How to set file owner/group when creating a file in Java

Question

I would like to set a (unix) owner and group of a file created from Java. I would like something like this:

Path file = ...;
Set<PosixFilePermission> perms = PosixFilePermissions.fromString("rwxr-x---");
FileAttribute<Set<PosixFilePermission>> attr = PosixFilePermissions.asFileAttribute(perms);
Files.createFile(file, attr);

-- it is an example how to set permissions, but I cannot find how to do the same with owner/group.

Please note that I'm not interested in changing the owner after the file is created (this has already been answered on SO [1] [2]), but when the file is created.

Motivation for this question is that I need to make sure that the file I'm creating is not modified by other users while I set proper owner and permissions.

Answer 1

The Oracle-Documentation describes how to set and get posix conform owner.

Path path = ...
 UserPrincipalLookupService lookupService =
     provider(path).getUserPrincipalLookupService();
 UserPrincipal joe = lookupService.lookupPrincipalByName("joe");
 Files.setOwner(path, joe);

Function Prototype looks like this:

public static Path setOwner(Path path,
        UserPrincipal owner)
                 throws IOException

Parameters:

• path - A file reference that locates the file
• owner - The new file owner

The group is indeed not mentioned in the Docs:

Retrieve the group owner of a file

File originalFile = new File("original.jpg"); // just as an example
GroupPrincipal group = Files.readAttributes(originalFile.toPath(), PosixFileAttributes.class, LinkOption.NOFOLLOW_LINKS).group();

Set the group owner of a file

File targetFile = new File("target.jpg");
Files.getFileAttributeView(targetFile.toPath(), PosixFileAttributeView.class, LinkOption.NOFOLLOW_LINKS).setGroup(group);

Answer 2

Setting ownership on file creation does not seem to be possible. When you look at the documentation of open() system call, it describes how to set file permissions, but the only mention of owner is:

If the file does not exist it will be created. The owner (user ID) of the file is set to the effective user ID of the process. The group ownership (group ID) is set either to the effective group ID of the process or to the group ID of the parent directory

See also this answer.

The solution I went for in the end was this:

1. Create the file with default owner but restrictive permissions 000:

   Path file = ...;
   Set<PosixFilePermission> perms = Collections.<PosixFilePermissions>emptySet();
   FileAttribute<Set<PosixFilePermission>> attr = PosixFilePermissions.asFileAttribute(perms);
   Files.createFile(file, attr);
   

2. Change the owner/group to the target user

3. The target user then sets permissions to what it needs.

This should ensure that no other user can modify the file at any point in time.