[Backport 2.x] Add support for Flat Objects

CHANGELOG.md

@@ -3,6 +3,7 @@ Inspired from [Keep a Changelog](https://keepachangelog.com/en/1.0.0/)
 ## [Unreleased 2.x]
 ### Added
 - Added support for icu_collation_keyword type ([#725](https://github.com/opensearch-project/opensearch-java/pull/725))
+- Added support for flat_object field property ([#735](https://github.com/opensearch-project/opensearch-java/pull/735))
 
 ### Dependencies
 

USER_GUIDE.md

@@ -7,6 +7,7 @@
     - [Creating an index](#creating-an-index)
       - [With default settings](#with-default-settings)
       - [With custom settings and mappings](#with-custom-settings-and-mappings)
+      - [With FlatObject mappings](#with-flat-object-mappings)
     - [Indexing data](#indexing-data)
     - [Searching for a document](#searching-for-a-document)
     - [Deleting a document](#deleting-a-document)
@@ -127,6 +128,18 @@ CreateIndexRequest createIndexRequest = new CreateIndexRequest.Builder()
         .build();
 client.indices().create(createIndexRequest);
 ```
+#### With flat object mappings
+OpenSearch supports FlatObject mappings from version 2.7.0 without additional parameters.
+
+```java
+final CreateIndexRequest createIndexRequest = new CreateIndexRequest.Builder().index(indexName)
+                    .mappings(m -> m.properties("issue", Property.of(p -> p.flatObject(new FlatObjectProperty.Builder().build()))))
+                    .build();
+client.indices().create(createIndexRequest);
+```
+
+You can find a working sample of the above code in [FlatObjectBasics.java](./samples/src/main/java/org/opensearch/client/samples/FlatObjectBasics.java).
+
 
 ### Indexing data
 

java-client/src/main/java/org/opensearch/client/opensearch/_types/mapping/FieldType.java

@@ -107,7 +107,7 @@ public enum FieldType implements JsonEnum {
 
     RankFeatures("rank_features"),
 
-    Flattened("flattened"),
+    FlatObject("flat_object"),
 
     Shape("shape"),
 

java-client/src/main/java/org/opensearch/client/opensearch/_types/mapping/FlatObjectProperty.java

@@ -0,0 +1,116 @@
+/*
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * The OpenSearch Contributors require contributions made to
+ * this file be licensed under the Apache-2.0 license or a
+ * compatible open source license.
+ */
+
+/*
+ * Licensed to Elasticsearch B.V. under one or more contributor
+ * license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright
+ * ownership. Elasticsearch B.V. 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.
+ */
+
+/*
+ * Modifications Copyright OpenSearch Contributors. See
+ * GitHub history for details.
+ */
+
+package org.opensearch.client.opensearch._types.mapping;
+
+import jakarta.json.stream.JsonGenerator;
+import java.util.function.Function;
+import org.opensearch.client.json.JsonpDeserializable;
+import org.opensearch.client.json.JsonpDeserializer;
+import org.opensearch.client.json.JsonpMapper;
+import org.opensearch.client.json.ObjectBuilderDeserializer;
+import org.opensearch.client.json.ObjectDeserializer;
+import org.opensearch.client.util.ObjectBuilder;
+
+// typedef: _types.mapping.FlatObjectProperty
+
+@JsonpDeserializable
+public class FlatObjectProperty extends PropertyBase implements PropertyVariant {
+
+    // ---------------------------------------------------------------------------------------------
+
+    private FlatObjectProperty(Builder builder) {
+        super(builder);
+
+    }
+
+    public static FlatObjectProperty of(Function<Builder, ObjectBuilder<FlatObjectProperty>> fn) {
+        return fn.apply(new Builder()).build();
+    }
+
+    /**
+     * Property variant kind.
+     */
+    @Override
+    public Property.Kind _propertyKind() {
+        return Property.Kind.FlatObject;
+    }
+
+    protected void serializeInternal(JsonGenerator generator, JsonpMapper mapper) {
+
+        generator.write("type", "flat_object");
+        super.serializeInternal(generator, mapper);
+    }
+
+    // ---------------------------------------------------------------------------------------------
+
+    /**
+     * Builder for {@link FlatObjectProperty}.
+     */
+
+    public static class Builder extends PropertyBase.AbstractBuilder<Builder> implements ObjectBuilder<FlatObjectProperty> {
+
+        @Override
+        protected Builder self() {
+            return this;
+        }
+
+        /**
+         * Builds a {@link FlatObjectProperty}.
+         *
+         * @throws NullPointerException
+         *             if some of the required fields are null.
+         */
+        public FlatObjectProperty build() {
+            _checkSingleUse();
+
+            return new FlatObjectProperty(this);
+        }
+    }
+
+    // ---------------------------------------------------------------------------------------------
+
+    /**
+     * Json deserializer for {@link FlatObjectProperty}
+     */
+    public static final JsonpDeserializer<FlatObjectProperty> _DESERIALIZER = ObjectBuilderDeserializer.lazy(
+        Builder::new,
+        FlatObjectProperty::setupFlatObjectPropertyDeserializer
+    );
+
+    protected static void setupFlatObjectPropertyDeserializer(ObjectDeserializer<FlatObjectProperty.Builder> op) {
+        PropertyBase.setupPropertyBaseDeserializer(op);
+
+        op.ignore("type");
+    }
+
+}