Beginning Java EE 6 with GlassFish 3, Second Edition

CHAPTER 6 ■ ENTERPRISE JAVA BEANS}}book.setIsbn("1-84023-742-2");book.setNbOfPage(354);bookEJB.createBook(book);One of the differences between a pure Java class and a session bean is shown in Listing 6-2; even ifthe code is similar, the Main class doesn’t create an instance of the BookEJB using the new keyword;instead, the class needs to first obtain a reference to the EJB, either by injection or by JNDI lookup,before calling a method. This mechanism is used because the EJB runs in a managed environment. Itneeds to be deployed in a container (embedded or not).Like most Java EE 6 components, EJBs need metadata to inform the container of required actions(transaction demarcation) or services to inject. Metadata can take the form of either annotations orXML. EJBs can be deployed with the optional ejb-jar.xml deployment descriptor, which will overrideannotations. With configuration by exception, a single annotation (@Stateless in this case) is enough toturn a POJO into an EJB because the container applies all the default behavior.EJB ContainerAs mentioned, an EJB is a server-side component and needs to be executed in a container. This runtimeenvironment provides core features common to many enterprise applications such as the following:• Remote client communication: Without writing any complex code, an EJB client(another EJB, a user interface, a batch process, etc.) can invoke methods remotelyvia standard protocols.• Dependency injection: The container can inject several resources into an EJB (JMSdestinations and factories, datasources, other EJBs, environment variables, and soon).• State management: For stateful session beans, the container manages their statetransparently. You can maintain state for a particular client, as if you weredeveloping a desktop application.• Pooling: For stateless beans and MDBs, the container creates a pool of instancesthat can be shared by multiple clients. Once invoked, an EJB returns to the pool tobe reused instead of being destroyed.• Component life cycle: The container is responsible for managing the life cycle ofeach component.• Messaging: The container allows MDBs to listen to destinations and consumemessages without too much JMS plumbing.183