What is the proper way to re-attach detached objects in Hibernate?

I have a situation in which I need to re-attach detached objects to a hibernate session, although an object of the same identity MAY already exist in the session, which will cause errors.

Right now, I can do one of two things.

1. getHibernateTemplate().update( obj ) This works if and only if an object doesn't already exist in the hibernate session. Exceptions are thrown stating an object with the given identifier already exists in the session when I need it later.

2. getHibernateTemplate().merge( obj ) This works if and only if an object exists in the hibernate session. Exceptions are thrown when I need the object to be in a session later if I use this.

Given these two scenarios, how can I generically attach sessions to objects? I don't want to use exceptions to control the flow of this problem's solution, as there must be a more elegant solution...

So it seems that there is no way to reattach a stale detached entity in JPA.

merge() will push the stale state to the DB, and overwrite any intervening updates.

refresh() cannot be called on a detached entity.

lock() cannot be called on a detached entity, and even if it could, and it did reattach the entity, calling 'lock' with argument 'LockMode.NONE' implying that you are locking, but not locking, is the most counterintuitive piece of API design I've ever seen.

So you are stuck. There's an detach() method, but no attach() or reattach(). An obvious step in the object lifecycle is not available to you.

Judging by the number of similar questions about JPA, it seems that even if JPA does claim to have a coherent model, it most certainly does not match the mental model of most programmers, who have been cursed to waste many hours trying understand how to get JPA to do the simplest things, and end up with cache management code all over their applications.

It seems the only way to do it is discard your stale detached entity and do a find query with the same id, that will hit the L2 or the DB.

Mik

All of these answers miss an important distinction. update() is used to (re)attach your object graph to a Session. The objects you pass it are the ones that are made managed.

merge() is actually not a (re)attachment API. Notice merge() has a return value? That's because it returns you the managed graph, which may not be the graph you passed it. merge() is a JPA API and its behavior is governed by the JPA spec. If the object you pass in to merge() is already managed (already associated with the Session) then that's the graph Hibernate works with; the object passed in is the same object returned from merge(). If, however, the object you pass into merge() is detached, Hibernate creates a new object graph that is managed and it copies the state from your detached graph onto the new managed graph. Again, this is all dictated and governed by the JPA spec.

In terms of a generic strategy for "make sure this entity is managed, or make it managed", it kind of depends on if you want to account for not-yet-inserted data as well. Assuming you do, use something like

if ( session.contains( myEntity ) ) {
    // nothing to do... myEntity is already associated with the session
}
else {
    session.saveOrUpdate( myEntity );
}

Notice I used saveOrUpdate() rather than update(). If you do not want not-yet-inserted data handled here, use update() instead...

Entity states

JPA defines the following entity states:

New (Transient)

A newly created object that hasn’t ever been associated with a Hibernate Session (a.k.a Persistence Context) and is not mapped to any database table row is considered to be in the New (Transient) state.

To become persisted we need to either explicitly call the EntityManager#persist method or make use of the transitive persistence mechanism.

Persistent (Managed)

A persistent entity has been associated with a database table row and it’s being managed by the currently running Persistence Context. Any change made to such an entity is going to be detected and propagated to the database (during the Session flush-time).

With Hibernate, we no longer have to execute INSERT/UPDATE/DELETE statements. Hibernate employs a transactional write-behind working style and changes are synchronized at the very last responsible moment, during the current Session flush-time.

Detached

Once the currently running Persistence Context is closed all the previously managed entities become detached. Successive changes will no longer be tracked and no automatic database synchronization is going to happen.

Entity state transitions

You can change the entity state using various methods defined by the EntityManager interface.

To understand the JPA entity state transitions better, consider the following diagram:

When using JPA, to reassociate a detached entity to an active EntityManager, you can use the merge operation.

When using the native Hibernate API, apart from merge, you can reattach a detached entity to an active Hibernate Sessionusing the update methods, as demonstrated by the following diagram:

Merging a detached entity

The merge is going to copy the detached entity state (source) to a managed entity instance (destination).

