Continuous query enables to listen to the modifications performed on specific map entries. It is an entry listener with predicates. Please see Entry Listener 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 will be listened.
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 Modify
, you will see the output similar to the one 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