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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 閱讀(826) 評論(0) 編輯收藏所屬分類: JavaEE

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