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Thread-safety when injecting JPA EntityManager

Injecting EJB 3 stateful beans into servlet instance fields is not thread-safe. Along the same line, injecting EntityManager with @PersistenceContext into servlet instance variables is not thread-safe, either. EntityManager is just not designed to be thread-safe.

For example, the following code snippet of a servlet class is incorrect:

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public class EMTestServlet extends HttpServlet {
  //This field injection is not thread-safe.
  //FIXME
  @PersistenceContext
  private EntityManager em;
...
}

One way to fix this is to inject EntityManagerFactory instead. EntityManagerFactory is guaranteed to be thread-safe. For example:
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public class EMTestServlet extends HttpServlet {
  //This field injection is thread-safe
  @PersistenceUnit
  private EntityManagerFactory emf;
  
  protected void doGet(HttpServletRequest request,
             HttpServletResponse response)
  throws ServletException, IOException {
  EntityManager em = emf.createEntityManager();
  //work with em
  }
}

Continuing container-managed EntityManager vs application-managed EntityManager.

There are important differences between the injected EntityManager, and the EntityManager created from an injected EntityManagerFactory. Basically, injected EntityManager is container-managed, meaning all of its lifecycle is controlled by the container (web container or EJB container). Application code cannot close it, or otherwise interfere with its life.

In addition, for a container-managed EntityManager, its associated PersistenceContext is automatically propagated along with the underlying JTA, from servlet A to servlet B, from servlet to EJB, from EJB a to EJB B, and so on. As such, EntityManager of the same configuration injected into various classes can share the same PersistenceContext in a call stack.

On the other hand, EntityManager created from EntityManagerFactory is application-managed EntityManager. Application code is responsible for managing its whole lifecycle. And there is no PersistenceContext propagation for application-managed EntityManager.

Are all EntityManager's obtained from EntityManagerFactory application-managed EntityManager? Yes.

Is it possible to get a container-managed EntityManager from EntityManagerFactory? No.

You may have read about the method EntityManagerFactory.getEntityManager(), which returns a container-managed EntityManager. This method was considered and included in the early draft version of Java Persistence API, but was eventually removed from its final release.

posted on 2013-02-04 10:49 gembin 阅读(862) 评论(0)  编辑  收藏 所属分类: JavaEE


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