Mind thread-safety when injecting EntityManager

As I wrote in post Why we need type-level injections in JavaEE, 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:

public class FooServlet 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:

public class FooServlet 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
}
}

Tags: resource injection, JavaEE, servlet, EntityManager, PersistenceContext, JPA