How to atomically rename a file in Java, even if the dest file already exists?

Question

I have a cluster of machines, each running a Java app.

These Java apps need to access a unique resource.txt file concurrently.

I need to atomically rename a temp.txt file to resource.txt in Java, even if resource.txt already exist.

Deleting resource.txt and renaming temp.txt doesn't work, as it's not atomic (it creates a small timeframe where resource.txt doesn't exist).

And it should be cross-platform...

Thanks !

Answer 1

For Java 1.7+, use java.nio.file.Files.move(Path source, Path target, CopyOption... options) with CopyOptions "REPLACE_EXISTING" and "ATOMIC_MOVE".

See API documentation for more information.

For example:

Files.move(src, dst, StandardCopyOption.ATOMIC_MOVE); 

Answer 2

On Linux (and I believe Solaris and other UNIX operating systems), Java's File.renameTo() method will overwrite the destination file if it exists, but this is not the case under Windows.

To be cross platform, I think you'd have to use file locking on resource.txt and then overwrite the data.

The behavior of the file lock is platform-dependent. On some platforms, the file lock is advisory, which means that unless an application checks for a file lock, it will not be prevented from accessing the file. On other platforms, the file lock is mandatory, which means that a file lock prevents any application from accessing the file.

try {     // Get a file channel for the file     File file = new File("filename");     FileChannel channel = new RandomAccessFile(file, "rw").getChannel();      // Use the file channel to create a lock on the file.     // This method blocks until it can retrieve the lock.     FileLock lock = channel.lock();      // Try acquiring the lock without blocking. This method returns     // null or throws an exception if the file is already locked.     try {         lock = channel.tryLock();     } catch (OverlappingFileLockException e) {         // File is already locked in this thread or virtual machine     }      // Release the lock     lock.release();      // Close the file     channel.close(); } catch (Exception e) { } 

Linux, by default, uses voluntary locking, while Windows enforces it. Maybe you could detect the OS, and use renameTo() under UNIX with some locking code for Windows?

There's also a way to turn on mandatory locking under Linux for specific files, but it's kind of obscure. You have to set the mode bits just right.

Linux, following System V (see System V Interface Definition (SVID) Version 3), lets the sgid bit for files without group execute permission mark the file for mandatory locking