How to get container-managed EntityManager in servlets

In this previous post, I wrote about the danger of using field/setter injection to get EntityManager in servlets A follow-up post compares container-managed EntityManager vs application-managed EntityManager.

What if I need a container-managed EntityManager in my servlet class? There are 2 ways to do that:

1. type-level injection + JNDI lookup

@PersistenceContext(unitName="my-pu", name="persistence/em")
public class FooServlet extends HttpServlet {
@Resource private UserTransaction ut;

protected void doGet(HttpServletRequest request,
                 HttpServletResponse response)
throws ServletException, IOException {
EntityManager em = null;
try {
 Context ic = new InitialContext();
 em = (EntityManager) ic.lookup
   ("java:comp/env/persistence/em");
 } catch (NamingException e) {
   throw new ServletException(e);
 }
 //call EntityManager methods inside transaction
 try {
   ut.begin();
   Employee employee = new Employee(...);
   em.persist(employee);
   ut.commit();
 } catch (Exception e) {
   throw new ServletException(e);
 }
}
}

2. web.xml + JNDI lookup

public class FooServlet extends HttpServlet {
@Resource private UserTransaction ut;
//the rest is the same as listed in bullet 1

The persistence-context-ref is declared in web.xml:

<?xml version="1.0" encoding="UTF-8"?>
<web-app version="2.5"
xmlns="http://java.sun.com/xml/ns/javaee"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd">
<servlet>
<servlet-name>FooServlet</servlet-name>
<servlet-class>com.foo.abc.FooServlet</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>FooServlet</servlet-name>
<url-pattern>/foo</url-pattern>
</servlet-mapping>
<persistence-context-ref>
<persistence-context-ref-name>persistence/em</persistence-context-ref-name>
  <persistence-unit-name>my-pu</persistence-unit-name>
</persistence-context-ref>
</web-app>

Tags: JPA, servlet, EntityManager, EntityManagerFactory, PersistenceContext, PersistenceUnit