J2EE Design Patterns

Finding Resources

A presentation tier is only as good as the data it has to present. In some applications, finding data is as simple as instantiating the appropriate business delegate and calling a method. But life gets complicated when an application grows to encompass a range of resources that are spread across a number of services. To help manage distributed components, J2EE specifies that they should be connected via the Java’s directory API, JNDI. JNDI is a simple, standard API that allows communication with a variety of directories such as LDAP and NDS. Using a directory server, clients are somewhat shielded from the complexities of distributed environments. The canonical J2EE picture is shown in Figure 9-10.

Figure 9-10. A distributed J2EE environment

The nice thing about a directory is that a client can get a reference to a resource anywhere on the network, remote or local, with a single lookup. This decouples the client from a particular server implementation. Of course, there is no such thing as a free lunch, and anyone who has actually built a distributed J2EE environment knows that all we have done is shift a development headache to a deployment headache.

Finding resources on the network has performance implications. Connecting to a remote EJB might first require querying the directory server for the EJB’s location. While the cost may not seem substantial, we effectively ...