loop writing 
one object (identical text for one entry) and an unused local. I ran 
`Grails8DomainRendering` on 8.0.x (232bb34006) myself, in UTC: all 11 scenarios 
give the `GRAILS_8_DOMAIN` text byte for byte, and on this head the spec passes 
with the default mapper and with the configured one.
   



##########
grails-web-common/src/main/groovy/org/grails/web/json/JSONWriter.java:
##########
@@ -297,65 +322,194 @@ public JSONWriter value(double d) {
      * @return this
      */
     public JSONWriter value(long l) {
-        return append(Long.toString(l));
+        return write(() -> generator.writeNumber(l), l);
     }
 
     /**
-     * Append a number value
+     * Append a number value.
      *
-     * @param number
-     * @return
+     * @param number A Number.
+     * @return this
      */
     public JSONWriter value(Number number) {
-        return number != null ? append(number.toString()) : valueNull();
+        return number != null ? write(() -> writeNumber(number), number) : 
valueNull();
     }
 
+    /**
+     * Append the value <code>null</code>.
+     *
+     * @return this
+     */
     public JSONWriter valueNull() {
-        return append(nullWritable);
+        return write(generator::writeNull, null);
     }
 
-    static Writable nullWritable = new NullWritable();
-
-    private static class NullWritable implements Writable {
-        @Override
-        public Writer writeTo(Writer out) throws IOException {
-            out.write("null");
-            return out;
+    /**
+     * Append an object value: a {@link JsonMapperValue} as its mapper writes 
it, {@code null} (and
+     * {@link JSONObject#NULL}), numbers and booleans as JSON literals, a 
{@code Date} as a JavaScript
+     * {@code new Date(...)} (for the javascript JSON date format), a {@link 
JSONElement} as the JSON text it writes, a
+     * {@code String} as a JSON string, and any other value as the quoted, 
JSON encoded text of {@link JSONObject}.
+     *
+     * @param o The object to append.
+     * @return this
+     */
+    public JSONWriter value(Object o) {
+        if (o == null || o.equals(null)) {
+            return valueNull();
+        }
+        if (o instanceof JsonMapperValue mapperValue && 
mapperValue.getJsonMapper() == jsonMapper) {
+            return write(() -> jsonMapper.writeValue(generator, 
mapperValue.getValue(), mapperValue.getNestedValueWriter()), o);
+        }
+        if (o instanceof Number number) {
+            return value(number);
+        }
+        if (o instanceof Boolean b) {
+            return value(b.booleanValue());
+        }
+        if (o instanceof Date date) {
+            return write(() -> generator.writeRawValue("new Date(" + 
date.getTime() + ")"), o);
+        }
+        if (o instanceof JSONElement element) {
+            return write(() -> generator.writeRawValue(text(element)), o);
+        }
+        if (o.getClass() == String.class || o.getClass() == 
StringBuilder.class || o.getClass() == StringBuffer.class) {
+            return write(() -> generator.writeString(o.toString()), o);
         }
+        return write(() -> generator.writeRawValue(quoted(o)), o);
     }
 
     /**
-     * Append an object value.
+     * Writes a value nested in a {@link JsonMapperValue} that this writer is 
writing, such as a value that a Jackson
+     * serializer writes with {@link JsonGenerator#writePOJO(Object)}, from 
the value's nested value writer. The writer
+     * accepts one value, as at the start of a JSON text, and then returns to 
the state it was in.
      *
-     * @param o The object to append. It can be null, or a Boolean, Number,
-     *          String, JSONObject, or JSONArray.
-     * @return this
+     * @param writeValue writes one value to this writer
+     * @throws JSONException if {@code writeValue} does not write one complete 
value
+     * @since 9.0
      */
-    public JSONWriter value(Object o) {
-        return o != null ? append(new QuotedWritable(o)) : valueNull();
+    public void writeNested(Runnable writeValue) {
+        Mode outerMode = this.mode;
+        Stack<Mode> outerStack = this.stack;
+        boolean outerComma = this.comma;
+        this.mode = INIT;
+        this.stack = new Stack<>();
+        this.nesting++;
+        try {
+            writeValue.run();
+            if (this.mode != DONE) {
+                throw new JSONException("Incomplete nested value: expected 
mode of DONE but was " + this.mode);
+            }
+        }
+        finally {
+            this.mode = outerMode;
+            this.stack = outerStack;
+            this.comma = outerComma;
+            this.nesting--;
+        }
     }
 
