Deploying EJB Technology

Introduction

As mentioned previously, the container handles persistence, transactions, concurrency, and access control automatically for the enterprise beans. The EJB specification describes a declarative mechanism for how these things will be handled, through the use of an XML deployment descriptor. When a bean is deployed into a container, the container reads the deployment descriptor to find out how transaction, persistence (entity beans), and access control should be handled. The person deploying the bean will use this information and specify additional information to hook the bean up to these facilities at run time.

A deployment descriptor has a predefined format that all EJB-compliant beans must use and all EJB-compliant servers must know how to read. This format is specified in an XML Document Type Definition, or DTD. The deployment descriptor describes the type of bean (session or entity) and the classes used for the remote, home, and bean class. It also specifies the transactional attributes of every method in the bean, which security roles can access each method (access control), and whether persistence in the entity beans is handled automatically or is performed by the bean. Below is an example of a XML deployment descriptor used to describe the Customer bean:


EJB-capable application servers usually provide tools that can be used to build the deployment descriptors; this greatly simplifies the process.

When a bean is to be deployed, its remote, home, and bean class files and the XML deployment descriptor must be packaged into a jar file. The deployment descriptor must be stored in the jar under the special name META-INF/ejb-jar.xml. This jar file, called an ejb-jar, is vendor neutral; it can be deployed in any EJB container that supports the complete EJB specification. When a bean is deployed in an EJB container, its XML deployment descriptor is read from the jar to determine how to manage the bean at run time. The person deploying the bean will map attributes of the deployment descriptor to the container's environment. This will include mapping access security to the environment's security system, adding the bean to the EJB container's naming system, and so on. Once the bean developer has finished deploying the bean, it will become available for client applications and other beans to use.

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