PersistenceHolder.java

package pw.osin.example.data;

import javax.persistence.EntityManager;
import javax.persistence.Persistence;

public enum PersistenceHolder {

    INSTANCE;

    private EntityManager entityManager;

    public EntityManager getEntityManager() {
        if (entityManager == null) {
             entityManager  = Persistence.createEntityManagerFactory("tweetPU").createEntityManager();
        }
        return entityManager;
    }
}