android6.0 读取storage文件夹需要手动检测读取权限

解决方案摘自： 

http://stackoverflow.com/questions/8854359/exception-open-failed-eacces-permission-denied-on-android

除了在AndroidManifest.xml中申请权限

还需要在读写文件中设置一下代码：

// Storage Permissions
private static final int REQUEST_EXTERNAL_STORAGE = 1;
private static String[] PERMISSIONS_STORAGE = {
        Manifest.permission.READ_EXTERNAL_STORAGE,
        Manifest.permission.WRITE_EXTERNAL_STORAGE
};

// Check if we have write permission
int permission = ActivityCompat.checkSelfPermission(activity, Manifest.permission.WRITE_EXTERNAL_STORAGE);
if (permission != PackageManager.PERMISSION_GRANTED) {
   // We don't have permission so prompt the user
   ActivityCompat.requestPermissions(
       activity,
       PERMISSIONS_STORAGE,
       REQUEST_EXTERNAL_STORAGE
     );
 }