Passing Parameters Between ClassLoaders Via Reflection API

Question

I'm using a custom Classloader to create and return an instance of a class, this seems to work ok however when I try to call a method (via the Reflection API) and pass in a custom object as described below I get a NoSuchMethodException:

Supposing that the custom class loader creates and returns an instance like so:

Object obj = customClassLoader.load(String className,Class[] paramTypes,Object[] param)

Then I call a method (via reflection) and pass in a custom object:

NOTE: THIS IS THE LINE CAUSING THE ERROR

Method m = obj.getClass.getDeclaredMethod("mName",new Class[]{aCustomObject.class}) 

m.invoke(obj,new Object[]{new CustomObject() })

I'm stumped as to what could be causing the exception since a method definitely does exist which takes the specified custom object, I have confirmed this by using reflection to list all methods.

Did you try print out all available methods from this class and see if your method present there? Using getDeclaredMethods. — Mikita Belahlazau
– Mikita Belahlazau, Commented Nov 8, 2012 at 16:50
Yes, as mentioned above, I have successfully been able to print out all methods. I'm wondering if its because I'm using two classloaders. — Giles Thompson
– Giles Thompson, Commented Nov 8, 2012 at 21:00

pholser · Accepted Answer · 2012-11-08 22:14:23Z

2

How is your custom loader's load() method instantiating the object it is to return? Maybe the NoSuchMethodException arises during trying to find the correct constructor?

This example seems to work out OK:

package com.pholser;

import java.lang.reflect.Method;

public class ClassLoading {
    public static class CustomLoader extends ClassLoader {
        public Object load(String className, Class<?>[] paramTypes, Object[] params) throws Exception {
            Class<?> loaded = loadClass(className);
            return loaded.getConstructor(paramTypes).newInstance(params);
        }
    }

    public static class ACustomObject {
    }

    public void foo(ACustomObject a) {
        System.out.println("foo");
    }

    public static Object newCustomObject() throws Exception {
        return new CustomLoader().load("com.pholser.ClassLoading$ACustomObject", new Class<?>[0], new Object[0]);
    }

    public static void main(String[] args) throws Exception {
        ClassLoading obj = new ClassLoading();

        Method m = obj.getClass().getDeclaredMethod("foo", ACustomObject.class);

        m.invoke(obj, newCustomObject());
    }
}

answered Nov 8, 2012 at 22:14

pholser

4,9471 gold badge29 silver badges39 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Giles Thompson Over a year ago

The last line should be: m.invoke(obj, new CustomObject()); I have updated my question to reflect this. Your example does work as expected and I suspect its because you are creating the new object with the SAME ClassLoader as the one that instantiated the ClassLoading class. In my example I try to pass in an object instantiated by the default classloader.

Giles Thompson Over a year ago

Additionally, thank you very much indeed for your detailed answer.

Collectives™ on Stack Overflow

Passing Parameters Between ClassLoaders Via Reflection API

1 Answer 1

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related