All Known Implementing Classes:
    CopyCommand, TestCommand, DelegatingCommand, DelegatingFilter, ExceptionFilter, CountCommand, ServletGetLocaleCommand, TestChain, TestAlternateContextCommand, RemoveCommand, TestCommand, ServletSetLocaleCommand, ChainBase, TestCommand, PathInfoMapper, NonDelegatingFilter, PortletGetLocaleCommand, RequestParameterMapper, FacesSetLocaleCommand, DispatchCommand, AddingCommand, AbstractSetLocaleCommand, ForwardCommand, Filter, Chain, FacesGetLocaleCommand, LookupCommand, AbstractGetLocaleCommand, NonDelegatingCommand, ExceptionCommand, PortletSetLocaleCommand, DispatchLookupCommand, ServletPathMapper

A Command encapsulates a unit of processing work to be performed, whose purpose is to examine and/or modify the state of a transaction that is represented by a Context . Individual Command s can be assembled into a Chain , which allows them to either complete the required processing or delegate further processing to the next Command in the Chain .

Command implementations should be designed in a thread-safe manner, suitable for inclusion in multiple Chain s that might be processed by different threads simultaneously. In general, this implies that Command classes should not maintain state information in instance variables. Instead, state information should be maintained via suitable modifications to the attributes of the Context that is passed to the execute() command.

Command implementations typically retrieve and store state information in the Context instance that is passed as a parameter to the execute() method, using particular keys into the Map that can be acquired via Context.getAttributes(). To improve interoperability of Command implementations, a useful design pattern is to expose the key values used as JavaBeans properties of the Command implementation class itself. For example, a Command that requires an input and an output key might implement the following properties:

private String inputKey = "input";
public String getInputKey() {
return (this.inputKey);
}
public void setInputKey(String inputKey) {
this.inputKey = inputKey;
}

private String outputKey = "output";
public String getOutputKey() {
return (this.outputKey);
}
public void setOutputKey(String outputKey) {
this.outputKey = outputKey;
}

And the operation of accessing the "input" information in the context would be executed by calling:

String input = (String) context.get(getInputKey());

instead of hard coding the attribute name. The use of the "Key" suffix on such property names is a useful convention to identify properties being used in this fashion, as opposed to JavaBeans properties that simply configure the internal operation of this Command .

author: Craig - R. McClanahan

version: $ - Revision: 480477 $ $Date: 2006-11-29 08:34:52 +0000 (Wed, 29 Nov 2006) $

 public boolean execute(Context context) throws Exception
Execute a unit of processing work to be performed. This
Command  may either complete the required processing
and return true, or delegate remaining processing
to the next Command  in a Chain  containing this
Command  by returning false