Enterprise JavaBeans, Third Edition

Component Models

The term "component model” has many different interpretations. Enterprise JavaBeans specifies a server-side component model. Using a set of classes and interfaces from the javax.ejb package, developers can create, assemble, and deploy components that conform to the EJB specification.

The original JavaBeans™ is also a component model, but it’s not a server-side component model like EJB. In fact, other than sharing the name “JavaBeans,” these two component models are completely unrelated. In the past, a lot of the literature referred to EJB as an extension of the original JavaBeans, but this is a misrepresentation. The two APIs serve very different purposes, and EJB does not extend or use the original JavaBeans component model.

JavaBeans is intended to be used for intraprocess purposes, while EJB is designed for inter process components. In other words, the original JavaBeans was not intended for distributed components. JavaBeans can be used to solve a variety of problems, but it is primarily used to build clients by assembling visual (GUI) and nonvisual widgets. It’s an excellent component model, possibly the best one ever devised for intraprocess development, but it’s not a server-side component model. EJB, on the other hand, is explicitly designed to address issues involved with managing distributed business objects in a three-tier architecture.

Given that JavaBeans and Enterprise JavaBeans are completely different, why are they both called component models? In this context, a component model defines a set of contracts between the component developer and the system that hosts the component. The contracts express how a component should be developed and packaged. Once a component is defined, it becomes an independent piece of software that can be distributed and used in other applications. A component is developed for a specific purpose but not a specific application. In the original JavaBeans, a component might be a push button or a spreadsheet that can be used in any GUI application according to the rules specified in the original JavaBeans component model. In EJB, a component might be a customer business object that can be deployed in any EJB server and used to develop any business application that needs a customer business object. Other types of Java component models include servlets, JSP pages ( JSPs), and applets.