Documentation Version: 3.4 - Publication Date : Dec 24, 2014

Continuous Query

Continuous query enables you to listen to the modifications performed on specific map entries. It is an entry listener with predicates. Please see the Entry Listener section for information on how to add entry listeners to a map.

As an example, let's listen to the changes made on an employee with the surname "Smith". First, let's create the Employee class.

import java.io.Serializable;

public class Employee implements Serializable {

    private final String surname;

    public Employee(String surname) {
        this.surname = surname;
    }

    @Override
    public String toString() {
        return "Employee{" +
                "surname='" + surname + '\'' +
                '}';
    }
}

Then, let's create the continuous query by adding the entry listener with the surname predicate.

import com.hazelcast.core.*;
import com.hazelcast.query.SqlPredicate;

public class ContinuousQuery {

    public static void main(String[] args) {
        HazelcastInstance hz = Hazelcast.newHazelcastInstance();
        IMap<String, String> map = hz.getMap("map");
        map.addEntryListener(new MyEntryListener(),
                new SqlPredicate("surname=smith"), true);
        System.out.println("Entry Listener 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 play with the employee "smith" and see how that employee will be listened to.

import com.hazelcast.core.Hazelcast;
import com.hazelcast.core.HazelcastInstance;
import com.hazelcast.core.IMap;

public class Modify {

    public static void main(String[] args) {
        HazelcastInstance hz = Hazelcast.newHazelcastInstance();
        IMap<String, Employee> map = hz.getMap("map");

        map.put("1", new Employee("smith"));
        map.put("2", new Employee("jordan"));
        System.out.println("done");
        System.exit(0);
    }
}

When you first run the class ContinuousQuery and then run Modify, you will see output similar to the listing below.

entryAdded:EntryEvent {Address[192.168.178.10]:5702} key=1,oldValue=null,
value=Person{name= smith }, event=ADDED, by Member [192.168.178.10]:5702