I have this entity bean:

import javax.persistence.*;

@Entity
public class CounterTest {
    private int id;
    private int counter;

    @Id
    public int getId() { return id; }
    public void setId(int id) { this.id = id; }

    public int getCounter() { return counter; }
    public void setCounter(int counter) { this.counter = counter; }
}

and this stateful bean to increment a counter:

import java.rmi.RemoteException;

import javax.ejb.*;
import javax.persistence.*;

@Stateful
public class CounterTestBean implements CounterTestRemote {
    @PersistenceContext(unitName = "JavaEE")
    EntityManager manager;

    public void initDB() {
        CounterTest ct = new CounterTest();
        ct.setNr(1);
        ct.setWert(1);
        manager.persist(ct);
    }

    public boolean testCounterWithLock() {
        try {
            CounterTest ct = manager.find(CounterTest.class, 1);
            manager.lock(ct, LockModeType.WRITE);
            int wert = ct.getWert();
            ct.setWert(wert + 1);
            manager.flush();
            return true;
        } catch (Throwable t) {
            return false;
        }
    }
}

When I call testCounterWithLock() from three threads 500 times each, the counter gets incremented between 13 and 1279 times. How do I fix this code so that it is incremented 1500 times?