Is it possible to replace groovy method for existing object?

Question

The following code tried to replace an existing method in a Groovy class:

class A {
  void abc()  {
     println \"original\"
  }
} 

x= new A()
x.abc()
A.me         


        

Answer 1

You can use the per-instance metaClass to change the value in the existing object like so:

x= new A()
x.abc()
x.metaClass.abc={-> println "new" }
x.abc()
x.metaClass.methods.findAll{it.name=="abc"}.each { println "Method $it"}

But as you have seen, x will have two methods associated with it (well, in actual fact, a method and the closure you added

If you change the definition of A so that the method becomes a closure definition like so:

class A {
  def abc = { ->
     println "original"  
  }
} 

Then you will only get a single closure in the metaClass and no method after the alteration

Answer 2

I fully agree with @tim_yates on this. But, there is a way around, if you want to avoid modifying the original class use MethodClosure instead, as shown below:

class A {
  void abc()  {
     println "original"
  }
} 

x = new A()

//Create a Method Closure or Method pointer
pointer = x.&abc

//Replace Original call with Method pointer
//x.abc()
pointer()

//Meta classed
A.metaClass.abc={-> println "new" }

//Call method pointer instead of original again
//x.abc()
pointer()

A.metaClass.methods.findAll{it.name=="abc"}.each { println "Method $it"}

new A().abc()

You should get what is expected:

original
new
Method org.codehaus.groovy.runtime.metaclass.ClosureMetaMethod@43094309
  [name: abc params: [] returns: class java.lang.Object owner: class A]
Method public void A.abc()
new

Based on Groovy 2.2.1. The question is too old though.

Answer 3

I'm sure your still having this issue ;) but... I'm running Groovy version 1.8.7 on Fedora 17. I've found that have you have to do this combination:

A.metaClass.abc = {-> println it}
A obj = new A()
obj.metaClass.abc = {-> println it}
obj.abc

From there on out it will act as you want. When you look for the methods, you'll still get two:

Method org.codehaus.groovy.runtime.metaclass.ClosureMetaMethod@103074e[name: abc params: [] returns: class java.lang.Object owner: class A]
Method public void A.abc()

But at least you don't have to change your public void declaration.
Not sure if this is a bug or what.