问如何在Java中创建父-最后/子-第一ClassLoader，或者如何覆盖已经加载在父CL中的旧Xerces版本？

EN

/**
 * 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.
 * 
 * For those not familiar with class loading trickery, be wary
 */
private static 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(List<URL> classpath)
    {
        super(Thread.currentThread().getContextClassLoader());

        URL[] urls = classpath.toArray(new URL[classpath.size()]);

        childClassLoader = new ChildURLClassLoader( urls, new FindClassClassLoader(this.getParent()) );
    }

    @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);
        }
    }
}

                Class<?> loaded = super.findLoadedClass(name);
                if( loaded != null )
                    return loaded;

/**
 * Creates a classloader for the given project using the classpath given.
 *
 * @param parent The parent classloader to which unsatisfied loading
 *               attempts are delegated. May be <code>null</code>,
 *               in which case the classloader which loaded this
 *               class is used as the parent.
 * @param project The project to which this classloader is to belong.
 *                Must not be <code>null</code>.
 * @param classpath the classpath to use to load the classes.
 *                  May be <code>null</code>, in which case no path
 *                  elements are set up to start with.
 * @param parentFirst If <code>true</code>, indicates that the parent
 *                    classloader should be consulted  before trying to
 *                    load the a class through this loader.
 */
public AntClassLoader(
    ClassLoader parent, Project project, Path classpath, boolean parentFirst) {
    this(project, classpath);
    if (parent != null) {
        setParent(parent);
    }
    setParentFirst(parentFirst);
    addJavaLibraries();
}