Java Shutdown Hook

What is a Shutdown Hook?

• A shutdown hook is a special Java feature that allows you to run a piece of code when the Java Virtual Machine (JVM) is shutting down.
• It can be useful for cleaning up resources, saving data, or performing other important tasks before the application exits.

Key Concepts

• JVM Shutdown: The JVM shuts down when the program ends, either due to normal execution, user termination (e.g., Ctrl+C), or a system shutdown.
• Thread for Cleanup: A shutdown hook is essentially a thread that runs when the JVM is shutting down.

How to Create a Shutdown Hook

1. Extend the Thread Class: Create a class that extends Thread and override its run() method with the code you want to execute during shutdown.
2. Register the Hook: Use Runtime.getRuntime().addShutdownHook(Thread thread) to register your shutdown hook.

Example:

public class MyShutdownHook extends Thread {
    public void run() {
        System.out.println("Shutdown Hook is running!");
        // Cleanup code here
    }
    
    public static void main(String[] args) {
        MyShutdownHook hook = new MyShutdownHook();
        Runtime.getRuntime().addShutdownHook(hook);
        
        System.out.println("Application is running...");
        
        // Simulating application work
        try {
            Thread.sleep(2000); // Sleep for 2 seconds
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        
        System.out.println("Application is ending...");
    }
}

Important Notes

• Order of Execution: If there are multiple shutdown hooks, they execute in the order they were registered.
• No Guarantees: There is no guarantee that the shutdown hook will execute in certain situations, such as a crash or forced termination.
• Keep it Short: The shutdown hook should complete quickly to avoid delaying the shutdown process.

Conclusion

• Shutdown hooks are a powerful feature in Java that help manage resource cleanup during application termination.
• They provide a way to ensure that important tasks are completed even as the application is shutting down.