INT-3335: Implement MongoDb Outbound Gateway

JIRA: https://jira.spring.io/browse/INT-3335

INT-3335: PR fixes

INT-3335: fix code style

INT-3335: Add queryExpressionString + minor fixes

* Polishing. Mostly code style

src/reference/asciidoc/mongodb.adoc

@@ -22,23 +22,23 @@ To connect to MongoDB you can use an implementation of the `MongoDbFactory` inte
 ----
 public interface MongoDbFactory {
 
-	/**
-	 * Creates a default {@link DB} instance.
-	 *
-	 * @return the DB instance
-	 * @throws DataAccessException
-	 */
-	DB getDb() throws DataAccessException;
+    /**
+     * Creates a default {@link DB} instance.
+     *
+     * @return the DB instance
+     * @throws DataAccessException
+     */
+    DB getDb() throws DataAccessException;
 
-	/**
-	 * Creates a {@link DB} instance to access the database with the given name.
-	 *
-	 * @param dbName must not be {@literal null} or empty.
-	 *
-	 * @return the DB instance
-	 * @throws DataAccessException
-	 */
-	DB getDb(String dbName) throws DataAccessException;
+    /**
+     * Creates a {@link DB} instance to access the database with the given name.
+     *
+     * @param dbName must not be {@literal null} or empty.
+     *
+     * @return the DB instance
+     * @throws DataAccessException
+     */
+    DB getDb(String dbName) throws DataAccessException;
 }
 ----
 
@@ -290,3 +290,120 @@ and other attributes that are common across all other inbound adapters (e.g., 'c
 The example above is relatively simple and static since it has a literal value for the `collection-name`.
 Sometimes you may need to change this value at runtime based on some condition.
 To do that, simply use `collection-name-expression` where the provided expression can be any valid SpEL expression.
+
+[[mongodb-outbound-gateway]]
+=== MongoDB Outbound Gateway
+
+Starting with _version 5.0_, the MongoDb Outbound Gateway is provided and it allows you to query a database by sending a Message to its request channel.
+The gateway will then send the response to the reply channel.
+The Message payload and headers can be used to specify the query, as well as collection name.
+
+[source,xml]
+----
+<int-mongodb:outbound-gateway id="gatewayQuery"
+    mongodb-factory="mongoDbFactory"
+    mongo-converter="mongoConverter"
+    query="{firstName: 'Bob'}"
+    collection-name="foo"
+    request-channel="in"
+    reply-channel="out"
+    entity-class="org.springframework.integration.mongodb.test.entity$Person"/>
+----
+
+* `collection-name` or `collection-name-expression` - identifies the name of the MongoDb collection to use;
+* `mongo-converter` - reference to an instance of `o.s.data.mongodb.core.convert.MongoConverter` to assist with converting a raw java object to a JSON document representation
+* `mongodb-factory` - reference to an instance of `o.s.data.mongodb.MongoDbFactory`
+* `mongo-template` - reference to an instance of `o.s.data.mongodb.core.MongoTemplate` (NOTE: you can not have both mongo-template and mongodb-factory set)
+* `entity-class` - the fully qualified name of the entity class to be passed to `find(..)` or `findOne(..)` method in MongoTemplate.
+If this attribute is not provided the default value is `org.bson.Document`;
+* `query` or `query-expression` - specifies the MongoDb query.
+Please refer to http://www.mongodb.org/display/DOCS/Querying[MongoDB documentation] for more query samples.
+
+==== Configuring with Java Configuration
+
+The following Spring Boot application provides an example of configuring the outbound gateway using Java configuration:
+
+[source, java]
+----
+@SpringBootApplication
+public class MongoDbJavaApplication {
+
+    public static void main(String[] args) {
+        new SpringApplicationBuilder(MongoDbJavaApplication.class)
+            .web(false)
+            .run(args);
+    }
+
+    @Autowired
+    private MongoDbFactory mongoDbFactory;
+
+    @Bean
+    public MessageChannel requestChannel() {
+        return new DirectChannel();
+    }
+
+    @Bean
+    public MessageChannel replyChannel() {
+        return new QueueChannel(5);
+    }
+
+    @Bean
+    @ServiceActivator(inputChannel = "requestChannel")
+    public MessageHandler mongoDbOutboundGateway() {
+        MongoDbOutboundGateway gateway = new MongoDbOutboundGateway(this.mongoDbFactory);
+        gateway.setCollectionNameExpressionString("'foo'");
+        gateway.setQueryExpressionString("'{''name'' : ''Bob''}'");
+        gateway.setExpectSingleResult(true);
+        gateway.setEntityClass(Person.class);
+        gateway.setOutputChannelName("replyChannel");
+
+        return gateway;
+    }
+
+    @Bean
+    @ServiceActivator(inputChannel = "replyChannel")
+    public MessageHandler handler() {
+        return message -> System.out.println(message.getPayload());
+    }
+}
+----
+
+==== Configuring with the Java DSL
+
+The following Spring Boot application provides an example of configuring the Outbound Gateway using the Java DSL:
+
+[source, java]
+----
+@SpringBootApplication
+public class MongoDbJavaApplication {
+
+    public static void main(String[] args) {
+        new SpringApplicationBuilder(MongoDbJavaApplication.class)
+            .web(false)
+            .run(args);
+    }
+
+    @Autowired
+    private MongoDbFactory;
+
+    @Autowired
+    private MongoConverter;
+
+
+    @Bean
+    public IntegrationFlow gatewaySingleQueryFlow() {
+        return f -> f
+                .handle(queryOutboundGateway())
+                .channel(c -> c.queue("retrieveResults"));
+    }
+
+    private MongoDbOutboundGatewaySpec queryOutboundGateway() {
+        return MongoDb.outboundGateway(this.mongoDbFactory, this.mongoConverter)
+                .query("{name : 'Bob'}")
+                .collectionNameFunction(m -> m.getHeaders().get("collection"))
+                .expectSingleResult(true)
+                .entityClass(Person.class);
+    }
+
+}
+----

src/reference/asciidoc/whats-new.adoc

@@ -9,6 +9,10 @@ development process.
 [[x5.0-new-components]]
 === New Components
 
+==== MongoDB Outbound Gateway
+
+The new `MongoDbOutboundGateway` allows you to make queries to the database on demand by sending a message to its request channel.
+See <<mongodb-outbound-gateway>> for more information.
 
 [[x5.0-general]]
 === General Changes