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 proposed 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). However, conditional compilation in the build script allows graceful creation of the underlying API JAR file even in the absence of the optional dependencies.
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.
The fundamental API contracts of the Chain of Responsibility
pattern are encapsulated in the following interfaces in package
org.apache.commons.chain:
execute() method is called to perform that
work.Chain is itself a
Command, so arbitrarily complex hierarchies of
execution may be composed.Command that requires any Chain that
executes it to promise a later call to the
postprocess() method if its execute()
method was ever called, even in the face of exceptions being
thrown by subsequently called commands.Command or
Chain. A Context instance is passed
to the execute() method of each
Command, which allows Command
instances to be easily shared in a multithread
environment.Commands (or Chains) that can be used
to symbolically identify a computation to be performed.In addition to the fundamental API contracts described above, additional packages are provided (some of them optional based on the availability of the corresponding APIs at compile time):
Command that are completely
generic across any execution environment.Catalog instance can be
populated with Command and Chain
instances configured from an XML document. Optional, compiled
only if commons-digester.jar is available.Context that represents the
fundamental characteristics of request, session, and
application scope objects in a web application environment,
without being tied specificaly to the Servlet or Portlet APIs.
These characteristics are exposed under property names that are
identical to the "implicit variables" of the expression
language that is defined by JSTL 1.0 and JSP 2.0.WebContext for the JavaServer
Faces API. Optional, compiled only if the JavaServer Faces API
classes are available.WebContext for the Portlet API.
Optional, compiled only if the Portlet API classes are
available.WebContext for the Servlet API.
Optional, compiled only if the Servlet API classes are
available.Over time, it is expected that additional generic commands and
specialized contexts will be developed for specific requirements.
However, conditional compilation capabilities in the build script
should be maintained so that a user of commons-chain need only
provide the APIs that he or she cares about. Likewise, for
maximum reuse, command implementations should be based on the
org.apache.commons.chain.Context API, rather than a
more specialized implementation class, if at all possible.
Chain relies on:
org.apache.commons.chain.config package.org.apache.commons.chain.config package.org.apache.commons.chain.config
package.org.apache.commons.chain.config package and
to build and use the
org.apache.commons.chain.web.servlet.config
package.org.apache.commons.web.faces package.org.apache.commons.web.portlet
package.org.apache.commons.web.servlet
package.This package represents a new approach to the Chain of Responsibility pattern, and the initial source is provided by Craig R. McClanahan. It was inspired by ideas from many sources -- in particular, the notion of a Chain being a Command was copied from the way that handlers are described in Axis.
chain in the
jakarta-commons CVS repository.The initial committers on the Chain component shall be: