看起来还没有人对此有一个答案，我已经解决了这个问题，并希望分享我的解决方法.因此，基本上，这是行不通的，因为我没有在运行时请求权限.

Look's like no one has an answer for this yet, I have already solved the issue and wanted to share how I solved it. So basically It wasn't working because I wasn't asking for permission on runtime.

我所做的是向运行时询问权限.在下面使用此代码.

What I did was ask the permission on runtime. Using this code below.

ActivityCompat.requestPermissions(MainActivity.this, new String[]{Manifest.permission.WRITE_EXTERNAL_STORAGE}, 1);