Daemon threads

Definition

A daemon thread is a background helper. The JVM does not wait for daemon threads: when only daemon threads are left, the program ends. Normal (non-daemon) threads keep the program alive until they finish.

Example code

A daemon thread does not keep the program alive

class Demo {
    public static void main(String[] args) {
        Thread background = new Thread(() -> {
            while (true) {                                  // would run forever...
                try { Thread.sleep(100); } catch (InterruptedException e) { return; }
            }
        });
        background.setDaemon(true);                         // ...but a daemon cannot block shutdown
        background.start();

        System.out.println("is daemon: " + background.isDaemon());
        System.out.println("main is done, the JVM exits even though the daemon is still running");
    }
}

// Output:
// is daemon: true
// main is done, the JVM exits even though the daemon is still running