org.apache.commons.chain
Interface Command

All Known Subinterfaces:
Chain, Filter
All Known Implementing Classes:
AbstractGetLocaleCommand, AbstractSetLocaleCommand, ChainBase, CopyCommand, DispatchCommand, DispatchLookupCommand, FacesGetLocaleCommand, FacesSetLocaleCommand, LookupCommand, PathInfoMapper, PortletGetLocaleCommand, PortletSetLocaleCommand, RemoveCommand, RequestParameterMapper, ServletGetLocaleCommand, ServletPathMapper, ServletSetLocaleCommand

public interface Command

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 Commands 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 Chains 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.

Version:
$Revision: 480477 $ $Date: 2006-11-29 08:34:52 +0000 (Wed, 29 Nov 2006) $
Author:
Craig R. McClanahan

Field Summary
static boolean CONTINUE_PROCESSING
          Commands should return CONTINUE_PROCESSING if the processing of the given Context should be delegated to a subsequent Command in an enclosing Chain.
static boolean PROCESSING_COMPLETE
          Commands should return PROCESSING_COMPLETE if the processing of the given Context has been completed.
 
Method Summary
 boolean execute(Context context)
          Execute a unit of processing work to be performed.
 

Field Detail

CONTINUE_PROCESSING

static final boolean CONTINUE_PROCESSING

Commands should return CONTINUE_PROCESSING if the processing of the given Context should be delegated to a subsequent Command in an enclosing Chain.

Since:
Chain 1.1
See Also:
Constant Field Values

PROCESSING_COMPLETE

static final boolean PROCESSING_COMPLETE

Commands should return PROCESSING_COMPLETE if the processing of the given Context has been completed.

Since:
Chain 1.1
See Also:
Constant Field Values
Method Detail

execute

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

Parameters:
context - The Context to be processed by this Command
Returns:
true if the processing of this Context has been completed, or false if the processing of this Context should be delegated to a subsequent Command in an enclosing Chain
Throws:
Exception - general purpose exception return to indicate abnormal termination
IllegalArgumentException - if context is null


Copyright 2003-2008 The Apache Software Foundation. All Rights Reserved.