How to Ensure Only One Instance of a Java Application Runs at a Time?

Achieving Single Instance Applications in Java

In contrast to certain applications like MSN and Windows Media Player, which only create a single instance when executed while running, Java applications may spawn multiple instances. In this article, we'll explore how to create single instance Java applications using the proven method described below.

Java Approach

Unlike C# where the Mutex class handles this functionality, Java utilizes a distinct approach:

1. Create a Lock File: Create a unique file to indicate the application is running, preventing multiple instances.
2. Acquire File Lock: Acquire a lock on the lock file using RandomAccessFile.getChannel().tryLock(). This ensures only one application has write access to the file.
3. Register Shutdown Hook: Register a shutdown hook to automatically release the lock and delete the lock file when the application exits.
4. Call lockInstance(): Invoke the lockInstance() method in the main method to enforce single instance functionality.

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;
}

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