9/29/2006

Inject EntityManagerFactory and EntityManagerFactory to Servlet

You can inject EntityManagerFactory and/or EntityManager into a Servlet, but be aware of the thread-safe issue. A servlet can service multiple concurrent requests using multiple threads. So all instance variables are shared by all these threads, which may cause undesired side-effect.

The good news is, all methods in EntityManagerFactory interface are thread-safe. Therefore, you can safely inject EntityManagerFactory into a servlet instance variable.

public class MyServlet extends HttpServlet {
//this is thread-safe
@PersistenceUnit(unitName="my-pu")
private EntityManagerFactory emf;
However, methods in EntityManager interface are not thread-safe, and may not be shared among multiple concurrent requests. Therefore, do not inject EntityManager into a servlet instance variable.
//this is not thread-safe, and avoid it
@PersistenceContext(unitName="my-pu")
private EntityManager em;
Having said that, you can still inject EntityManager at the servlet class type level, and look it up when needed during request processing.
@PersistenceContext(unitName="my-pu", name="persistence/em")
public class MyServlet extends HttpServlet {
protected void doGet(
HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException {
try {
InitialContext ic = new InitialContext();
EntityManager em =
(EntityManager) ic.lookup("java:comp/env/persistence/em");
} catch (NamingException ex) {
...
}
}
}

3 comments:

Steve Smith said...

Great and Useful Article.

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Carla said...

Great answer, very nice that you give such answer here.

eMexo Technologies said...

Good Post! Thank you so much for sharing this pretty post, it was so good to read and useful to improve my knowledge as updated one, keep blogging.

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