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JPD builds on Java Language Metadata technology (JSR-175) to provide an easy-to-use syntax for describing a business process at the source code level for the J2EE platform. Metadata will bind data and tasks within the process definition to variables, classes, and tasks in the source code. The metadata will be amenable to manipulation by tools.
Java Process Definitions behave as regular java files but are available only to Oracle WebLogic Server. Not only do they behave as regular java but they can access regular java classes as well and all the same object-oriented programming principles and practices apply to these jpd files. Since they are not reusable only process specific code should be placed in there and the rest in specific java classes.
Process definitions should only be used for a specific process and their use for writing business logic is a grave violation of the separation of concerns principle. Process definitions by no means are a substitute of business logic. Process definitions are designed to serve as an integration point for the communication between various business logic components.