Daemon Threads in Java

A daemon thread in Java is a background helper thread that does not keep the program running. The JVM exits as soon as all user (non-daemon) threads have finished, even if daemon threads are still working; those daemon threads are simply stopped. The garbage collector is an example of a daemon thread. You make a thread a daemon with setDaemon(true) before starting it. In this lesson, you will learn how daemon threads behave and when to use them.

User Threads vs Daemon Threads

Methods

Example: The JVM Doesn't Wait for Daemons

Let's see an example where a daemon thread would print forever, but the program still ends.

public class DaemonDemo {
    public static void main(String[] args) throws InterruptedException {
        Thread heartbeat = new Thread(() -> {
            int beat = 1;
            while (true) {
                System.out.println("heartbeat " + beat++);
                try {
                    Thread.sleep(200);
                } catch (InterruptedException e) {
                    return;
                }
            }
        });
        heartbeat.setDaemon(true);
        System.out.println("Is daemon? " + heartbeat.isDaemon());
        heartbeat.start();
 
        Thread.sleep(500);
        System.out.println("Main ends now");
    }
}

Is daemon? true
heartbeat 1
heartbeat 2
heartbeat 3
Main ends now

Explanation: The heartbeat thread has an endless loop. Because it is a daemon, the JVM doesn't wait for it: when main, the last user thread, finishes, the program exits and the heartbeat stops. Without setDaemon(true), the program would never end.

Example: setDaemon After start()

public class TooLate {
    public static void main(String[] args) throws InterruptedException {
        Thread t = new Thread(() -> { });
        t.start();
        try {
            t.setDaemon(true);
        } catch (IllegalThreadStateException e) {
            System.out.println("setDaemon after start: "
                + e.getClass().getSimpleName());
        }
        t.join();
    }
}

setDaemon after start: IllegalThreadStateException

Explanation: A thread's daemon status is fixed once it starts. Calling setDaemon afterwards throws IllegalThreadStateException.

Common Mistakes

Calling setDaemon after start(). It throws.

Using a daemon thread to save data. It may be killed before finishing.

Wondering why a program never exits. A forgotten non-daemon background thread is keeping it alive.

Interview Corner

Practice

1. Easy: Change heartbeat to a normal thread and observe that the program never ends (stop it with Ctrl+C).
2. Harder: Create a daemon thread that prints memory usage every second while the main thread does three seconds of work.

Key Takeaways