Consider we have persisted the following Book entity, and now the entity is detached as the EntityManager that was used to persist the entity got closed:

Book _book = doInJPA(entityManager -> {
    Book book = new Book()
    .setIsbn("978-9730228236")
    .setTitle("High-Performance Java Persistence")
    .setAuthor("Vlad Mihalcea");
 
    entityManager.persist(book);
 
    return book;
});

While the entity is in the detached state, we modify it as follows:

_book.setTitle(
    "High-Performance Java Persistence, 2nd edition"
);

Now, we want to propagate the changes to the database, so we can call the merge method:

doInJPA(entityManager -> {
    Book book = entityManager.merge(_book);
 
    LOGGER.info("Merging the Book entity");
 
    assertFalse(book == _book);
});

And Hibernate is going to execute the following SQL statements:

SELECT
    b.id,
    b.author AS author2_0_,
    b.isbn AS isbn3_0_,
    b.title AS title4_0_
FROM
    book b
WHERE
    b.id = 1
 
-- Merging the Book entity
 
UPDATE
    book
SET
    author = 'Vlad Mihalcea',
    isbn = '978-9730228236',
    title = 'High-Performance Java Persistence, 2nd edition'
WHERE
    id = 1

If the merging entity has no equivalent in the current EntityManager, a fresh entity snapshot will be fetched from the database.

Once there is a managed entity, JPA copies the state of the detached entity onto the one that is currently managed, and during the Persistence Context flush, an UPDATE will be generated if the dirty checking mechanism finds that the managed entity has changed.

> So, when using merge, the detached object instance will continue to remain detached even after the merge operation.

Reattaching a detached entity

Hibernate, but not JPA supports reattaching through the update method.

A Hibernate Session can only associate one entity object for a given database row. This is because the Persistence Context acts as an in-memory cache (first level cache) and only one value (entity) is associated with a given key (entity type and database identifier).

An entity can be reattached only if there is no other JVM object (matching the same database row) already associated with the current Hibernate Session.

Considering we have persisted the Book entity and that we modified it when the Book entity was in the detached state:

Book _book = doInJPA(entityManager -> {
    Book book = new Book()
    .setIsbn("978-9730228236")
    .setTitle("High-Performance Java Persistence")
    .setAuthor("Vlad Mihalcea");
 
    entityManager.persist(book);
 
    return book;
});
      
_book.setTitle(
    "High-Performance Java Persistence, 2nd edition"
);

We can reattach the detached entity like this:

doInJPA(entityManager -> {
    Session session = entityManager.unwrap(Session.class);
 
    session.update(_book);
 
    LOGGER.info("Updating the Book entity");
});

And Hibernate will execute the following SQL statement:

-- Updating the Book entity
 
UPDATE
    book
SET
    author = 'Vlad Mihalcea',
    isbn = '978-9730228236',
    title = 'High-Performance Java Persistence, 2nd edition'
WHERE
    id = 1

> The update method requires you to unwrap the EntityManager to a Hibernate Session.

Unlike merge, the provided detached entity is going to be reassociated with the current Persistence Context and an UPDATE is scheduled during flush whether the entity has modified or not.

To prevent this, you can use the @SelectBeforeUpdate Hibernate annotation which will trigger a SELECT statement that fetched loaded state which is then used by the dirty checking mechanism.

@Entity(name = "Book")
@Table(name = "book")
@SelectBeforeUpdate
public class Book {
 
    //Code omitted for brevity
}

Beware of the NonUniqueObjectException

One problem that can occur with update is if the Persistence Context already contains an entity reference with the same id and of the same type as in the following example:

Book _book = doInJPA(entityManager -> {
    Book book = new Book()
    .setIsbn("978-9730228236")
    .setTitle("High-Performance Java Persistence")
    .setAuthor("Vlad Mihalcea");
 
    Session session = entityManager.unwrap(Session.class);
    session.saveOrUpdate(book);
 
    return book;
});
 
