Ok I found the right way to find zip file extra data in the unzip source files... didn't found any documentation (if someone has, please send us a link)
The two first bytes array is an universal timestamp. The next two bytes array is the size of the next data which contains pkware data as access time, modify time eso.
The next two bytes array is also the header id, and the next two one is the data size and last bytes are extra informations.
But even I interpret them, and even the java code is running on a Unix file system, I don't get any information about the file is a symlink or not.