ParentLastURLClassLoader.java

package com.mojn.jmx;

import java.net.URL;
import java.net.URLClassLoader;
import java.util.List;

/**
 * A parent-last classloader that will try the child classloader first and then the parent.
 * This takes a fair bit of doing because java really prefers parent-first.
 * <p/>
 * For those not familiar with class loading trickery, be wary
 */
public class ParentLastURLClassLoader extends ClassLoader {
    private ChildURLClassLoader childClassLoader;

    /**
     * This class allows me to call findClass on a classloader
     */
    private static class FindClassClassLoader extends ClassLoader {
        public FindClassClassLoader(ClassLoader parent) {
            super(parent);
        }

        @Override
        public Class<?> findClass(String name) throws ClassNotFoundException {
            return super.findClass(name);
        }
    }

    /**
     * This class delegates (child then parent) for the findClass method for a URLClassLoader.
     * We need this because findClass is protected in URLClassLoader
     */
    private static class ChildURLClassLoader extends URLClassLoader {
        private FindClassClassLoader realParent;

        public ChildURLClassLoader(URL[] urls, FindClassClassLoader realParent) {
            super(urls, null);

            this.realParent = realParent;
        }

        @Override
        public Class<?> findClass(String name) throws ClassNotFoundException {
            try {
                // first try to use the URLClassLoader findClass
                return super.findClass(name);
            } catch (ClassNotFoundException e) {
                // if that fails, we ask our real parent classloader to load the class (we give up)
                return realParent.loadClass(name);
            }
        }
    }

    public ParentLastURLClassLoader(URL[] urls) {
        super(Thread.currentThread().getContextClassLoader());
        childClassLoader = new ChildURLClassLoader(urls, new FindClassClassLoader(this.getParent()));
    }
    public ParentLastURLClassLoader(List<URL> classpath) {
        this(classpath.toArray(new URL[classpath.size()]));

    }

    @Override
    protected synchronized Class<?> loadClass(String name, boolean resolve) throws ClassNotFoundException {
        try {
            // first we try to find a class inside the child classloader
            return childClassLoader.findClass(name);
        } catch (ClassNotFoundException e) {
            // didn't find it, try the parent
            return super.loadClass(name, resolve);
        }
    }
}