FilenameUtils.getPrefixLength is used in FilenameUtils.doNormalize.
FilenameUtils.normalize("////I don't want to become null!") returns null.
Expected was: "I don't want to become null!"
The method FilenameUtils.getPrefixLength returns -1 for the mentioned string.
The root problem is found in following lines of code:
All leading slashes should be ignored at all, but considering the rest of the string.