_book.setTitle(
    "High-Performance Java Persistence, 2nd edition"
);
 
try {
    doInJPA(entityManager -> {
        Book book = entityManager.find(
            Book.class,
            _book.getId()
        );
 
        Session session = entityManager.unwrap(Session.class);
        session.saveOrUpdate(_book);
    });
} catch (NonUniqueObjectException e) {
    LOGGER.error(
        "The Persistence Context cannot hold " +
        "two representations of the same entity",
        e
    );
}

Now, when executing the test case above, Hibernate is going to throw a NonUniqueObjectException because the second EntityManager already contains a Book entity with the same identifier as the one we pass to update, and the Persistence Context cannot hold two representations of the same entity.

org.hibernate.NonUniqueObjectException:
    A different object with the same identifier value was already associated with the session : [com.vladmihalcea.book.hpjp.hibernate.pc.Book#1]
    at org.hibernate.engine.internal.StatefulPersistenceContext.checkUniqueness(StatefulPersistenceContext.java:651)
    at org.hibernate.event.internal.DefaultSaveOrUpdateEventListener.performUpdate(DefaultSaveOrUpdateEventListener.java:284)
    at org.hibernate.event.internal.DefaultSaveOrUpdateEventListener.entityIsDetached(DefaultSaveOrUpdateEventListener.java:227)
    at org.hibernate.event.internal.DefaultSaveOrUpdateEventListener.performSaveOrUpdate(DefaultSaveOrUpdateEventListener.java:92)
    at org.hibernate.event.internal.DefaultSaveOrUpdateEventListener.onSaveOrUpdate(DefaultSaveOrUpdateEventListener.java:73)
    at org.hibernate.internal.SessionImpl.fireSaveOrUpdate(SessionImpl.java:682)
    at org.hibernate.internal.SessionImpl.saveOrUpdate(SessionImpl.java:674)

Conclusion

The merge method is to be preferred if you are using optimistic locking as it allows you to prevent lost updates.

The update is good for batch updates as it can prevent the additional SELECT statement generated by the merge operation, therefore reducing the batch update execution time.

Undiplomatic answer: You're probably looking for an extended persistence context. This is one of the main reasons behind the Seam Framework... If you're struggling to use Hibernate in Spring in particular, check out this piece of Seam's docs.

Diplomatic answer: This is described in the Hibernate docs. If you need more clarification, have a look at Section 9.3.2 of Java Persistence with Hibernate called "Working with Detached Objects." I'd strongly recommend you get this book if you're doing anything more than CRUD with Hibernate.

If you are sure that your entity has not been modified (or if you agree any modification will be lost), then you may reattach it to the session with lock.

session.lock(entity, LockMode.NONE);

It will lock nothing, but it will get the entity from the session cache or (if not found there) read it from the DB.

It's very useful to prevent LazyInitException when you are navigating relations from an "old" (from the HttpSession for example) entities. You first "re-attach" the entity.

Using get may also work, except when you get inheritance mapped (which will already throw an exception on the getId()).

entity = session.get(entity.getClass(), entity.getId());

I went back to the JavaDoc for org.hibernate.Session and found the following:

> Transient instances may be made persistent by calling save(), persist() or saveOrUpdate(). Persistent instances may be made transient by calling delete(). Any instance returned by a get() or load() method is persistent. Detached instances may be made persistent by calling update(), saveOrUpdate(), lock() or replicate(). The state of a transient or detached instance may also be made persistent as a new persistent instance by calling merge().

Thus update(), saveOrUpdate(), lock(), replicate() and merge() are the candidate options.

update(): Will throw an exception if there is a persistent instance with the same identifier.

saveOrUpdate(): Either save or update

lock(): Deprecated

replicate(): Persist the state of the given detached instance, reusing the current identifier value.

merge(): Returns a persistent object with the same identifier. The given instance does not become associated with the session.

Hence, lock() should not be used straightway and based on the functional requirement one or more of them can be chosen.

