Topic
Hazelcast provides a distribution mechanism for publishing messages that are delivered to multiple subscribers. This is also known as a publish/subscribe (pub/sub) messaging model. Publishing and subscribing operations are cluster wide. When a member subscribes to a topic, it is actually registering for messages published by any member in the cluster, including the new members that joined after you add the listener.
NOTE: Publish operation is async. It does not wait for operations to run in
remote nodes, it works as fire and forget.
Getting a Topic and Publishing Messages
Use the HazelcastInstance getTopic method to get the Topic, then use the topic publish method to publish your messages (messageObject).
import com.hazelcast.core.Topic;
import com.hazelcast.core.Hazelcast;
import com.hazelcast.core.MessageListener;
public class Sample implements MessageListener<MyEvent> {
public static void main( String[] args ) {
Sample sample = new Sample();
HazelcastInstance hazelcastInstance = Hazelcast.newHazelcastInstance();
ITopic topic = hazelcastInstance.getTopic( "default" );
topic.addMessageListener( sample );
topic.publish( new MyEvent() );
}
public void onMessage( Message<MyEvent> message ) {
MyEvent myEvent = message.getMessageObject();
System.out.println( "Message received = " + myEvent.toString() );
if ( myEvent.isHeavyweight() ) {
messageExecutor.execute( new Runnable() {
public void run() {
doHeavyweightStuff( myEvent );
}
} );
}
}
// ...
private final Executor messageExecutor = Executors.newSingleThreadExecutor();
}
Hazelcast Topic uses MessageListener to listen to events which occur when a message is received. Please refer to the Listening for Topic Messages section for information on how to create a message listener class and register it.