Groovy runtime method interception

Question

I'm playing with Groovy and I wonder, why doesn't this piece of code works?

package test

interface A {
    void myMethod()
}

class B implements A {
    void myMethod() {
        println "No catch"
    }
}

B.metaClass.myMethod = {
    println "Catch!"
}

(new B()).myMethod()

It prints out No catch, while I expect it to print Catch! instead.

Related [Groovy Meta Programming](http://stackoverflow.com/q/11892620/462015) — Arturo Herrero, Sep 05 '12 at 13:36

score 9 · Accepted Answer · edited Jun 30 '15 at 16:24

9

It's a bug in Groovy, there is an open issue in JIRA: Cannot override methods via metaclass that are part of an interface implementation, GROOVY-3493.

edited Jun 30 '15 at 16:24

brunobowden

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answered Sep 05 '12 at 13:35

Arturo Herrero

12,772
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73

score 1 · Answer 2 · answered Sep 05 '12 at 13:36

1

Instead of rewriting B.metaClass.myMethod, try following:

 B.metaClass.invokeMethod = {String methodName, args ->
    println "Catch!"
 }

This blog post describes it quite well.

answered Sep 05 '12 at 13:36

Anton Arhipov

6,479
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score 0 · Answer 3 · answered Jun 30 '15 at 19:49

There is a workaround but it only applies to all classes and not specific instances.

metaclass modification BEFORE construction:

interface I {
    def doIt()
}

class T implements I {
    def doIt() { true }
}

I.metaClass.doIt = { -> false }
T t = new T()
assert !t.doIt()

metaclass modification AFTER construction:

interface I {
    def doIt()
}

class T implements I {
    def doIt() { true }
}

T t = new T()
// Removing either of the following two lines breaks this
I.metaClass.doIt = { -> false }
t.metaClass.doIt = { -> false }
assert !t.doIt()

Groovy runtime method interception

3 Answers3