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<p>In the <a href="http://en.wikipedia.org/wiki/Chain_of_responsibility_pattern">Chain of Responsibility Pattern</a>, objects using and implementing an interface (one or more methods) are intentionally loosely coupled. A set of objects that <em>implement</em> the interface are organised in a list (or in rare cases a tree). Objects using the interface make requests from the first <em>implementor</em> object. It will decide whether to perform any action itself and whether to pass the request further down the line in the list (or tree). Sometimes a default implementation for some request is also coded into the pattern if none of the implementors respond to the request.</p>

<h3>Example</h3>

<p>In this example, the script sends requests to the <code>lister</code> object. The <code>lister</code> points to a <code>UnixLister</code> object. If it can't handle the request, it sends the request to the <code>WindowsLister</code>. If it can't handle the request, it sends the request to the <code>DefaultLister</code>.</p>

<table class="wysiwyg-macro" data-macro-name="code" style="background-image: url(/plugins/servlet/confluence/placeholder/macro-heading?definition=e2NvZGV9&amp;locale=en_GB&amp;version=2); background-repeat: no-repeat;" data-macro-body-type="PLAIN_TEXT"><tr><td class="wysiwyg-macro-body"><pre>
class UnixLister {
    private nextInLine
    UnixLister(next) { nextInLine = next }
    def listFiles(dir) {
        if (System.getProperty('os.name') == 'Linux')
            println "ls $dir".execute().text
        else
            nextInLine.listFiles(dir)        
    }
}

class WindowsLister {
    private nextInLine
    WindowsLister(next) { nextInLine = next }
    def listFiles(dir) {
        if (System.getProperty('os.name') == 'Windows XP')
            println "cmd.exe /c dir $dir".execute().text
        else
            nextInLine.listFiles(dir)        
    }
}

class DefaultLister {
    def listFiles(dir) {
        new File(dir).eachFile{ f -> println f }
    }
}

def lister = new UnixLister(new WindowsLister(new DefaultLister()))

lister.listFiles('Downloads')
</pre></td></tr></table>

<p>The output will be a list of files (with slightly different format depending on the operating system).</p>

<p>Here is a UML representation:</p>

<p><img class="confluence-embedded-image" src="/download/attachments/231080197/ChainOfResponsibilityClasses.gif?version=1&amp;modificationDate=1369483117935" data-image-src="/download/attachments/231080197/ChainOfResponsibilityClasses.gif?version=1&amp;modificationDate=1369483117935" data-linked-resource-id="231376101" data-linked-resource-type="attachment" data-linked-resource-default-alias="ChainOfResponsibilityClasses.gif" data-base-url="http://docs.codehaus.org" data-linked-resource-container-id="231080197" title="null > ChainOfResponsibilityClasses.gif"></p>

<p>Variations to this pattern:</p>
<ul>
	<li>we could have an explicit interface, e.g. <code>Lister</code>, to statically type the implementations but because of <em>duck-typing</em> this is optional</li>
	<li>we could use a chain tree instead of a list, e.g. <code>if (animal.hasBackbone())</code> delegate to <code>VertebrateHandler</code> else delegate to <code>InvertebrateHandler</code></li>
	<li>we could always pass down the chain even if we processed a request</li>
	<li>we could decide at some point to not respond and not pass down the chain</li>
	<li>we could use Groovy's meta-programming capabilities to pass unknown methods down the chain</li>
</ul>