Example of a simple lock file in Java

Main.java

package YOUR_PACKAGE_NAME;

import java.io.File;
import java.io.IOException;

/**
 * Minimal reproducible example (MRE) - Example of a simple lock file.
 * @author Remzi Cavdar - ict@remzi.info - <a href="https://github.com/Remzi1993">@Remzi1993</a>
 */
public class Main {
    public static void main(String[] args) {
        /*
         * Prevents the user of starting multiple instances of the application.
         * This is done by creating a temporary file in the app directory.
         * The temp file should be excluded from git and is called App.lock in this example.
         */
        final File FILE = new File("App.lock");

        try {
            if (FILE.createNewFile()) {
                System.out.println("Starting application");
            } else {
                System.err.println("The application is already running!");
                return;
            }
        } catch (IOException e) {
            throw new RuntimeException(e);
        }

        /*
         * Register a shutdown hook to delete the lock file when the application is closed. Even when forcefully closed
         * with the task manager. (Tested on Windows 11 with JavaFX 19)
         */
        FILE.deleteOnExit();
        // Whatever your app is supposed to do
    }
}

Interested in a slightly more advanced locking system? Check this gist out: https://gist.github.com/Remzi1993/234ea98293c65ffec4d10b7c70b88645