How to Ensure a Java Program Runs as a Single Instance?

Question

How can I ensure that only one instance of my Java application runs at a time?

// Java singleton pattern using a lock file
import java.io.*;
import java.nio.file.*;

public class SingleInstanceApp {
    public static void main(String[] args) {
        // Create a lock file to ensure single instance
        File lockFile = new File("/tmp/single_instance.lock");
        try {
            if (!lockFile.createNewFile()) {
                System.out.println("Application already running.");
                return;
            }
            // Your application logic here
            System.out.println("Running the application...");

            // Simulate application work
            Thread.sleep(10000);
        } catch (IOException | InterruptedException e) {
            e.printStackTrace();
        } finally {
            // Delete the lock file on exit
            lockFile.delete();
        }
    }
}

Answer

To prevent multiple instances of a Java application from running simultaneously, you can use a lock mechanism. One effective method is to create a lock file that acts as a flag indicating whether the application is currently running. If the lock file already exists, it means another instance is active, and the new instance can exit gracefully.

// Java singleton pattern using a lock file
import java.io.*;
import java.nio.file.*;

public class SingleInstanceApp {
    public static void main(String[] args) {
        // Create a lock file to ensure single instance
        File lockFile = new File("/tmp/single_instance.lock");
        try {
            if (!lockFile.createNewFile()) {
                System.out.println("Application already running.");
                return;
            }
            // Your application logic here
            System.out.println("Running the application...");

            // Simulate application work
            Thread.sleep(10000);
        } catch (IOException | InterruptedException e) {
            e.printStackTrace();
        } finally {
            // Delete the lock file on exit
            lockFile.delete();
        }
    }
}

Causes

Multiple instances spawned by user mistakenly starting the application again.
Background processes inadvertently launched multiple times.

Solutions

Implement a lock file mechanism.
Use a singleton design pattern to manage instance creation.
Check for running processes before launching a new instance.

Common Mistakes

Mistake: Not handling exceptions properly while creating or deleting the lock file.

Solution: Make sure to include proper exception handling to manage scenarios where file creation fails.

Mistake: Hardcoding the lock file path.

Solution: Consider making the lock file location configurable or platform-independent.

Helpers

Java single instance
Java prevent multiple instances
Java application lock file
Java singleton pattern