Help Needed: how to use readFile2String

[cs-usst]

Describe the feature

How to use FileIOUtils.readFile2String function?
I wrote this:

String externalString = PathUtils.getExternalStoragePath();
String testFileString = FileIOUtils.readFile2String(PathUtils.join(externalString,"test.txt"));
# i also tried:
String testFileString = FileIOUtils.readFile2String("/storage/emulated/0/test.txt");

And the testFileString is null.
Could anyone tell me why?
Thanks.

Reference

None

[shouzhong]

Maybe you don't have the permission to read the file from the '/storage/emulated/0/' dir.