JAVA如何动态加载jar，并调用对应的函数？

private static void proguard(final String proguardDir)
    {
        //直接加载lib，执行语句如下：
        //proguard.ProGuard.main(new String[] {CONFIG_FILE});
        try
        {
            URL url1 = new URL("file:D:/lib/proguard.jar");
            URLClassLoader classLoader = new URLClassLoader(new URL[] { url1 }, Thread.currentThread().getContextClassLoader());
            Class<?> proguard = classLoader.loadClass("proguard.ProGuard");
            Method mainMethod = proguard.getDeclaredMethod("main", String[].class);
            mainMethod.invoke(null, new Object[] {new String[] { CONFIG_FILE}});
            classLoader.close();
        }
        catch (Exception e)
        {
            e.printStackTrace();
        }
    }