I did it that way in C# with NHibernate, but it should work the same way in Java:

public virtual void Attach()
{
    if (!HibernateSessionManager.Instance.GetSession().Contains(this))
    {
        ISession session = HibernateSessionManager.Instance.GetSession();
        using (ITransaction t = session.BeginTransaction())
        {
            session.Lock(this, NHibernate.LockMode.None);
            t.Commit();
        }
    }
}

First Lock was called on every object because Contains was always false. The problem is that NHibernate compares objects by database id and type. Contains uses the equals method, which compares by reference if it's not overwritten. With that equals method it works without any Exceptions:

public override bool Equals(object obj)
{
    if (this == obj) { 
        return true;
    } 
    if (GetType() != obj.GetType()) {
        return false;
    }
    if (Id != ((BaseObject)obj).Id)
    {
        return false;
    }
    return true;
}

Session.contains(Object obj) checks the reference and will not detect a different instance that represents the same row and is already attached to it.

Here my generic solution for Entities with an identifier property.

public static void update(final Session session, final Object entity)
{
	// if the given instance is in session, nothing to do
	if (session.contains(entity))
		return;

	// check if there is already a different attached instance representing the same row
	final ClassMetadata classMetadata = session.getSessionFactory().getClassMetadata(entity.getClass());
	final Serializable identifier = classMetadata.getIdentifier(entity, (SessionImplementor) session);

	final Object sessionEntity = session.load(entity.getClass(), identifier);
	// override changes, last call to update wins
	if (sessionEntity != null)
		session.evict(sessionEntity);
	session.update(entity);
}

This is one of the few aspects of .Net EntityFramework I like, the different attach options regarding changed entities and their properties.

I came up with a solution to "refresh" an object from the persistence store that will account for other objects which may already be attached to the session:

public void refreshDetached(T entity, Long id)
{
	// Check for any OTHER instances already attached to the session since
	// refresh will not work if there are any.
	T attached = (T) session.load(getPersistentClass(), id);
	if (attached != entity)
	{
		session.evict(attached);
		session.lock(entity, LockMode.NONE);
	}
	session.refresh(entity);
}

Sorry, cannot seem to add comments (yet?).

Using Hibernate 3.5.0-Final

Whereas the Session#lock method this deprecated, the javadoc does suggest using Session#buildLockRequest(LockOptions)#lock(entity)and if you make sure your associations have cascade=lock, the lazy-loading isn't an issue either.

So, my attach method looks a bit like

MyEntity attach(MyEntity entity) {
    if(getSession().contains(entity)) return entity;
    getSession().buildLockRequest(LockOptions.NONE).lock(entity);
    return entity;

Initial tests suggest it works a treat.

Perhaps it behaves slightly different on Eclipselink. To re-attach detached objects without getting stale data, I usually do:

Object obj = em.find(obj.getClass(), id);

and as an optional a second step (to get caches invalidated):

em.refresh(obj)

try getHibernateTemplate().replicate(entity,ReplicationMode.LATEST_VERSION)

In the original post, there are two methods, update(obj) and merge(obj) that are mentioned to work, but in opposite circumstances. If this is really true, then why not test to see if the object is already in the session first, and then call update(obj) if it is, otherwise call merge(obj).

The test for existence in the session is session.contains(obj). Therefore, I would think the following pseudo-code would work:

if (session.contains(obj))
{
    session.update(obj);
}
else 
{
    session.merge(obj);
}

to reattach this object, you must use merge();

this methode accept in parameter your entity detached and return an entity will be attached and reloaded from Database.

Example :
    Lot objAttach =	em.merge(oldObjDetached);
	objAttach.setEtat(...);
	em.persist(objAttach);

calling first merge() (to update persistent instance), then lock(LockMode.NONE) (to attach the current instance, not the one returned by merge()) seems to work for some use cases.

Property hibernate.allow_refresh_detached_entity did the trick for me. But it is a general rule, so it is not very suitable if you want to do it only in some cases. I hope it helps.

Tested on Hibernate 5.4.9

SessionFactoryOptionsBuilder

try getHibernateTemplate().saveOrUpdate()