A popular technique for organizing the execution of complex processing flows is the "Chain of Responsibility" pattern, as described (among many other places) in the classic "Gang of Four" design patterns book. Although the fundamental API contracts required to implement this design patten are extremely simple, it is useful to have a base API that facilitates using the pattern, and (more importantly) encouraging composition of command implementations from multiple diverse sources.
Towards that end, the Chain API models a computation as a
series of "commands" that can be combined into a "chain". The API
for a command consists of a single method
execute()), which is passed a "context" parameter
containing the dynamic state of the computation, and whose return
value is a boolean that determines whether or not processing for
the current chain has been completed (true), or whether
processing should be delegated to the next command in the chain
The "context" abstraction is designed to isolate command
implementations from the environment in which they are run (such
as a command that can be used in either a Servlet or Portlet,
without being tied directly to the API contracts of either of
these environments). For commands that need to allocate resources
prior to delegation, and then release them upon return (even if a
delegated-to command throws an exception), the "filter" extension
to "command" provides a
postprocess() method for
this cleanup. Finally, commands can be stored and looked up in a
"catalog" to allow deferral of the decision on which command (or
chain) is actually executed.
To maximize the usefulness of the Chain of Responsibility pattern APIs, the fundamental interface contracts are defined in a manner with zero dependencies other than an appropriate JDK. Convenience base class implementations of these APIs are provided, as well as more specialized (but optional) implementations for the web environment (i.e. servlets and portlets).
Given that command implementations are designed to conform with these recommendations, it should be feasible to utilize the Chain of Responsibility APIs in the "front controller" of a web application framework (such as Struts), but also be able to use it in the business logic and persistence tiers to model complex computational requirements via composition. In addition, separation of a computation into discrete commands that operate on a general purpose context allows easier creation of commands that are unit testable, because the impact of executing a command can be directly measured by observing the corresponding state changes in the context that is supplied.
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