-    private static class QuotedWritable implements Writable {
-        Object o;
+    /**
+     * The location of the nested value that {@link #writeNested(Runnable)} is 
about to write, relative to the
+     * {@link JsonMapperValue} it is nested in, such as {@code .inner} or 
{@code [0]}.
+     */
+    String nestedPath() {
+        return generator != null ? NestedValueSerializer.nestedPath(generator) 
: "";
+    }
 
-        QuotedWritable(Object o) {
-            this.o = o;
+    /**
+     * Writes the JSON text buffered so far to the {@link Writer}, without 
flushing the {@code Writer}.
+     *
+     * @since 9.0
+     */
+    public void flush() {
+        if (generator != null) {
+            generate(generator::flush);
         }
+    }
 
-        @Override
-        public Writer writeTo(Writer out) throws IOException {
-            JSONObject.writeValue(out, o);
-            return out;
+    private JSONWriter write(Runnable writeValue, Object value) {
+        if (this.mode == INIT) {
+            generate(writeValue);
+            this.mode = DONE;
+            closeIfDone();
+            return this;
         }
-
-        public String toString() {
-            return String.valueOf(o);
+        if (this.mode == OBJECT || this.mode == ARRAY) {
+            generate(writeValue);
+            if (this.mode == OBJECT) {
+                this.mode = KEY;
+            }
+            this.comma = true;
+            return this;
         }
+        throw new JSONException("Value out of sequence: expected mode to be 
OBJECT or ARRAY when writing '" + value + "' but was " + this.mode);
     }
 
     /**
-     * Enumeration of the possible modes of the JSONWriter
+     * Once the JSON text is complete, closes the generator, which writes the 
rest of the text to the {@link Writer}
+     * and returns its buffers to Jackson.
      */
+    private void closeIfDone() {
+        if (this.mode == DONE && this.nesting == 0) {
+            generate(generator::close);
+        }
+    }
+
+    private void writeNumber(Number number) {
+        if (number instanceof Integer || number instanceof Short || number 
instanceof Byte) {
+            generator.writeNumber(number.intValue());
+        }
+        else if (number instanceof Long) {
+            generator.writeNumber(number.longValue());
+        }
+        else if (number instanceof Double) {
+            generator.writeNumber(number.doubleValue());
+        }
+        else if (number instanceof Float) {
+            generator.writeNumber(number.floatValue());
+        }
+        else if (number instanceof BigDecimal bigDecimal) {
+            generator.writeNumber(bigDecimal);
+        }
+        else if (number instanceof BigInteger bigInteger) {
+            generator.writeNumber(bigInteger);
+        }
+        else {
+            jsonMapper.writeValue(generator, number);

Review Comment:
   Verified at 27a86d86b8: `value(Number)` writes any other `Number` with 
`writeNumber(String.valueOf(number))`, so it never goes back through the 
mapper, and in the new mode `JSON.value` short-circuits only the eight JDK 
number types, so a `Number` with a module serializer reaches 
`JsonMapperValueMarshaller` as a top-level `JsonMapperValue`. 
`JsonMapperRenderingSpec` has the `AtomicInteger` re-entry case under a 
timeout, and `Money` as a map value, inside a module-written value and as a 
single value.
   



