JDK1.7 ClassLoader Memory Leak

I have child-first UrlClassLoader to load jar file dynamically. Then I do a reflection to invoke a method within loaded jar file. Once it is finished, I prefer to unload the classloader. Then I try to do some stress test code to make sure my code run smoothly. Basically, what I try to do is load and unload jar within looping statement. Here is my code:

    for (int i = 0; i < 1000; i++) {
        //Just to show the progress
        System.out.println("LOAD NUMBER : " + i);

        ChildFirstURLClassLoader classLoader = null;
        try {
            File file = new File("C:\\library.jar");
            String classToLoad = "com.test.MyClass";
            URL jarUrl = new URL("file:" + file.getAbsolutePath());

            classLoader = new ChildFirstURLClassLoader(new URL[] {jarUrl}, null);
            Class<?> loadedClass = classLoader.loadClass(classToLoad);
            Method method = loadedClass.getDeclaredMethod("execute",
                    new Class[] {});

            ClassLoader currCl= Thread.currentThread().getContextClassLoader();
            Thread.currentThread().setContextClassLoader(classLoader);
            method.invoke(null);
            Thread.currentThread().setContextClassLoader(currCl);

            method = null;
            loadedClass = null;
        } finally {
            if (classLoader != null) {
                classLoader.close();
                classLoader = null;
            }
        }
    }

When I'm running this code under JDK1.6, without classLoader.close(); statement, that code runs perfectly. But when I turn into JDK1.7, sometimes I get java.lang.OutOfMemoryError: PermGen space error. Unfortunately it occurs in inconsistent manner.