Documentation Version: 3.3 - Publication Date : Sep 07, 2014

Entry Listener

Map entry events can be listened. Hazelcast distributed map offers the method addEntryListener to add an entry listener to the map and listen to the entry events.

Let's take a look at the below sample code.

public class Listen {

  public static void main( String[] args ) {
    HazelcastInstance hz = Hazelcast.newHazelcastInstance();
    IMap<String, String> map = hz.getMap( "somemap" );
    map.addEntryListener( new MyEntryListener(), true );
     System.out.println( "EntryListener registered" );
  }

  static class MyEntryListener implements EntryListener<String, String> {
    @Override
    public void entryAdded( EntryEvent<String, String> event ) {
      System.out.println( "Entry Added:" + event );
    }

    @Override
    public void entryRemoved( EntryEvent<String, String> event ) {
      System.out.println( "Entry Removed:" + event );
    }

    @Override
    public void entryUpdated( EntryEvent<String, String> event ) {
      System.out.println( "Entry Updated:" + event );
    }

    @Override
    public void entryEvicted( EntryEvent<String, String> event ) {
      System.out.println( "Entry Evicted:" + event );
    }

    @Override
    public void mapEvicted( MapEvent event ) {
      System.out.println( "Map Evicted:" + event );
    }
  }
}

And, now let's perform some modifications on the map entries using the below sample code.

public class Modify {

  public static void main( String[] args ) {
    HazelcastInstance hz = Hazelcast.newHazelcastInstance();
    IMap<String, String> map = hz.getMap( "somemap");
    String key = "" + System.nanoTime();
    String value = "1";
    map.put( key, value );
    map.put( key, "2" );
    map.delete( key );
  }
}

Now, let's first execute the class Listen and then execute Modify. Check out the below output produced by Listen.

entryAdded:EntryEvent {Address[192.168.1.100]:5702} key=251359212222282,
    oldValue=null, value=1, event=ADDED, by Member [192.168.1.100]:5702

entryUpdated:EntryEvent {Address[192.168.1.100]:5702} key=251359212222282,
    oldValue=1, value=2, event=UPDATED, by Member [192.168.1.100]:5702

entryRemoved:EntryEvent {Address[192.168.1.100]:5702} key=251359212222282,
    oldValue=2, value=2, event=REMOVED, by Member [192.168.1.100]:5702

Entry Listener runs on event threads which are also used by other listeners (e.g. collection listeners, pub/sub message listeners, etc.). This means entry listeners can access to other partitions. So, consideration should be given when running long tasks since listening to those tasks may cause other event listeners to starve.