##########
grails-converters/src/main/groovy/org/grails/web/converters/jackson/DomainClassSerializer.java:
##########
@@ -0,0 +1,321 @@
+/*
+ *  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
+ *
+ *    https://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.grails.web.converters.jackson;
+
+import java.lang.reflect.AnnotatedElement;
+import java.lang.reflect.Field;
+import java.lang.reflect.Method;
+import java.util.ArrayList;
+import java.util.Collection;
+import java.util.Collections;
+import java.util.HashSet;
+import java.util.IdentityHashMap;
+import java.util.LinkedHashMap;
+import java.util.LinkedHashSet;
+import java.util.Map;
+import java.util.Set;
+import java.util.SortedMap;
+import java.util.SortedSet;
+import java.util.TreeMap;
+import java.util.TreeSet;
+
+import com.fasterxml.jackson.annotation.JsonIgnore;
+import com.fasterxml.jackson.annotation.JsonProperty;
+import tools.jackson.core.JsonGenerator;
+import tools.jackson.databind.JacksonSerializable;
+import tools.jackson.databind.SerializationContext;
+import tools.jackson.databind.jsontype.TypeSerializer;
+import tools.jackson.databind.ser.std.StdSerializer;
+
+import org.springframework.beans.BeanWrapper;
+import org.springframework.beans.BeanWrapperImpl;
+
+import org.grails.core.exceptions.GrailsConfigurationException;
+import org.grails.datastore.mapping.model.PersistentEntity;
+import org.grails.datastore.mapping.model.PersistentProperty;
+import org.grails.datastore.mapping.model.config.GormProperties;
+import org.grails.datastore.mapping.model.types.Association;
+import org.grails.datastore.mapping.model.types.ManyToOne;
+import org.grails.datastore.mapping.model.types.OneToOne;
+import org.grails.datastore.mapping.reflect.NameUtils;
+import org.grails.datastore.mapping.reflect.ReflectionUtils;
+
+/**
+ * Serializes domain class instances as {@code grails.converters.JSON} renders 
them: an object of the id, the version
+ * and the class name when they are included, and the persistent properties. 
An associated domain class instance is a
+ * reference of its id, such as {@code {"id":1}}, or, when rendering deep, the 
instance in full. Embedded instances and
+ * collections of basic values are written in full.
+ *
+ * <p>Property values are written with {@link 
JsonGenerator#writePOJO(Object)}, so a mapper writes them as it writes any
+ * other value, and a converter renders them with its marshallers. When 
rendering deep, an instance that is already
+ * being written, further up, is written as a reference.
+ *
+ * <p>A property annotated with {@code @JsonIgnore}, or with {@code 
@JsonProperty(access = WRITE_ONLY)}, on its field
+ * or getter, is not written. The {@link DomainClassRendering} decides about 
the others.
+ *
+ * @since 9.0
+ */
+public final class DomainClassSerializer extends StdSerializer<Object> {
+
+    private static final String RENDERING_INSTANCES = 
DomainClassSerializer.class.getName() + ".instances";
+
+    private static final ClassValue<Set<String>> IGNORED_PROPERTIES = new 
ClassValue<>() {
+        @Override
+        protected Set<String> computeValue(Class<?> type) {
+            Set<String> ignored = new HashSet<>();
+            for (Class<?> current = type; current != null && current != 
Object.class; current = current.getSuperclass()) {
+                for (Field field : current.getDeclaredFields()) {
+                    if (ignoredForSerialization(field)) {
+                        ignored.add(field.getName());
+                    }
+                }
+                for (Method method : current.getDeclaredMethods()) {
+                    if (ReflectionUtils.isGetter(method.getName(), 
method.getParameterTypes()) &&
+                            ignoredForSerialization(method)) {
+                        
ignored.add(NameUtils.getPropertyNameForGetterOrSetter(method.getName()));
+                    }
+                }
+            }
+            return Set.copyOf(ignored);
+        }
+    };
+
+    private final DomainClassRendering rendering;
+
+    /**
+     * @param rendering how to render domain class instances
+     */
+    public DomainClassSerializer(DomainClassRendering rendering) {
+        super(Object.class);
+        this.rendering = rendering;
+    }
+
+    /**
+     * @return how this serializer renders domain class instances
+     */
+    public DomainClassRendering getRendering() {
+        return rendering;
+    }
+
+    /**
+     * A value that a mapper writes as a domain class instance rendered in a 
particular way, whether or not the mapper
+     * has a {@link DomainClassJacksonModule}.
+     *
+     * @param object a domain class instance
+     * @param rendering how to render it
+     * @return the value to write
+     */
+    public static Object value(Object object, DomainClassRendering rendering) {
+        return new JacksonSerializable.Base() {
+            @Override
+            public void serialize(JsonGenerator generator, 
SerializationContext context) {
+                write(object, generator, context, rendering);
+            }
+
+            @Override
+            public void serializeWithType(JsonGenerator generator, 
SerializationContext context,
+                    TypeSerializer typeSerializer) {
+                serialize(generator, context);
+            }
+        };
+    }
+
+    @Override
+    public void serialize(Object value, JsonGenerator gen, 
SerializationContext ctxt) {

