android – 如何挂载加密的APK扩展文件？

final StorageManager storageManager = (StorageManager) getSystemservice(STORAGE_serviCE);
    String obbPath = Environment.getExternalStorageDirectory() + "/Android/obb";
    final String obbFilePath = obbPath + "/com.example/main.1.com.example.obb";
    storageManager.mountObb(obbFilePath,"SecretKey",new OnObbStatechangelistener() {
        @Override
        public void onObbStateChange(String path,int statE) {
            super.onObbStateChange(path,statE);
            if (state == OnObbStatechangelistener.MOUNTED) {
                onObbMounted();
            } else {
                Log.d("##","Path: " + path + "; state: " + statE);
            }
        }
    });

但是在运行时我得到状态21：ERROR_Could_NOT_MOUNT：

Path: /storage/sdcard0/Android/obb/com.example/main.1.com.example.obb; state: 21

我添加了这个：

File f = new File(obbFilePath);
    if (!f.exists()) {
        Log.e("OBB","FILE NOT FOUND!!!");
    }

而logcat说该文件存在.我不知道,为什么我能得到这个状态21？