| 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: 411876 $ $Date: 2006-06-05 19:02:19 +0100 (Mon, 05 Jun 2006) $ |