Review Comment:
   Verified at 27a86d86b8: `serializeWithType` writes the type prefix and 
suffix around the properties, asks the rendering's `writeReference` for a cycle 
before anything is written, and the unwrapping variant reports a bad definition 
under `FAIL_ON_UNWRAPPED_TYPE_IDENTIFIERS` with `UnwrappingBeanSerializer`'s 
message. `DomainClassJacksonModuleSpec` covers the root, the superclass-typed 
property, the deep cycle through the hook and both unwrapped outcomes.
   



##########
grails-doc/src/en/guide/upgrading/upgrading90x.adoc:
##########
@@ -0,0 +1,189 @@
+////
+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
+
+https://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.
+////
+
+=== Upgrade Instructions for Grails and Related Dependencies
+
+This guide outlines the changes to review when you upgrade a Grails project 
from Grails 8 to Grails 9.
+
+==== 1. `grails.converters.JSON` Writes JSON With Spring Boot's JsonMapper
+
+`render ... as JSON`, `respond` for a JSON request and every other use of 
`grails.converters.JSON` now write JSON through
+the application's Jackson `JsonMapper`, the one Spring Boot auto-configures. 
The mapper writes the JSON text (numbers and
+pretty printing), and writes every single value that it has a dedicated 
serializer for: dates and times, `Month`,
+`UUID`, `Locale`, `byte[]`, types with a `@JsonValue`, and the types of any 
Jackson module registered with the
+application. Those values render as Spring Boot renders them, and 
`spring.jackson.*` properties apply to them. For
+example, `spring.jackson.time-zone` writes `Date` and `Calendar` values in 
that time zone, and converts `OffsetDateTime`
+and `ZonedDateTime` values to it. When the application context has no 
`JsonMapper`, or has several and none is primary,
+as in a unit test, a default `JsonMapper` is used. To keep rendering JSON as 
Grails 8 did while you migrate, see
+<<renderingJsonAsGrails8,Rendering JSON As Grails 8 Did>>.
+
+Parsing JSON does not change. `JSON.parse`, `request.JSON` and `new 
JSONObject(String)` parse with Grails' own
+`JSONTokener`, as before: they accept the same syntax beyond strict JSON 
(single quotes, comments, unquoted values,
+uppercase literals, trailing commas, `new Date(...)`), return the same number 
types (`Integer`, `Long`, `BigInteger`,
+`Double` or `BigDecimal`, depending on the number) and the same `JSONObject` 
and `JSONArray` containers, which implement
+`Map` and `List`.
+
+Grails marshallers still render domain classes, beans, enums, collections, 
maps and arrays, and now records (as an
+object of their components) and `Optional` (as its value), so the values they 
contain are rendered as any other value
+is. A marshaller that an application registers, with 
`JSON.registerObjectMarshaller(...)` or an
+`ObjectMarshallerRegisterer`, still takes precedence over the mapper for the 
types it supports.
+
+A Jackson module registered with the application, such as a `JacksonModule` 
bean or a `@JacksonComponent` serializer,
+now applies to `grails.converters.JSON`, where Grails 8 ignored it. When a 
module provides a serializer for a type that
+Grails 8 rendered as a bean, the serializer renders it instead. The values 
that the serializer writes, such as a property
+it writes with `JsonGenerator#writePOJO`, are rendered by the converter as any 
other value is, so registered marshallers,
+enum names and the circular reference behaviour apply to them. A marshaller 
registered for the type itself takes
+precedence over the module's serializer.
+
+Two things deliberately differ from Spring Boot, as they did in Grails 8:
+
+* Enums render by `name()`, where Jackson 3 writes `toString()`, so that the 
JSON binds back to the enum.
+* Strings are escaped for an HTML `<script>` element: `</` is written as 
`<\u002f`, and U+2028 and U+2029 as `\u2028`
+and `\u2029`. The parsed values are the same as Spring Boot's.
+
+The following values render differently than in Grails 8:
+
+[cols="2,2,3", options="header"]
+|===
+| Value
+| Grails 8
+| Grails 9
+
+| `OffsetTime`
+| `"03:00:00-03:00"`, `"03:00:00.5+05:30"`
+| `"03:00-03:00"`, `"03:00:00.500+05:30"`, from `OffsetTime#toString()`
+
+| `Month`
+| `"SEPTEMBER"`
+| `9`, a JSON number. A `Month` binds back from its number.
+
+| `java.util.Locale`
+| `"zh_TW_#Hant"`, from `Locale#toString()`
+| `"zh-Hant-TW"`, its language tag
+
+| `char`, `Character`
+| `{}`
+| `"c"`
+
+| `byte[]`
+| `[1,2,3]`
+| `"AQID"`, Base64
+
+| `UUID`, `URI`, a type with a `@JsonValue`
+| an object of its bean properties
+| its value, such as `"123e4567-e89b-12d3-a456-426614174000"`
+
+| `Optional`
+| `{"empty":false,"present":true}`
+| its value, or `null` when it is empty
+
+| A `NaN` or infinite `double`
+| `NaN`, which is not valid JSON
+| `"NaN"`
+
+| Pretty printed JSON (`grails.converters.json.pretty.print`, 
