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It is difficult to appreciate the amount of menial code the container takes care of until you see the alternative. Keep this in mind when looking at applicationmanaged EntityManagers in the coming section. Note that container-managed EntityManagers are available to all components in a Java EE container, including JSF-backing beans and servlets. However, be aware of the fact that EntityManagers are not thread-safe; thus, injecting them frivolously into presentationlayer components can readily land you in some trouble. We ll discuss this nuance in the sidebar EntityManagers and thread safety.

an EJB reference because of the nature of EJB deployment. EJB references point to stub objects that may be different stub instances yet point to the same EJB. An equals() invocation is not guaranteed to return the correct value.

It is very easy to forget the fact that web-tier components are meant to be used by multiple concurrent threads. Servlet-based components like JSPs are deliberately designed this way because they are intended to achieve high throughput through statelessness. However, this fact means that you cannot use resources that are not thread-safe as servlet instance variables. The EntityManager falls under this category, so injecting it into a web component is a big no-no. One way around this problem is to use the SingleThreadModel interface to make a servlet thread-safe. In practice, this technique severely degrades application performance and is almost always a bad idea. Some vendors might try to solve this problem. If you are extremely comfortable with your container vendor, you could count on this. Remember, though, one very important benefit of using EJB 3 is portability across container implementations. You shouldn t give up this advantage frivolously. If you must use container-managed EntityManagers from your servlet, the best option is to look up an instance of the EntityManager inside your method as follows:

@PersistenceContext(name="pu/actionBazaar", unitName="ActionBazaar") public class ItemServlet extends HttpServlet { @Resource private UserTransaction ut; public void service (HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {

The importance of OOP for developing complex programs is undeniable. However, we will show that writing clear and elegant programs using only OOP is impossible in at least two cases: when the application contains crosscutting functionalities, and when the application includes code scattering.

Use the isIdentical() method from the remote or local interface to compare EJB references. The client program in listing 1.15 creates and compares several EquityBean entity EJB instances. A stock symbol makes an EquityBean instance unique.

Context ctx = new InitialContext(); EntityManager em =(EntityManager) ctx.lookup( "java:comp/env/pu/actionBazaar"); ut.begin(); em.persist(item); ut.commit(); ... } }

The other alternative is to use an application-managed EntityManager with a JTA transaction. It is worth noting that EntityManagerFactory is thread-safe.

Now that you are familiar with working with EntityManager inside the container, let s move on to the application-managed EntityManager.

public void { Equity Equity String String try { equity1 = findEquity( symbol, description ); equity2 = findEquity( symbol, description ); runExample() equity1 = null; equity2 = null; symbol = "ACME"; description = "ACME Corp";

Previously, we mentioned that when you analyze how to organize an application into classes, the analysis must be driven by the need for separating and encapsulating the data and its associated processing into coherent entities Although the classes are programmed independently of one another, they are sometimes behaviorally interdependent Typically, this is the case when you implement rules of referential integrity For example, a customer object must not be deleted while an outstanding order remains unpaid; otherwise, the program risks losing the contact details for that customer To enforce this rule, you could modify the customer-deletion method so that it initially determines whether all the orders have been paid However, this solution is deficient for several reasons: Determining whether an order has been paid does not belong to customer management but to order management Therefore, the customer class should not have to manage this functionality.