Java Magazine - November/December 2011

//fix this /

Hint: Check the HttpServlet API in the Java EE 6 API.

In the last issue, Arun Gupta, a Java evangelist at Oracle, posed a challenge about the CDI specification and asked readers for a fix.

2 THE CODE

The correct answer is #2: “beans.xml” is required to enable injection. The CDI specification requires

Consider the following piece of code to have database access within a Servlet:

“beans.xml” in the WEB-INF directory in order for bean injection to work. So even though the code is fine, a missing file did not allow the EJB injection to go through.

@WebServlet(name = "TestServlet", urlPatterns = {"/TestServlet"})
public class TestServlet extends HttpServlet {

JAVA IN ACTION

@PersistenceContext
EntityManager em;

And now, for Guptas latest challenge.

1 THE PROBLEM

 protected void doGet(HttpServletRequest request,
HttpServletResponse response) {
 // Invoke methods on Entity Manager
 }
}

Java EE 6 allows you to create Web applications very
3 WHAT S THE FIX?
easily using annotations on POJOs. A POJO can be
Is this code guaranteed to work in a multithreaded environment?
easily converted to a Servlet by adding @WebServlet
1) Of course. All the samples are written this way.

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and extending HttpServlet. A POJO becomes a JPA
2) No. Servlets are re-entrant, and EntityManager is not thread-
safe. Instead inject as
bean by adding @Entity. Each method of a Java class

@PersistenceUnit EntityManagerFactory em;

becomes implicitly transactional by adding @Stateless.
and then get EntityManager within the doGet() method.
Following the convention-over-configuration paradigm,
the deployment descriptors are optional for most of the
3) Database access in Servlets must be done through EJBs only. The
correct way is to move the database code in an EJB, inject the
EJB in this Servlet, and invoke methods on the EJB.
common cases.
4) EntityManager injection will not work in Servlets. Instead use
Persistence.createEntityManager to obtain EntityManager in the
doGet() method and then invoke operations on it.