EJB.16.1 Overview

The deployment descriptor is part of the contract between the EJB JAR file producer and consumer. This contract covers the passing of enterprise beans from the bean provider to the application assembler and from the application assembler to the deployer.

An EJB JAR file produced by the bean provider contains one or more enterprise beans and typically does not contain application assembly instructions. An EJB JAR file produced by an application assembler contains one or more enterprise beans, plus application assembly information describing how the enterprise beans are combined into a single application deployment unit.

The J2EE specification defines how enterprise beans and other application components contained in multiple EJB JAR files can be assembled into an application.

The role of the deployment descriptor is to capture the declarative information (i.e., information that is not included directly in the enterprise beans' code) that is intended for the consumer of the EJB JAR file.

There are two basic kinds of information in the deployment descriptor:

• Enterprise beans' structural information . Structural information describes the structure of an enterprise bean and declares an enterprise bean's external dependencies. Providing structural information in the deployment descriptor is mandatory for the EJB JAR file producer. The structural information cannot, in general, be changed because doing so could break the enterprise bean's function.

• Application assembly information . Application assembly information describes how the enterprise bean (or beans) in the EJB JAR file is composed into a larger application deployment unit. Providing assembly information in the deployment descriptor is optional for the EJB JAR file producer. Assembly level information can be changed without breaking the enterprise bean's function, although doing so may alter the behavior of an assembled application.