`JSON#toString(true)`)
+| `"a": 1`, with every array element on its own line, or, from 
`toString(true)`, indented by three spaces
+| `"a" : 1`, the layout of the mapper's default pretty printer
+
+| A single value, such as `render Role.HEAD as JSON`
+| a `ConverterException`
+| the JSON value, `"HEAD"`
+|===
+
+The other default date and time formats are unchanged from Grails 8.0.0, which 
already renders `Date`, `Calendar`,
+`java.sql.Date` and `java.sql.Timestamp` with millisecond precision and 
supports the additional date and time types.
+Map keys now use the mapper's key serializers, including its configured date 
format and time zone, instead of Grails'
+fixed date formats or the key's `toString()`.
+
+The JSON marshallers that rendered these values are no longer registered, and 
are deprecated, for removal in Grails 10:
+`DateMarshaller`, `CalendarMarshaller`, `ToStringBeanMarshaller`, 
`ByteArrayMarshaller`, `InstantMarshaller`,
+`LocalDateMarshaller`, `LocalDateTimeMarshaller`, `OffsetDateTimeMarshaller` 
and `ZonedDateTimeMarshaller`, in the
+`org.grails.web.converters.marshaller.json` package. To render one of these 
types differently, register a marshaller for
+it, or configure the `JsonMapper`. The XML marshallers of the same names are 
not changed. The default JSON marshallers have implicit priorities that follow 
their order (-1, -2, and so on),
+and that order has changed, so a marshaller registered with an explicit 
negative priority may now come before or after
+different defaults.
+
+`org.grails.web.json.JSONWriter`, which the converter and its marshallers 
write to, now writes through a Jackson
+`JsonGenerator`, and `new JSONWriter(writer)` writes with a default 
`JsonMapper`:
+
+* It keeps its public methods, rejects the same misplaced calls, and writes 
the same JSON, except as listed above. It
+also accepts a single value outside any object or array, where it threw a 
`JSONException`.
+* The generator buffers the text. It reaches the `Writer` as the buffer fills, 
once the JSON text is complete, and when
+`flush()` is called, rather than as each value is written. The `Writer` itself 
is not flushed or closed.
+* `array()`, `object()`, `key()` and the `value` methods no longer go through 
the protected `append` and `comma`
+methods. A subclass that overrides those to change the output, or writes to 
the `writer` field directly, must use the
+public methods instead.
+* `PrettyPrintJSONWriter` still indents as Grails 8 did, and is deprecated, 
for removal in Grails 10. Use
+`new JSONWriter(writer, new JsonMapperSupport(jsonMapper), true)`, which 
indents with the mapper's default pretty printer.
+
+The JavaCC JSON parser in `org.grails.web.json.parser` (`JSONParser`), which 
Grails does not use, is deprecated, for
+removal in Grails 10. Use `JSON.parse` or `new JSONObject(String)`.
+
+With the `PATH` circular reference behaviour 
(`grails.converters.json.circular.reference.behaviour`), an object with a
+`null` property, or a single value, now renders instead of failing, and a 
reference that follows a number property now
+has the right path.
+
+JSON views, the XML converter and the HAL renderer are not changed.
+
+[[renderingJsonAsGrails8]]
+===== Rendering JSON As Grails 8 Did
+
+To move to the new rendering at your own pace, set 
`grails.converters.json.legacy` to `true`:
+
+[source,yaml]
+----
+grails:
+    converters:
+        json:
+            legacy: true
+----
+
+The JSON converter then renders values with the marshallers of Grails 8, and 
indents pretty printed JSON as Grails 8 did,
+so `render ... as JSON` and `JSON#toString` return the same text as in Grails 
8. The application's `JsonMapper` renders
+domain classes as Jackson beans, as it did in Grails 8. What Grails 8 failed 
to render, or rendered as invalid JSON, such
+as a single value or `NaN`, renders as in Grails 9. The setting is deprecated, 
and will be removed in Grails 10.
+
+To keep the Grails 8 rendering of some types only, register a marshaller for 
each, for example in `BootStrap.init`:
+
+[source,groovy]
+----
+JSON.registerObjectMarshaller(Month) { Month month -> month.name() }
+JSON.registerObjectMarshaller(Locale) { Locale locale -> locale.toString() }
+JSON.registerObjectMarshaller(byte[]) { byte[] bytes -> bytes as List }
+----
+
+==== 2. Domain Classes Render Through Jackson
+
+The JSON converter renders domain classes with a Jackson serializer, and 
Grails registers it, as a Jackson module, with
+the application's `JsonMapper`. Writing a domain class instance with the 
application's `JsonMapper`, including returning
+one from a Spring MVC controller, now renders it as `render ... as JSON` does: 
associated instances as references of their
+id, such as `{"id":3}`, rather than in full, and proxies as the instances they 
proxy. Properties that Jackson annotations

