GH-1433: Fix DMLC Monitor Thread Name

Resolves #1433

Used `beanName` instead of `listenerId` (which falls back to `beanName` if `null`).
Containers for annotations are not beans per se.

**cherry-pick to 2.4.x, 2.3.x**

spring-rabbit/src/main/java/org/springframework/amqp/rabbit/listener/AbstractMessageListenerContainer.java

@@ -156,7 +156,7 @@ public abstract class AbstractMessageListenerContainer extends RabbitAccessor
 	private TransactionAttribute transactionAttribute = new DefaultTransactionAttribute();
 
 	@Nullable
-	private String beanName;
+	private String beanName = "not.a.Spring.bean";
 
 	private Executor taskExecutor = new SimpleAsyncTaskExecutor();
 

spring-rabbit/src/main/java/org/springframework/amqp/rabbit/listener/DirectMessageListenerContainer.java

@@ -391,7 +391,7 @@ private void checkStartState() {
 	protected void doInitialize() {
 		if (this.taskScheduler == null) {
 			ThreadPoolTaskScheduler threadPoolTaskScheduler = new ThreadPoolTaskScheduler();
-			threadPoolTaskScheduler.setThreadNamePrefix(getBeanName() + "-consumerMonitor-");
+			threadPoolTaskScheduler.setThreadNamePrefix(getListenerId() + "-consumerMonitor-");
 			threadPoolTaskScheduler.afterPropertiesSet();
 			this.taskScheduler = threadPoolTaskScheduler;
 		}

spring-rabbit/src/test/java/org/springframework/amqp/rabbit/listener/DirectMessageListenerContainerMockTests.java

@@ -360,6 +360,29 @@ public void testMonitorCancelsAfterTargetChannelChanges() throws Exception {
 		container.stop();
 	}
 
+	@Test
+	void monitorTaskThreadName() {
+		DirectMessageListenerContainer container = new DirectMessageListenerContainer(mock(ConnectionFactory.class));
+		assertThat(container.getListenerId()).isEqualTo("not.a.Spring.bean");
+		container.setBeanName("aBean");
+		assertThat(container.getListenerId()).isEqualTo("aBean");
+		container.setListenerId("id");
+		assertThat(container.getListenerId()).isEqualTo("id");
+		container.afterPropertiesSet();
+		assertThat(container).extracting("taskScheduler")
+				.extracting("threadNamePrefix")
+				.asString()
+				.startsWith("id-consumerMonitor");
+
+		container = new DirectMessageListenerContainer(mock(ConnectionFactory.class));
+		container.setBeanName("aBean");
+		container.afterPropertiesSet();
+		assertThat(container).extracting("taskScheduler")
+				.extracting("threadNamePrefix")
+				.asString()
+				.startsWith("aBean-consumerMonitor");
+	}
+
 	private Envelope envelope(long tag) {
 		return new Envelope(tag, false, "", "");
 	}