Publish events about broker's availability

Components that are using a StompBrokerRelayMessageHandler may want
to know whether or not the broker's unavailable. If they're sending
messages to the relay via an asynchronous channel there's currently
no way for them to find this out.

This commit enhances StompBrokerRelayMessageHandler to publish
application events when the broker's availability changes:
BrokerBecameAvailableEvent and BrokerBecameUnavailableEvent.
Irrespective of the number of relay sessions only a single event is
published for each change in the broker's availability.

spring-messaging/src/main/java/org/springframework/messaging/simp/stomp/BrokerAvailabilityEvent.java

@@ -0,0 +1,33 @@
+/*
+ * Copyright 2002-2013 the original author or authors.
+ *
+ * Licensed 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.
+ */
+
+package org.springframework.messaging.simp.stomp;
+
+import org.springframework.context.ApplicationEvent;
+
+
+/**
+ * Base class for application events relating to broker availability.
+ *
+ * @author Andy Wilkinson
+ */
+public abstract class BrokerAvailabilityEvent extends ApplicationEvent {
+
+
+	protected BrokerAvailabilityEvent(Object source) {
+		super(source);
+	}
+}

spring-messaging/src/main/java/org/springframework/messaging/simp/stomp/BrokerBecameAvailableEvent.java

@@ -0,0 +1,38 @@
+/*
+ * Copyright 2002-2013 the original author or authors.
+ *
+ * Licensed 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.
+ */
+
+package org.springframework.messaging.simp.stomp;
+
+/**
+ * Event raised when the broker being used by a {@link
+ * StompBrokerRelayMessageHandler} becomes available
+ *
+ * @author Andy Wilkinson
+ */
+public class BrokerBecameAvailableEvent extends BrokerAvailabilityEvent {
+
+	/**
+	 * Creates a new BrokerBecameAvailableEvent
+	 *
+	 * @param source the {@code StompBrokerRelayMessageHandler} that is acting
+	 * as a relay for the broker that has become available. Must not be {@code
+	 * null}.
+	 */
+	public BrokerBecameAvailableEvent(StompBrokerRelayMessageHandler source) {
+		super(source);
+	}
+
+}

spring-messaging/src/main/java/org/springframework/messaging/simp/stomp/BrokerBecameUnavailableEvent.java

@@ -0,0 +1,39 @@
+/*
+ * Copyright 2002-2013 the original author or authors.
+ *
+ * Licensed 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.
+ */
+
+package org.springframework.messaging.simp.stomp;
+
+
+/**
+ * Event raised when the broker being used by a {@link
+ * StompBrokerRelayMessageHandler} becomes unavailable
+ *
+ * @author Andy Wilkinson
+ */
+public class BrokerBecameUnavailableEvent extends BrokerAvailabilityEvent {
+
+	/**
+	 * Creates a new BrokerBecameUnavailableEvent
+	 *
+	 * @param source the {@code StompBrokerRelayMessageHandler} that is acting
+	 * as a relay for the broker that has become unavailable. Must not be {@code
+	 * null}.
+	 */
+	public BrokerBecameUnavailableEvent(StompBrokerRelayMessageHandler source) {
+		super(source);
+	}
+
+}

spring-messaging/src/main/java/org/springframework/messaging/simp/stomp/StompBrokerRelayMessageHandler.java

@@ -24,9 +24,13 @@ import java.util.Map;
 import java.util.concurrent.BlockingQueue;
 import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.LinkedBlockingQueue;
+import java.util.concurrent.atomic.AtomicBoolean;
 
 import org.apache.commons.logging.Log;
 import org.apache.commons.logging.LogFactory;
+import org.springframework.beans.BeansException;
+import org.springframework.context.ApplicationEventPublisher;
+import org.springframework.context.ApplicationEventPublisherAware;
 import org.springframework.context.SmartLifecycle;
 import org.springframework.messaging.Message;
 import org.springframework.messaging.MessageChannel;
@@ -51,9 +55,11 @@ import reactor.tcp.spec.TcpClientSpec;
 
 /**
  * @author Rossen Stoyanchev
+ * @author Andy Wilkinson
+ *
  * @since 4.0
  */
-public class StompBrokerRelayMessageHandler implements MessageHandler, SmartLifecycle {
+public class StompBrokerRelayMessageHandler implements MessageHandler, SmartLifecycle, ApplicationEventPublisherAware {
 
 	private static final Log logger = LogFactory.getLog(StompBrokerRelayMessageHandler.class);
 
@@ -63,6 +69,8 @@ public class StompBrokerRelayMessageHandler implements MessageHandler, SmartLife
 
 	private final String[] destinationPrefixes;
 
+	private ApplicationEventPublisher applicationEventPublisher;
+
 	private String relayHost = "127.0.0.1";
 
 	private int relayPort = 61613;
@@ -83,6 +91,7 @@ public class StompBrokerRelayMessageHandler implements MessageHandler, SmartLife
 
 	private boolean running = false;
 
+	private AtomicBoolean brokerAvailable = new AtomicBoolean(false);
 
 	/**
 	 * @param messageChannel the channel to send messages from the STOMP broker to
@@ -197,6 +206,12 @@ public class StompBrokerRelayMessageHandler implements MessageHandler, SmartLife
 		}
 	}
 
+	@Override
+	public void setApplicationEventPublisher(ApplicationEventPublisher applicationEventPublisher)
+			throws BeansException {
+		this.applicationEventPublisher = applicationEventPublisher;
+	}
+
 	/**
 	 * Open a "system" session for sending messages from parts of the application
 	 * not associated with a client STOMP session.
@@ -342,6 +357,18 @@ public class StompBrokerRelayMessageHandler implements MessageHandler, SmartLife
 		return false;
 	}
 
+	private void brokerAvailable() {
+		if (this.brokerAvailable.compareAndSet(false, true)) {
+			this.applicationEventPublisher.publishEvent(new BrokerBecameAvailableEvent(this));
+		}
+	}
+
+	private void brokerUnavailable() {
+		if (this.brokerAvailable.compareAndSet(true, false)) {
+			this.applicationEventPublisher.publishEvent(new BrokerBecameUnavailableEvent(this));
+		}
+	}
+
 
 	private class RelaySession {
 
@@ -356,7 +383,7 @@ public class StompBrokerRelayMessageHandler implements MessageHandler, SmartLife
 		private final Object monitor = new Object();
 
 
-		public RelaySession(String sessionId) {
+		private RelaySession(String sessionId) {
 			Assert.notNull(sessionId, "sessionId is required");
 			this.sessionId = sessionId;
 		}
@@ -404,6 +431,7 @@ public class StompBrokerRelayMessageHandler implements MessageHandler, SmartLife
 			if (StompCommand.CONNECTED == headers.getCommand()) {
 				synchronized(this.monitor) {
 					this.isConnected = true;
+					brokerAvailable();
 					flushMessages(this.promise.get());
 				}
 				return;
@@ -419,6 +447,8 @@ public class StompBrokerRelayMessageHandler implements MessageHandler, SmartLife
 		}
 
 		private void sendError(String sessionId, String errorText) {
+			brokerUnavailable();
+
 			StompHeaderAccessor headers = StompHeaderAccessor.create(StompCommand.ERROR);
 			headers.setSessionId(sessionId);
 			headers.setMessage(errorText);