JPA: check whether an entity object has been persisted or not

Question

Is there a general method that can

 if(entity is persisted before){
     entity = entity.merge();
 }else{
     entity.persist();
 }

So the method contain above logic is safe everywhere?

Answer 1

If you need to know is object already in persistence context you should use contains method of EntityManager .

Only EntityManager can tell you is entity persisted or not, entity does not have such information.

Here you can check javadoc for contains method .

if (!em.contains(entity)) {
  em.persist(entity);
} else {
  em.merge(entity);
}

Answer 2

要检查当前 PersistenceContext 是否已持久化实体对象，您可以使用 EntityManager 方法contains(Object entity)

Answer 3

Maybe it's too late, but here are my findings! If you have an entity with a generate value, you can use it to check if the entity is already in DB, assuming you are not modifying this value manually.

@Entity
public class MyEntity {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "id")
    private Integer id;
   
    // getter...
}

public class Main {
    public static void main() {
        MyEntity myEntity1 = new MyEntity();
        MyEntity myEntity2 = em.find(MyEntity.class, 4);
        em.detach(myEntity2); // or em.close()

        // other stuff and modifications

        // begin transaction
        persistEntity(myEntity1); // will use persist()
        persistEntity(myEntity2); // will use merge()
        // commit transaction

    }

    // This will manage correctly entities in different state
    public void persistEntity(MyEtity entity) {
        if (myEntity.getId() != null) em.merge(entity);
        else em.persist(entity);
    }
}

Using em.contains(entity) will fail in this scenario:

public static void main(){
    MyEntity myEntity = em.find(MyEntity.class, 5);
    
    em.detach(myEntity); // or em.close()
  
    // We are going to execute persist() because the entity is detached
    if (!em.contains(myEntity)) 
        // This call will produce an exception org.hibernate.PersistentObjectException
        em.persist(myEntity);
    else 
        em.merge(myEntity);
}

There are a performance reasons to try to achieve what OP is trying to do. You surely can use em.merge() instead of em.persist() , but not without a cost.

A call to em.merge() is trying to retrieve an existing entity from DB with a SELECT query and update it. So if the entity was never persisted, this will waste some CPU cycles. On the other side em.persist() will only produce one INSERT query.