How to Resolve Hibernate 'Different Object with the Same Identifier Value Already Associated with the Session' Error

What causes the error 'Different object with the same identifier value was already associated with the session' in Hibernate?

The error message 'Different object with the same identifier value was already associated with the session' in Hibernate indicates that there is an issue with how entities are managed within the session. It typically occurs when trying to persist or merge an entity that has the same identifier (primary key) as another entity being managed by the current session. Understanding and resolving this problem requires knowledge of Hibernate's session management and the entity lifecycle.

// Example to clear session before persisting
Session session = sessionFactory.openSession();
Transaction tx = null;
try {
    tx = session.beginTransaction();
    // Assuming 'MyEntity' is the entity class and '1' is the ID
    MyEntity entity = session.get(MyEntity.class, 1);
    session.evict(entity); // Remove the old instance from session

    MyEntity newEntity = new MyEntity();
    newEntity.setId(1); // Same ID
    newEntity.setData("New Data");
    session.save(newEntity); // Save new instance
    tx.commit();
} catch (Exception e) {
    if (tx != null) tx.rollback();
    e.printStackTrace();
} finally {
    session.close();
}

Causes

• Two different entity instances have been instantiated with the same identifier value and are being managed by the same Hibernate session.
• An entity object was attempted to be saved or updated while another object with the same ID was already present in the session context.
• Trying to merge an object into the session which is already represented by a different instance with the same ID.

Solutions

• Verify that you are not mistakenly creating two instances of the same entity with the same identifier in your code.
• Use the appropriate 'merge()' method to combine detached entities, ensuring that only one instance with a given identifier exists in the session.
• Clear the session using 'session.clear()' or 'session.evict(entity)' to remove the current instance of the entity before attempting to save a new instance with the same ID.

Mistake: Not managing entity states properly (e.g., mixing transient, persistent, and detached entities)

Solution: Ensure that you are aware of the lifecycle of entities and use merging correctly when necessary.

Mistake: Forgetting to clear the session when needed, leading to stale or duplicate entity conflicts.

Solution: Use 'session.clear()' or 'session.evict(entity)' wisely between operations that may introduce conflicts.