Why does a method closure on a Class object fail?

closures, groovy

Solution

When you use a method pointer on instance of `Class` then it has to explicitly use the `doCall()` method provided by `MethodClosure` instead of using the default `call()` of `Closure`.

`doCall` method from `MethodClosure` overrides Closure's `doCall` and intercepts the method call by using `invokeMethod` instead of calling `call()` from `Closure`.

The `MethodClosure` will also work if you explicitly use the `InvokerHelper` which is synonymous to `doCall` in `MethodClosure` or simply `metaClass` List.

import org.codehaus.groovy.runtime.InvokerHelper

t = List.&isInstance

assert t.owner.simpleName == 'List'
assert t.doCall([]) == true    
assert InvokerHelper.getMetaClass(t.owner).
              invokeStaticMethod(t.owner, 'isInstance', []) == true
assert List.metaClass.invokeStaticMethod(t.owner, 'isInstance', []) == true

invokeStaticMethod of MOP is used if the object is an instance of `Class`.

On the other hand, `&plus` works appropriately because method pointer is created on a POJO.

Problem

In groovy it's pretty easy to create a method closure, for example: ``` groovy:000> p = 1.&plus ===> org.codehaus.groovy.runtime.MethodClosure@a7981d5 groovy:000> p(3) ===> 4 ``` However, for some reason it fails when trying to use an instance of `Class`: ``` groovy:000> List.isInstance([]) ===> true groovy:000> t = List.&isInstance ===> org.codehaus.groovy.runtime.MethodClosure@31ca1a68 groovy:000> t([]) ERROR groovy.lang.MissingMethodException: No signature of method: java.util.List.isInstance() is applicable for argument types: (java.util.ArrayList) values: [[]] at groovysh_evaluate.run (groovysh_evaluate:2) ... groovy:000> t = List.class.&isInstance ===> org.codehaus.groovy.runtime.MethodClosure@7b34c5ff groovy:000> t([]) ERROR groovy.lang.MissingMethodException: No signature of method: java.util.List.isInstance() is applicable for argument types: (java.util.ArrayList) values: [[]] at groovysh_evaluate.run (groovysh_evaluate:2) ... ``` It's pretty easy to work around this, but I'd like to understand why this happens. Is there something in the MOP that stops this from working, etc?

Original source