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Almost every application requires some means of keeping data across program runs. Most applications use a file or database for the storage or persistence of data. However, databases are not typically used to store objects, particularly Java objects. On the other hand, flat files alone do not cope well with object structure. What is required is some means to preserve the state of a Java object so that it may be easily stored and subsequently restored to its original state.

In the last chapter we looked at how an applet could communicate with a server application using sockets. This approach is relatively straight forward to use and exploits the widely adopted Sockets communication model. However, Java offers another way of enabling Java programs (both applets and applications) to communicate — Remote Method Invocation (or RMI). Indeed, RMI is surprisingly simple to use and may well be preferable to sockets for Java-to-Java communication. This is because the resulting software is simpler and easier to maintain than using sockets. For example, a distributed software system resembles a software system executing within a single virtual machine except for the addition of one line to a client and two lines to a server! Note that all RMI classes are in the java.rmi package or one of its subpackages, such as server.

A DB2 program can invoked in one of two ways: within the DB2 environment; independent of DB2, but the program must connect to a DB2 subsystem.