In the Chain of Responsibility Pattern, objects using and implementing an interface (one or more methods) are intentionally loosely coupled. A set of objects that implement the interface are organised in a list (or in rare cases a tree). Objects using the interface make requests from the first implementor 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.

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')