Hazelcast supports the addition of listeners when a given collection, for example, queue, set, list, etc. is updated. Typical events include entry added an entry removed.
Let’s see how to implement a set listener via an example. So, let’s say we want to implement a listener which tracks the number of elements in a set.
Example
So, let’s first implement the Producer −
public class SetTimedProducer{
public static void main(String... args) throws IOException,
InterruptedException {
//initialize hazelcast instance
HazelcastInstance hazelcast = Hazelcast.newHazelcastInstance();
Thread.sleep(5000);
// create a set
ISet<String> hzFruits = hazelcast.getSet("fruits");
hzFruits.add("Mango");
Thread.sleep(2000);
hzFruits.add("Apple");
Thread.sleep(2000);
hzFruits.add("Banana");
System.exit(0);
}
}
Now let’s implement the listener −
package com.example.demo;
import java.io.IOException;
import com.hazelcast.core.ISet;
import com.hazelcast.core.ItemEvent;
import com.hazelcast.core.ItemListener;
import com.hazelcast.core.Hazelcast;
import com.hazelcast.core.HazelcastInstance;
public class SetListener{
public static void main(String... args) throws IOException, InterruptedException {
//initialize hazelcast instance
HazelcastInstance hazelcast = Hazelcast.newHazelcastInstance();
// create a set
ISet<String> hzFruits = hazelcast.getSet("fruits");
ItemListener<String> listener = new FruitListener<String>();
hzFruits.addItemListener(listener, true);
System.exit(0);
}
private static class FruitListener<String> implements ItemListener<String> {
private int count = 0;
@Override
public void itemAdded(ItemEvent<String> item) {
System.out.println("item added" + item);
count ++;
System.out.println("Total elements" + count);
}
@Override
public void itemRemoved(ItemEvent<String> item) {
count --;
}
}
}
We will first run the producer −
java -cp .\target\demo-0.0.1-SNAPSHOT.jar com.example.demo.SetTimedProducer
And then, we run the listeners and let it run indefinitely −
java -cp .\target\demo-0.0.1-SNAPSHOT.jar com.example.demo.SetListener
Output
The output from the Listener is as follows −
item added: ItemEvent{
event=ADDED, item=Mango, member=Member [localhost]:5701-c28a60b7-3259-44bf-8793-54063d244394 this}
Total elements: 1
item added: ItemEvent{
event=ADDED, item=Apple, member=Member [localhost]:5701-c28a60b7-3259-44bf-8793-54063d244394 this}
Total elements: 2
item added: ItemEvent{
event=ADDED, item=Banana, member=Member [localhost]:5701-c28a60b7-3259-44bf-8793-54063d244394 this}
Total elements: 3
The call with hzFruits.addItemListener(listener, true) tells Hazelcast to provide member information. If set to false, we will just be notified that an entry was added/removed. This helps in avoiding the need to serialize and deserialize the entry to make it accessible to the listener.
