added "idleConsumerLimit" bean property to DefaultMessageListenerContainer (SPR-7189)

org.springframework.jms/src/main/java/org/springframework/jms/listener/DefaultMessageListenerContainer.java

@@ -172,6 +172,8 @@ public class DefaultMessageListenerContainer extends AbstractPollingMessageListe
 
 	private int maxMessagesPerTask = Integer.MIN_VALUE;
 
+	private int idleConsumerLimit = 1;
+
 	private int idleTaskExecutionLimit = 1;
 
 	private final Set<AsyncMessageListenerInvoker> scheduledInvokers = new HashSet<AsyncMessageListenerInvoker>();
@@ -237,9 +239,8 @@ public class DefaultMessageListenerContainer extends AbstractPollingMessageListe
 	 * However, if you want to optimize for a specific server, consider switching
 	 * this setting to at least CACHE_CONNECTION or CACHE_SESSION even in
 	 * conjunction with an external transaction manager.
-	 * <p>Currently known servers that absolutely require CACHE_NONE for XA
-	 * transaction processing: JBoss 4. For any others, consider raising the
-	 * cache level.
+	 * <p>Currently known servers that absolutely require CACHE_NONE for XA transaction
+	 * processing: JBoss 4. For any others, consider raising the cache level.
 	 * @see #CACHE_NONE
 	 * @see #CACHE_CONNECTION
 	 * @see #CACHE_SESSION
@@ -393,17 +394,43 @@ public class DefaultMessageListenerContainer extends AbstractPollingMessageListe
 	/**
 	 * Return the maximum number of messages to process in one task.
 	 */
-	public int getMaxMessagesPerTask() {
+	public final int getMaxMessagesPerTask() {
 		synchronized (this.lifecycleMonitor) {
 			return this.maxMessagesPerTask;
 		}
 	}
 
 	/**
-	 * Specify the limit for idle executions of a receive task, not having
+	 * Specify the limit for the number of consumers that are allowed to be idle
+	 * at any given time.
+	 * <p>This limit is used by the {@link #scheduleNewInvokerIfAppropriate} method
+	 * to determine if a new invoker should be created. Increasing the limit causes
+	 * invokers to be created more aggressively. This can be useful to ramp up the
+	 * number of invokers faster.
+	 * <p>The default is 1, only scheduling a new invoker (which is likely to
+	 * be idle initially) if none of the existing invokers is currently idle.
+	 */
+	public void setIdleConsumerLimit(int idleConsumerLimit) {
+		Assert.isTrue(idleConsumerLimit > 0, "'idleConsumerLimit' must be 1 or higher");
+		synchronized (this.lifecycleMonitor) {
+			this.idleConsumerLimit = idleConsumerLimit;
+		}
+	}
+
+	/**
+	 * Return the limit for the number of idle consumers. 
+	 */
+	public final int getIdleConsumerLimit() {
+		synchronized (this.lifecycleMonitor) {
+			return this.idleConsumerLimit;
+		}
+	}
+
+	/**
+	 * Specify the limit for idle executions of a consumer task, not having
 	 * received any message within its execution. If this limit is reached,
 	 * the task will shut down and leave receiving to other executing tasks.
-	 * <p>Default is 1, closing idle resources early once a task didn't
+	 * <p>The default is 1, closing idle resources early once a task didn't
 	 * receive a message. This applies to dynamic scheduling only; see the
 	 * {@link #setMaxConcurrentConsumers "maxConcurrentConsumers"} setting.
 	 * The minimum number of consumers
@@ -434,9 +461,9 @@ public class DefaultMessageListenerContainer extends AbstractPollingMessageListe
 	}
 
 	/**
-	 * Return the limit for idle executions of a receive task.
+	 * Return the limit for idle executions of a consumer task.
 	 */
-	public int getIdleTaskExecutionLimit() {
+	public final int getIdleTaskExecutionLimit() {
 		synchronized (this.lifecycleMonitor) {
 			return this.idleTaskExecutionLimit;
 		}
@@ -662,18 +689,19 @@ public class DefaultMessageListenerContainer extends AbstractPollingMessageListe
 	 * Schedule a new invoker, increasing the total number of scheduled
 	 * invokers for this listener container, but only if the specified
 	 * "maxConcurrentConsumers" limit has not been reached yet, and only
-	 * if this listener container does not currently have idle invokers
-	 * that are waiting for new messages already.
-	 * <p>Called once a message has been received, to scale up while
+	 * if the specified "idleConsumerLimit" has not been reached either.
+	 * <p>Called once a message has been received, in order to scale up while
 	 * processing the message in the invoker that originally received it.
 	 * @see #setTaskExecutor
 	 * @see #getMaxConcurrentConsumers()
+	 * @see #getIdleConsumerLimit()
 	 */
 	protected void scheduleNewInvokerIfAppropriate() {
 		if (isRunning()) {
 			resumePausedTasks();
 			synchronized (this.lifecycleMonitor) {
-				if (this.scheduledInvokers.size() < this.maxConcurrentConsumers && getIdleInvokerCount() == 0) {
+				if (this.scheduledInvokers.size() < this.maxConcurrentConsumers &&
+						getIdleInvokerCount() < this.idleConsumerLimit) {
 					scheduleNewInvoker();
 					if (logger.isDebugEnabled()) {
 						logger.debug("Raised scheduled invoker count: " + this.scheduledInvokers.size());