How Can I Implement a Single-Instance Java Application Using File Locking?

Implementing Single Instance Java Applications

Many applications such as MSN and Windows Media Player are designed to run as single instances, preventing multiple instances from being created while the application is running. While the Mutex class provides this functionality in C#, Java developers have a different approach.

Java Solution using File Locking

The recommended method for implementing a single instance Java application is through file locking. Here's the code:

private static boolean lockInstance(final String lockFile) {
    try {
        final File file = new File(lockFile);
        final RandomAccessFile randomAccessFile = new RandomAccessFile(file, "rw");
        final FileLock fileLock = randomAccessFile.getChannel().tryLock();
        if (fileLock != null) {
            Runtime.getRuntime().addShutdownHook(new Thread() {
                public void run() {
                    try {
                        fileLock.release();
                        randomAccessFile.close();
                        file.delete();
                    } catch (Exception e) {
                        log.error("Unable to remove lock file: " + lockFile, e);
                    }
                }
            });
            return true;
        }
    } catch (Exception e) {
        log.error("Unable to create and/or lock file: " + lockFile, e);
    }
    return false;
}

This method relies on file locking to prevent multiple instances from running simultaneously. It creates a lock file that is exclusively acquired by the first instance of the application. Subsequent instances attempting to acquire the lock will fail, effectively preventing them from running.

When the application terminates, a shutdown hook is executed to release the lock and delete the lock file, ensuring cleanup.

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