Commons Chain - Overview
Last Published: 12 April 2010
| Version: 1.3-SNAPSHOT
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Commons Chain
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
(false).
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.
Documentation
The
Javadoc
of the latest Release.
The
Cookbook
containing "recipes" for using the chain.
The
SVN repository
can be browsed.
Downloading Chain
See the
Downloads
page for current/previous
releases.
Support
The
commons mailing lists
act as the main support forum.
The user list is suitable for most library usage queries.
The dev list is intended for the development discussion.
Please remember that the lists are shared between all commons components,
so prefix your email by [chain].
Issues may be reported via
ASF JIRA
2003-2010
The Apache Software Foundation