Java Persistence API

Using Entities in Java EE Applications

EntityManagers are obtained via "Dependency Injection"
Within the context of an EJB method, the JTA transaction will be started for you

EJB Example

In a servlet environment, you must initiate the JTA transactions yourself (a servlet filter maybe)

Servlet Example

  public class EmployeeListServlet extends HttpServlet {
    @PersistenceContext
    private EntityManager entityManager;

    @Resource
    UserTransaction utx;

    protected void doGet(HttpServletRequest req,
                          HttpServletResponse res) {
      try {
        utx.begin();
        Query query = entityManager.createQuery( "select x from Employee x" );
        req.setAttribute( "employees", query.getResultList() );
        req.getRequestDispatcher("/employeeList.jsp" ).forward( req, res );
        utx.commit();
      } catch( Exception e ) {
        try {
          utx.rollback();
        } catch (Exception ee) {
          // Log exception!
        }
        throw new ServletException(e);
      }
    }
  }