iTechNote

An EntityManager can be injected directly into an EJB using the @javax.persistence.PersistenceContext annotation. By default, a transaction-scoped persistence context is injected. You can override the default with type() attribute. When the transaction is completed, the EJB container will perform the cleanup. The extended entity manager can only be injected into a stateful session bean.

As mentioned in part 1 of the article, the EntityManager is the central service for all persistence actions. The persistence actions can be summarized as follow:

• Persisting Entities: persist() method takes any entity bean instance and create a record in the database. The persist() method throws IllegalArgumentException if the parameter is not an entity bean. It also throws TransactionRequiredException if the method is invoked on a transaction-scoped persistence context. However, it is legal to call persist() outside of a transaction scope for extended entity manager.
• Finding Entities by Primary Key: Both find() and getReference() can be used to locate an entity. The find() method returns null if entity is not found in the database. It initializes the state based on the lazy-loading policies of each property. getReference() on the other hand throws a javax.persistence.EntityNotFound and the state may not be initialized. Both methods are safe to invoke outside transaction scope.
• Finding Entities by Query: Entities can also be located using EJB QL. You must first construct a javax.persistence.Query object using one of the five methods provided by EntityManager.
• Updating Entities: Updates to the state of an entity bean is automatically synchronized to the database depending on the flush mode or you can call flush() method directly.
• Merging Entities: State changes to a detached entity bean can be saved to the database by invoking the merge() method.
• Removing Entities: The remove() method deletes the entity bean from the database.
• Refreshing Entities: The refresh() method refreshes the state of the entity from the database, overwriting any state changes that are not flushed. If the entity bean is no longer in the database, EntityNotFoundException is thrown.
• Locking Entities: The EntityManager APIs support read and write locks.
• Determining Persistence Context: The contains() method returns true if the entity bean is managed by the persistence context.
• Detaching All Entities: The clear() method detaches all entity beans from the persistence context, losing changes that are not flushed.
• Saving Entities’ States: A method call of persist(), merge() or remove() takes effect when the EntityManager decides to flush or by manually invoking flush() method. Auto flushing only happens before a correlated query is executed and at transaction commit time. The behavior can be changed using FlushModeType, which takes valid values of AUTO and COMMIT. AUTO is the default flush mode as described before. COMMIT limits the flushing only at transaction commit time for performance reasons.
• Vendor-specific APIs: The getDelegate() method obtains a reference to the Java Persistence provider