Review Comment:
   Verified at 27a86d86b8: section 2 now lists what the serializer applies on 
the mapper (ignores, names, type ids, unwrapping, and the cases left to 
Jackson) and says it applies nothing else, naming inclusion, order, views, 
filters and property-level `@JsonFormat`/`@JsonSerialize`; the REST guide 
matches.
   



##########
grails-converters/src/main/groovy/org/grails/web/converters/marshaller/json/RecordMarshaller.java:
##########
@@ -0,0 +1,57 @@
+/*
+ *  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
+ *
+ *    https://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.grails.web.converters.marshaller.json;
+
+import java.lang.reflect.InvocationTargetException;
+import java.lang.reflect.RecordComponent;
+
+import grails.converters.JSON;
+import org.grails.web.converters.exceptions.ConverterException;
+import org.grails.web.converters.marshaller.ObjectMarshaller;
+import org.grails.web.json.JSONException;
+import org.grails.web.json.JSONWriter;
+
+/**
+ * JSON ObjectMarshaller which renders a record as an object of its 
components, in declaration order, as Spring Boot's
+ * JsonMapper does. Each component value is rendered by the converter, as any 
other value is.
+ *
+ * @since 9.0
+ */
+public class RecordMarshaller implements ObjectMarshaller<JSON> {
+
+    public boolean supports(Object object) {
+        return object.getClass().isRecord();
+    }
+
+    public void marshalObject(Object object, JSON converter) throws 
ConverterException {
+        JSONWriter writer = converter.getWriter();
+        try {
+            writer.object();
+            for (RecordComponent component : 
object.getClass().getRecordComponents()) {

Review Comment:
   Verified at 27a86d86b8: the javadoc promises the components by name in 
declaration order and says the Jackson annotations are not applied, 
`JsonMapperRenderingSpec` pins `@JsonProperty` and `@JsonIgnore` on components 
being ignored, and `InaccessibleObjectException` is caught with the others.
   



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