Designing applications with EJB 3.0 in Java

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Designing applications with EJB 3.0
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In practice, an architecture that relies on the Command pattern works nicely. In the next section, we discuss how EJB 3.0 components can further simplify a layered application architecture.
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16.4 Designing applications with EJB 3.0
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We ve focused on the Java Persistence standard in this book and discussed only a few examples of other EJB 3.0 programming constructs. We wrote some EJB session beans, enabled container-managed transactions, and used container injection to get an EntityManager. There is much more to be discovered in the EJB 3.0 programming model. In the following sections, we show you how to simplify some of the previous patterns with EJB 3.0 components. However, we again only look at features that are relevant for a database application, so you need to refer to other documentation if you want to know more about timers, EJB interceptors, or message-driven EJBs. First you ll implement an action in a web application with a stateful session bean, a conversational controller. Then you ll simplify data-access objects by turning them into EJBs to get container-managed transactions and injection of dependencies. You ll also switch from any Hibernate interfaces to Java Persistence, to stay fully compatible with EJB 3.0. You start by implementing a conversation with EJB 3.0 components in a web application.
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16.4.1 Implementing a conversation with stateful beans
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A stateful session bean (SFSB) is the perfect controller for a potentially long-running conversation between the application and the user. You can write an SFSB that implements all the steps in a conversation for example, a PlaceItem conversation:
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User enters item information User can add images for an item User submits the completed form
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Step 2 of this conversation can be executed repeatedly, if more than one image must be added. Let s implement this with an SFSB that uses Java Persistence and the EntityManager directly. A single SFSB instance is responsible for the whole conversation. First, here s the business interface:
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Creating and testing layered applications
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public interface PlaceItem { public Item createItem(Long userId, Map itemData); public void addImage(String filename); public void submit(); }
In the first step of the conversation, the user enters the basic item details and supplies a user identifier. From this, an Item instance is created and stored in the conversation. The user can then execute addImage() events several times. Finally, the user completes the form, and the submit() method is called to end the conversation. Note how you can read the interface like a story of your conversation. This is a possible implementation:
@Stateful @TransactionAttribute(TransactionAttributeType.NEVER) public class PlaceItemBean implements PlaceItem { @PersistenceContext(type = PersistenceContextType.EXTENDED) private EntityManager em; private Item item; private User seller; public Item createItem(Long userId, Map itemData) { // Load seller into conversation seller = em.find(User.class, userId); // Create item for conversation item = new Item(itemData, seller); user.addItem(item); return item; } public void addImage(String filename) { item.getImages().add(filename); } @Remove @TransactionAttribute(TransactionAttributeType.REQUIRED) public void submit() { em.persist(item); } }
An instance of this stateful session bean is bound to a particular EJB client, so it also acts as a cache during the conversation. You use an extended persistence context that is flushed only when submit() returns, because this is the only method
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that executes inside a transaction. All data access in other methods runs in autocommit mode. So em.find(User.class, userId) executes nontransactional, whereas em.persist(item) is transactional. Because the submit() method is also marked with @Remove, the persistence context is closed automatically when this method returns, and the stateful session bean is destroyed. A variation of this implementation doesn t call the EntityManager directly, but data-access objects.
16.4.2 Writing DAOs with EJBs
A data-access object is the perfect stateless session bean. Each data-access method doesn t require any state; it only needs an EntityManager. So, when you implement a GenericDAO with Java Persistence, you require an EntityManager to be set:
public abstract class GenericEJB3DAO<T,ID extends Serializable> implements GenericDAO<T, ID> { private Class<T> entityBeanType; private EntityManager em; public GenericEJB3DAO() { this.entityBeanType = (Class<T>) ( (ParameterizedType) getClass().getGenericSuperclass() ) .getActualTypeArguments()[0]; } @PersistenceContext public void setEntityManager(EntityManager em) { this.em = em; } protected EntityManager getEntityManager() { return em; } public Class<T> getEntityBeanType() { return entityBeanType; } ... }
This is really the same implementation you created earlier for Hibernate in section 16.2.2, Implementing the generic CRUD interface. However, you mark the setEntityManager() method with @PersistenceContext, so you get automatic injection of the right EntityManager when this bean executes inside a container. If it s executed outside of an EJB 3.0 runtime container, you can set the EntityManager manually.
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