Enable auto-startup for `DefaultMessageListenerContainer`.

This change modifies DefaultMessageListenerContainer to automatically start when the Spring application context is initialized, by default.

The 'autoStartup' property defaults to true. A setter 'setAutoStartup(boolean)' has been added to allow users to disable this behavior if needed.

Closes #4403
Original pull request: #4976

Signed-off-by: Junhyeok Lee <jhl221123@naver.com>

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/messaging/DefaultMessageListenerContainer.java

@@ -43,6 +43,7 @@ import org.springframework.util.ObjectUtils;
  *
  * @author Christoph Strobl
  * @author Mark Paluch
+ * @author Junhyeok Lee
  * @since 2.1
  */
 public class DefaultMessageListenerContainer implements MessageListenerContainer {
@@ -62,6 +63,7 @@ public class DefaultMessageListenerContainer implements MessageListenerContainer
 	private final Lock subscriptionWrite = Lock.of(subscriptionMonitor.writeLock());
 
 	private boolean running = false;
+	private boolean autoStartup = true;
 
 	/**
 	 * Create a new {@link DefaultMessageListenerContainer}.
@@ -105,7 +107,16 @@ public class DefaultMessageListenerContainer implements MessageListenerContainer
 
 	@Override
 	public boolean isAutoStartup() {
-		return false;
+		return this.autoStartup;
+	}
+
+	/**
+	 * Set whether to auto-start this container.
+	 * <p>Default is {@code true}.
+	 * @param autoStartup {@code true} to auto-start.
+	 */
+	public void setAutoStartup(boolean autoStartup) {
+		this.autoStartup = autoStartup;
 	}
 
 	@Override

spring-data-mongodb/src/test/java/org/springframework/data/mongodb/core/messaging/DefaultMessageListenerContainerUnitTests.java

@@ -36,6 +36,7 @@ import org.springframework.util.ErrorHandler;
  * Unit tests for {@link DefaultMessageListenerContainer}.
  *
  * @author Christoph Strobl
+ * @author Junhyeok Lee
  */
 @ExtendWith(MockitoExtension.class)
 class DefaultMessageListenerContainerUnitTests {
@@ -80,6 +81,17 @@ class DefaultMessageListenerContainerUnitTests {
 		runOnce(new RemoveSubscriptionWhileRunning(container));
 	}
 
+	@Test // GH-4403
+	void shouldHaveAutoStartupEnabledByDefault() {
+		assertThat(container.isAutoStartup()).isTrue();
+	}
+
+	@Test // GH-4403
+	void shouldAllowDisablingAutoStartup() {
+		container.setAutoStartup(false);
+		assertThat(container.isAutoStartup()).isFalse();
+	}
+
 	private static class RemoveSubscriptionWhileRunning extends MultithreadedTestCase {
 
 		DefaultMessageListenerContainer container;