Understanding Daemon Threads in Java with Example

What is the Daemon Thread in Java? Give an example?

In Java, a daemon thread is a thread that runs in the background to perform tasks that do not prevent the JVM from exiting when the program finishes. Daemon threads are typically used for background tasks such as garbage collection, monitoring, or other housekeeping tasks. When all user (non-daemon) threads finish executing, the JVM will terminate, even if daemon threads are still running.

Characteristics of Daemon Threads:

1. Background Service: Daemon threads are usually used for background services.
2. Lifecycle: The JVM does not wait for daemon threads to finish before it exits. If only daemon threads are left running, the JVM will terminate.
3. Set as Daemon: You can set a thread as a daemon by calling the setDaemon(true) method before starting the thread.

Example of a Daemon Thread:

Here’s a simple example demonstrating how to create and use a daemon thread in Java:

class DaemonThreadExample extends Thread {
    public void run() {
        while (true) {
            System.out.println("Daemon thread is running...");
            try {
                Thread.sleep(1000); // Sleep for 1 second
            } catch (InterruptedException e) {
                System.out.println("Daemon thread interrupted.");
            }
        }
    }
}

public class Main {
    public static void main(String[] args) {
        DaemonThreadExample daemonThread = new DaemonThreadExample();
        daemonThread.setDaemon(true); // Set the thread as a daemon
        daemonThread.start(); // Start the daemon thread

        // Main thread sleeps for 5 seconds
        try {
            Thread.sleep(5000);
        } catch (InterruptedException e) {
            System.out.println("Main thread interrupted.");
        }

        System.out.println("Main thread is finishing...");
        // When the main thread finishes, the JVM will exit, and the daemon thread will be terminated.
    }
}

Explanation:

1. DaemonThreadExample: This class extends Thread and overrides the run() method to print a message in an infinite loop.
2. setDaemon(true): This line sets the thread as a daemon thread.
3. Main Class: The main method starts the daemon thread and then sleeps for 5 seconds. After the main thread finishes, the JVM exits, and the daemon thread is terminated.

Output:

When you run the above code, you will see the message from the daemon thread printed every second for 5 seconds, after which the main thread finishes, and the program exits. The daemon thread will not continue running after the main thread has completed.