大家好，如何从android中的目录中获取所有文件

Question

如何从android中的目录中获取所有文件？例如，如果我想从DCIM / Camera读取文件。

@Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        File dir = Environment.getExternalStorageDirectory();
        Log.e("dir", dir.toString());
        Log.e("dir", Arrays.toString(dir.list()));   
    }

日志

03-12 22:21:33.370 14224-14224/com.example.kirill.javagallery E/dir: /storage/emulated/0
03-12 22:21:33.370 14224-14224/com.example.kirill.javagallery E/dir: null

为什么ExternalStorageDirectory为空？

Manifest
    <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
    <uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE" />