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1.4. 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 packages, developers can create, assemble, and deploy components that conform to the EJB specification.

The original JavaBeans™, the java.beans package in the core Java API, 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. A lot of the literature has referred to EJB as an extension of the original JavaBeans, but this is a misrepresentation. Other than the shared name, and the fact that they are both Java component models, the two APIs serve very different purposes. EJB does not extend or use the original JavaBeans component model.

The original JavaBeans (the java.beans package) is intended to be used for intraprocess purposes, while EJB (the javax.ejb package) is designed to be used for interprocess components. In other words, the original JavaBeans was not intended for distributed components. JavaBeans can be used to solve a variety of problems, but is primarily used to build clients by assembling visual (GUI) and nonvisual widgets. It's an excellent component model, possibly the best component model for intraprocess development ever devised, but it's not a server-side component model. EJB is 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 interfaces and classes in the form of Java packages that must be used in a particular way to isolate and encapsulate a set of functionality. 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 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.

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