Is there a way to write file atomically in Java on Linux?

Question

Linux Api has O_TMPFILE flag to be specified with open system call creating unnamed temporary file which cannot be opened by any path. So we can use this to write data to the file "atmoically" and the linkat the given file to the real path. According to the open man page it can be implemented as simple as

char path[1000];
int fd = open("/tmp", O_TMPFILE | O_WRONLY, S_IWUSR);
write(fd, "123456", sizeof("123456"));
sprintf(path, "/proc/self/fd/%d", fd);
linkat(AT_FDCWD, path, AT_FDCWD, "/tmp/1111111", AT_SYMLINK_FOLLOW);

Is there a Java alternative (probably non crossplatform) to do atomic write to a file without writing Linux-specific JNI function? Files.createTempFile does completely different thing.

By atomic write I mean that either it cannot be opened and be read from or it contains all the data required to be writted.

Answer 1

I don't believe Java has an API for this, and it seems to depend on both the OS and filesystem having support, so JNI might be the only way, and even then only on Linux.

I did a quick search for what Cygwin does, seems to be a bit of a hack just to make software work, creating a file with a random name then excluding it only from their own directory listing.

I believe the closest you can get in plain Java is to create a file in some other location (kinda like a /proc/self/fd/... equivalent), and then when you are done writing it, either move it or symbolic link it from the final location. To move the file, you want it on the same filesystem partition so the file contents don't actually need to be copied. Programs watching for the file in say /tmp/ wouldn't see it until the move or sym link creation.

You could possibly play around with user accounts and filesystem permissions to ensure that no other (non SYSTEM/root) program can see the file initially even if they tried to look wherever you hid it.