(0) Rationale

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.

(1) Scope of the Package

The fundamental API contracts of the Chain of Responsibility pattern are encapsulated in the following interfaces in package org.apache.commons.chain:

• Command - An individual unit of work whose execute() method is called to perform that work.
• Chain - A set of commands to which work will be delegated, in a well-defined order, until one of the commands indicates that work on a request has been completed. Note that a Chain is itself a Command, so arbitrarily complex hierarchies of execution may be composed.
• Filter - A specialized 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.
• Context - A container for attributes and properties that represent the dynamic state of the computation being performed by a 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.
• Catalog - A collection of named 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):

• org.apache.commons.chain.impl - Convenience base class implementations of the fundamental API contracts.
• org.apache.commons.chain.generic - Implementations of Command that are completely generic across any execution environment.
• org.apache.commons.chain.config - Implementation of XML parsing rules (via the use of Commons Digester) so that a Catalog instance can be populated with Command and Chain instances configured from an XML document. Optional, compiled only if commons-digester.jar is available.
• org.apache.commons.chain.web - Abstract implementation of 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.
• org.apache.commons.chain.web.faces - Concrete implementation of WebContext for the JavaServer Faces API. Optional, compiled only if the JavaServer Faces API classes are available.
• org.apache.commons.chain.web.portlet - Concrete implementation of WebContext for the Portlet API. Optional, compiled only if the Portlet API classes are available.
• org.apache.commons.chain.web.servlet - Concrete implementation of 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.

(2) Initial Source of the 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.

(3) Required Jakarta-Commons Resources

• CVS Repository - New directory chain in the jakarta-commons CVS repository.
• Mailing List - Discussions will take place on the general dev@commons.apache.org mailing list. To help list subscribers identify messages of interest, it is suggested that the message subject of messages about this component be prefixed with [Chain].
• Bugzilla - New component “Chain” under the “Commons” product category, with appropriate version identifiers as needed.

(4) Initial Committers

The initial committers on the Chain component shall be:

• Craig R. McClanahan
• Ted Husted
• TBD