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
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
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) {