Defining Enterprise JavaBeans

Enterprise JavaBeans is an architecture for transactional, component-based distributed computing. The specification for EJBs lays out not just the format of a bean itself, but also a set of services that must be provided by the container in which the bean runs. This makes EJBs a powerful development methodology for distributed application development. Neither the bean developer nor the client application programmer needs to be concerned with service details such as transaction support, security, remote object access, and many other complicated and error-prone issues. These are provided transparently for the developers by the EJB server and container.

The effect of the EJB architecture is to make server-side development much easier for the Java application programmer. Since the implementation details are hidden from the developer, and since services such as transaction support and security are provided in an easy-to-use manner, EJBs can be developed relatively quickly. Furthermore, EJBs offer portability. A bean that is developed on one EJB server should run on other EJB servers that meet the EJB specification. Portability has not been tested yet for most servers, but it is a bright promise for the future.

EJB Development Roles

The EJB specification describes enterprise bean development in terms of five roles:

• The EJB developer writes the code that implements individual EJBs. This code is the business logic of the application, usually involving database access.

  The EJB developer is a Java applications programmer, and is familiar with both SQL and with database access using SQLJ or JDBC.

• The EJB deployer installs and publishes the EJBs. This involves interaction with the EJB developer, so that the transactional nature of the EJBs are understood. The EJB deployer writes the deployment descriptor files that specify the properties of each bean to be deployed. See "Deploying an EJB" for specific information about this phase of development.

  The EJB deployer must be familiar with the runtime environment of the EJBs, including database-specific matters such as network ports, database roles required, and other schema-specific requirements. For the Oracle8i server, the EJB deployer is responsible for publishing the EJB home interfaces in a database, and communicating this information to the client-side application developer.

• The EJB server vendor implements the framework in which the EJB containers run. For Oracle, the Oracle8i data server is the framework that supports the EJB containers.

• The EJB container vendor supplies the services that support the EJB at runtime. For example, when a client expects the bean to handle transaction support automatically, the container framework together with the data resource supports this.

• The application developer writes the client-side code that calls methods on server EJBs.

The roles of the EJB server and EJB container developers are not clearly distinguished. There is, for example, no standardized API between the container and the server. For this reason, initial implementations of EJB servers and containers are likely to be done by the same vendor. This is the case for Oracle8i.

EJBs as Distributed Components

While the EJB specification is based on concepts developed for the Remote Method Invocation interface (RMI), EJB server vendors are not required to use the RMI transport. Oracle8i uses the Internet Inter-ORB Protocol (IIOP). Using IIOP means that a server can support EJBs whose methods can be invoked by other IIOP clients.

Enterprise beans can also call out to CORBA objects. See Figure .

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