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A first taste of EJB
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Listing 2.1 PlaceBid stateless session bean code
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package ejb3inaction.example.buslogic; import javax.ejb.Stateless; import ejb3inaction.example.persistence.Bid; @Stateless public class PlaceBidBean implements PlaceBid { ... public PlaceBidBean() {}
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Marks POJO as stateless session bean
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public Bid addBid(Bid bid) { System.out.println("Adding bid, bidder ID=" + bid.getBidderID() + ", item ID=" + bid.getItemID() + ", bid amount=" + bid.getBidAmount() + "."); return save(bid); } ... } ... package ejb3inaction.example.buslogic; import javax.ejb.Local; import ejb3inaction.example.persistence.Bid; @Local public interface PlaceBid { Bid addBid(Bid bid); }
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The first thing that you have probably noticed is how plain this code looks. The PlaceBidBean class is just a plain old Java object (POJO) and the PlaceBid interface is a plain old Java interface (POJI). There is no cryptic EJB interface to implement, class to extend, or confusing naming convention to follow. In fact, the only notable features in listing 2.1 are the two EJB 3 annotations: @Stateless and @Local:
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@Stateless The @Stateless annotation tells the EJB container that PlaceBidBean is a stateless session bean. This means that the container automat-
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ically provides such services to the bean as automatic concurrency control, thread safety, pooling, and transaction management. In addition, you can add other services for which stateless beans are eligible, such as transparent security and interceptors.
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Building business logic with session beans
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@Local The @Local annotation on the PlaceBid interface tells the container that the PlaceBid EJB can be accessed locally through the interface. Since EJB and servlets are typically collocated in the same application, this
is probably perfect. Alternatively, we could have marked the interface with the @Remote annotation. Remote access through the @Remote annotation is provided under the hood by Java Remote Method Invocation (RMI), so this is the ideal means of remote access from Java clients. If the EJB needs to be accessed by non-Java clients like Microsoft .NET applications, web services based remote access can be enabled using the @WebService annotation applied either on the interface or the bean class. That s pretty much all we re going to say about stateless session beans for now. Let s now turn our attention to the client code for using the PlaceBid EJB.
2.3.2 The stateless bean client
Virtually any client can use the PlaceBid EJB in listing 2.1. However, the most likely scenario for EJB usage is from a Java-based web tier. In the ActionBazaar scenario, the PlaceBid EJB is probably called from a JavaServer Page (JSP) or servlet. For simplicity, let s assume that the PlaceBid EJB is used by a servlet named PlaceBidServlet. Listing 2.2 shows how the code might look. The servlet s service method is invoked when the user wants to place a bid. The bidder s ID, item ID, and the bid amount are passed in as HTTP request parameters. The servlet creates a new Bid object, sets it up, and passes it to the EJB addBid method.
Listing 2.2 A simple servlet client for the PlaceBid EJB
package ejb3inaction.example.buslogic; import import import import import javax.ejb.EJB; javax.servlet.*; javax.servlet.http.*; java.io.*; ejb3inaction.example.persistence.Bid;
public class PlaceBidServlet extends HttpServlet { @EJB Injects instance private PlaceBid placeBid; of PlaceBid EJB public void service(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { int bidderID = Integer.parseInt( request.getParameter("bidder_id"));
A first taste of EJB
int itemID = Integer.parseInt( request.getParameter("item_id")); double bidAmount = Double.parseDouble( request.getParameter("bid_amount")); Bid bid = new Bid(); bid.setBidderID(bidderID); bid.setItemID(itemID); bid.setBidAmount(bidAmount); placeBid.addBid(bid); ... } ... }
As you can see in listing 2.2, EJB from the client side looks even simpler than developing the component code. Other than the @EJB annotation on the placeBid private variable, the code is no different than using a local POJO.
NOTE
When the servlet container sees the @EJB annotation as the servlet is first loaded, it looks up the PlaceBid EJB behind the scenes and sets the placeBid variable to the retrieved EJB reference. If necessary, the container will look up the EJB remotely over RMI.
The @EJB annotation works in any component that is registered with the Java EE container, such as a servlet or JavaServer Faces (JSF) backing bean. As long as you are using the standard Java EE stack, this is probably more than sufficient. There are a couple other interesting items in this code that illustrate concepts we introduced earlier. Let s take a closer look. EJB 3 dependency injection Although we mentioned DI in the beginning of the chapter, if you are not familiar with it you may think that what the @EJB annotation is doing is a little unusual in a nifty, black-magic kind of way. In fact, if we didn t tell you anything about the code, you might have been wondering if the placeBid private variable is even usable in the servlet s service method since it is never set! If fact, if the container didn t intervene we d get the infamous java.lang.NullPointerException when we tried to call the addBid method in listing 2.2 since the placeBid variable would still be null. One interesting way to understand DI is to think of it as custom Java variable instantiation. The @EJB annotation in listing 2.2 makes the container
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