Can you use Groovy meta programming to override a private method on a Java class

Question

I'm trying to override a private method on a Java class using meta programming. The code looks something like this:

// Java class
public class MyClass{

    private ClassOfSomeSort property1;
    private ClassOfSomeOtherSort property2;

    public void init(){

        property1 = new ClassOfSomeSort();
        property2 = new ClassOfSomeOtherSort();

        doSomethingCrazyExpensive();
    }

    private void doSomethingCrazyExpensive(){
        System.out.println("I'm doing something crazy expensive");
    }
}

// Groovy class
public class MyClassTest extends Specification{

    def "MyClass instance gets initialised correctly"(){

        given:
        ExpandoMetaClass emc = new ExpandoMetaClass( MyClass, false )
        emc.doSomethingCrazyExpensive = { println "Nothing to see here..." }
        emc.initialize()
        def proxy = new groovy.util.Proxy().wrap( new MyClass() )
        proxy.setMetaClass( emc )
        when:
        proxy.init()
        then:
        proxy.property1 != null
        proxy.property2 != null     
    }
}

The problem is that the overridden implementation of doSomethingCrazyExpensive isn't called - I think that this is because the private method is called by the init() method internally and not called through the metaClass. If I call myProxy.doSomethingCrazyExpensive() directly, the overridden method is invoked, so the meta-programming does work to some degree.

Is there a way to use meta programming to override a method on a Java class (or instance) in such a way that the overridden implementation is called when it is invoked internally?

Answer 1

Groovy as operator is quite powerful, and can create proxies out of concrete types whose changes are visible in Java. Sadly, seems like it can't override private methods, though i managed to change a public method:

Java class:

public class MyClass{

    public void init(){
        echo();
        doSomethingCrazyExpensive();
    }

    public void echo() { System.out.println("echo"); }

    private void doSomethingCrazyExpensive(){
        System.out.println("I'm doing something crazy expensive");
    }
}

Groovy test:

class MyClassTest extends GroovyTestCase {
    void "test MyClass instance gets initialised correctly"(){

        def mock = [
          doSomethingCrazyExpensive: { println 'proxy crazy' },
          echo: { println 'proxy echo' }
        ] as MyClass

        mock.init()

        mock.doSomethingCrazyExpensive()
    }
}

It prints:

proxy echo
I'm doing something crazy expensive
proxy crazy

So the public method got intercepted and changed, even when being called from Java, but not the private one.