Question:

I am aware that it's possible to set the owner of a file using <a href="https://docs.oracle.com/javase/7/docs/api/java/nio/file/Files.html#setOwner%28java.nio.file.Path,%20java.nio.file.attribute.UserPrincipal%29" rel="nofollow">Files.setOwner</a>:

Files.setOwner(Path path, UserPrincipal owner);

However what about the POSIX <em>group</em>? There is no such Files.setGroup() method in the API, and not even using the FileOwnerAttributeView works, as there is only setOwner too.

Is it possible at all?

If your app is working on POSIX systems exclusively, there's <a href="http://docs.oracle.com/javase/7/docs/api/java/nio/file/attribute/PosixFileAttributeView.html#setGroup%28java.nio.file.attribute.GroupPrincipal%29" rel="nofollow">PosixFileAttributeView.setGroup()</a>.