barcode font vb.net Getting inside EJB in Java

Encoder Data Matrix in Java Getting inside EJB

The Java Persistence Query Language JPA provides a specialized SQL -like query language called the Java Persistence Query Language (JPQL) to search for entities saved into the database. With a robust and flexible API such as JPQL, you won t lose anything by choosing automated persistence instead of handwritten JDBC. In addition, JPA supports native, database-specific SQL, in the rare cases where it is worth using. At this point, you should have a decent high-level view of the various parts of EJB. You also know that you need an EJB container to execute session beans and MDBs as well as a persistence provider to run your entities, so that these components can access the services EJB 3 provides. The container, the persistence provider, and the services are the central concepts in EJB 3, and we ll address them next.

When you build a simple Java class, you need a Java Virtual Machine (JVM) to execute it. In a similar way (as you learned in the previous section) to execute session beans and MDBs you need an EJB container, and to run your entities you need a persistence provider. In this section we give you a bird s-eye view of containers and persistence providers and explain how they are related. In the Java world, containers aren t just limited to the realm of EJB 3. You re probably familiar with a web container, which allows you to run web-based applications using Java technologies such as servlets, JSP, or JSF. A Java EE container is an application server solution that supports EJB 3, a web container, and other Java EE APIs and services. BEA WebLogic Server, Sun Microsystems s GlassFish, IBM WebSphere, JBoss Application Server, and Oracle Application Server 10g are examples of Java EE containers. The relationship between the Java EE container, web container, EJB container, and JPA persistence provider is shown in figure 1.7. If you install a Java EE compliant application server such as GlassFish, it will contain a preconfigured web container, EJB container, and a JPA provider. However, some vendors and open source projects may provide only a web container such as Tomcat or an EJB 3 compliant persistence provider such as Hibernate. These containers provide limited functionality compared to what you get with a complete Java EE 5 container. In this section, we ll focus on how the EJB container and the persistence provider work, and we ll finish with a more complete discussion of EJB services. First, let s tackle the EJB container.

Figure 1.7 Java EE container typically contains web and EJB containers and a persistence provider. The stateless session bean (Credit Check EJB) and stateful session bean (Cart EJB) are deployed and run in the EJB container. Entities (Customer and Catalog) are deployed and run within an EJB persistence provider and can be accessed by either web or EJB container components.

Think of the container as simply an extension of the basic idea of a JVM. Just as the JVM transparently manages memory on your behalf, the container transparently provides EJB component services such as transactions, security management, remoting, and web services support. As a matter of fact, you might even think of the container as a JVM on steroids, whose purpose is to execute EJBs. In EJB 3, the container provides services applicable to session beans and MDBs only. The task of putting an EJB 3 component inside a container is called deployment. Once an EJB is successfully deployed in a container, it can be used in your applications. The persistence provider is the container counterpart in JPA. We ll briefly talk about it next.