What does .delegate mean in groovy?

Question

I found this code snippet:

def say = {println m} say.delegate = [m:2] say() 

That apperantly prints 2. How does it work? Where can find documentation about .delegate? Google led me to the Delegate Transformation page that doesn't mention .delegate at all.

Answer 1

The delegate of a closure is an object that is used to resolve references that cannot be resolved within the body of the closure itself. If your example was written like this instead:

def say = {   def m = 'hello'   println m } say.delegate = [m:2] say() 

It prints 'hello', because m can be resolved within the closure. However, when m is not defined within the closure,

def say = {   println m } say.delegate = [m:2] say() 

the delegate is used to resolve the reference, and in this case the delegate is a Map that maps m to 2.

Answer 2

Three property of closures, are this, owner, and delegate, In general delegate is set to owner

def testClosure(closure) {   closure() } testClosure() {   println "this is " + this + ", super:" + this.getClass().superclass.name   println "owner is " + owner + ", super:" + owner.getClass().superclass.name   println "delegate is " + delegate + ", super:" + delegate.getClass().superclass.name    testClosure() {     println "this is " + this + ", super:" + this.getClass().superclass.name     println "owner is " + owner + ", super:" + owner.getClass().superclass.name     println "delegate is " + delegate + ", super:" + delegate.getClass().superclass.name   } } 

prints

this is ConsoleScript0@11d20d3, super:groovy.lang.Script owner is ConsoleScript0@11d20d3, super:groovy.lang.Script delegate is ConsoleScript0@11d20d3, super:groovy.lang.Script this is ConsoleScript0@11d20d3, super:groovy.lang.Script owner is ConsoleScript0$_run_closure1@caea19, super:groovy.lang.Closure delegate is ConsoleScript0$_run_closure1@caea19, super:groovy.lang.Closure