Aspects of Enterprise Java Beans

Enterprise Java Beans (EJB), a specification for a Java component framework recently released by Sun Microsystems, immediately attracted attention of several major software vendors, including IBM, Oracle and Sybase. Analysts agree that EJB has a potential to replace CORBA as a standard for enterprise level applications. Aspect Oriented Programming fits naturally into EJB paradigm, but requires a few enhancements to be useful in this environment. Enterprise Java Beans EJB ties together other specs from Sun, including Java Transaction Service (JTS), Java Naming and Directory Interface (JNDI), Java Database Connectivity (JDBC) and Java Messaging Service (JMS). It can be seen as a modern version of CORBA, made more accessible through the use of a single language and modern component methodologies and patterns. A mapping to CORBA was released along with the very first version of EJB. The main concepts are: • EJB server • EJB container • EJ Beans • EJB JAR files • Deployment descriptors Accordingly, the following roles are differentiated for developers: • server provider • container provider • bean provider • bean deployer and assembler EJB server is a Java component/application server that houses containers and provides services to them: connection and thread management, etc. Containers house EJ beans and are able to take over transactions, persistence and security that the beans do not wish to handle themselves. Containers can be specialized for beans with special needs. The spec differentiates EJ beans of two types: non-persistent session beans and persistent entity beans, corresponding to records in a database, CICS transactions or other remote sources. EJB JAR file is a zip Java archive that provides an export/import standard for the EJ beans. Besides classes themselves it contains various meta-information files, including deployment descriptors, transaction and security attributes in the form of serialized Java classes. The spec describes the contracts between EJ Beans and containers, and promises to add container-server contracts. A typical application bean would consist of: • implementation classes • a remote interface; required by the spec RMI-style interfaces, the only public “face” of the bean • a home interface; also required, factory and finder methods • a deployment descriptor(DD) The responsibilities of container include generation of classes for access to the bean (EJB Object), for lifecycle operations (EJB Home), and possibly remote stubs. The spec also mentions the possibility of modification of the application code by the container at deployment time.