PathClassLoader和DexClassLoader

动态加载.apk、.jar和.dex文件而且路径不受限制

4.1之后的版本中，不允许直接加载外部jar包

 final File dexOutputPath =new File(Environment.getExternalStorageDirectory().toString()        + File.separator + "dextest/Demojar-dex.jar");        File dir = this.getDir("out-dex", 0);        DexClassLoader cl = new DexClassLoader(dexOutputPath.getAbsolutePath(),        dir.getAbsolutePath(), null, getClassLoader());