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Design patterns and web technologies
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Figure 12.1 BidEAO is the interface for the EAO object. BidEAOImpl is the EAO class that implements the logic for CRUD operations for the Bid entity.
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Implementing the entity access object The following code for the BidEAO interface exposes the operations that can be performed on a Bid:
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public interface BidEAO { public Bid addBid(Item item, String bidderId, double bidPrice); public Bid cancelBid(Long bidId); ... }
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Some developers believe that using an interface for the entity access object is too much trouble. However, you ll learn just how advantageous it is do so when we discuss using the EJB 3 JPA with Spring in chapter 16. An interface allows you to change the underlying persistence provider with no effect on the classes that use the EAO interface. Listing 12.1 shows the implementation class for the EAO that performs the CRUD operations.
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Listing 12.1 JPA-specific implementation class for BidEAO
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public class BidEAOImpl implements BidEAO { private static String EM_NAME = "java:comp/env/actionBazaar"; public BidEAOImpl() { } private EntityManager getEntityManager() { try {
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Looks up container-managed EntityManager
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Effectively integrating EJB 3 across your application tiers
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Context ctx = new InitialContext(); return (EntityManager)ctx.lookup(EM_NAME); } catch (Exception e) { e.printStackTrace(); return null; } } public Bid addBid(Item item, String bidderId, double bidPrice) throws BidException { EntityManager em = getEntityManager(); if (em != null) { Gets EntityManager Bid bid = new Bid(); instance ... em.persist(bid); Persists Entity return bid; } ... }
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public Bid cancelBid(Long bidId) { ... } }
The code in the implementation class is straightforward. The getEntityManager method b uses JNDI lookup to grab an instance of a container-managed EntityManager. Note that because the EAO is a regular POJO, you cannot use DI, so we have used JNDI to look up and grab an instance of an EntityManager. You ll need to make appropriate changes if you want to use an application-managed entity manager. The rest of the code is nothing but moving the entity access code into the EAO instead of embedding it in your business logic. The addBid method uses the getEntityManager to get an instance of EntityManager c and then persist an instance of Bid entity d. Let s assume that the client for your EAO code is the PlaceBid EJB. Listing 12.2 shows how the code will look.
Listing 12.2 Using BidEAO from the PlaceBid EJB
@PersistenceContext(unitName = "actionBazaar", Declares dependency on name = "actionBazaar") persistence unit @Stateless public class PlaceBidBean implements PlaceBid { public Long addBid(String userId, Long itemId, double bidPrice) throws BidException { ...
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BidEAO bidEAO = EAOFactory.jpa.getBidEAO(); Bid bid = bidEAO.addBid(item, userId, bidPrice); return bid.getBidId(); } }
Creates an instance of EAO
Uses EAO to add a new Bid
Listing 12.2 references the persistence unit using the @PersistenceContext annotation b because we used JNDI lookup in the EAO to grab an instance of EntityManager. We then used the EAOFactory to create an instance of an EAO c. After creating an instance of the EAO, we can use it to perform entity operations d. In the code c we used an EAOFactory to create EAO instances. Here is a simple EAO factory that you can use to create instances of EAOs via JPA:
public abstract class EAOFactory { public static final EAOFactory jpa = new JPAEAOFactory(); public abstract ItemEAO getItemEAO(); public abstract BidEAO getBidEAO(); ... } public class JPAEAOFactory extends EAOFactory { public JPAEAOFactory(){} public BidEAO getBidEAO() { return (new BidEAOImpl()); } }
The advantage of having loose coupling between your business logic and your persistence code should be obvious. If you want to change to a different persistence tier, then you only have to modify your EAO implementation classes. Many tools and utilities such as FireStorm/DAO and Hibernate s hbm2java are available that will help generate EAOs for your entities, so adopting EAOs should be relatively painless. This is a good practice to follow, although it may require some additional coding to manage a few extra classes. Using session beans as EAOs Since EJB 3 session beans are POJOs, they are clear candidates for EAOs if you are deploying your enterprise applications to a Java EE container. EAOs can simplify things via injection and do not require using an EAOFactory. If you decide to implement your EAOs using session beans, then the code will look like this:
@Stateless public class BidEAOImpl implements BidEAO { @PersistenceContext(unitName = "actionBazaar") private EntityManager em;
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