Daemon Threads

A “daemon” thread is one that is supposed to provide a general service in the background as long as the program is running, but is not part of the essence of the program. Thus, when all of the non-daemon threads complete, the program is terminated. Conversely, if there are any non-daemon threads still running, the program doesn’t terminate. There is, for instance, a non-daemon thread that runs main( ).

//: c13:SimpleDaemons.java
// Daemon threads don't prevent the program from ending.

public class SimpleDaemons extends Thread {
  public SimpleDaemons() {
    setDaemon(true); // Must be called before start()
    start();
  }
  public void run() {
    while(true) {
      try {
        sleep(100);
      } catch (InterruptedException e) {
        throw new RuntimeException(e);
      }
      System.out.println(this);
    }
  }
  public static void main(String[] args) {
    for(int i = 0; i < 10; i++)
      new SimpleDaemons();
  }
} ///:~

You must set the thread to be a daemon by calling setDaemon( ) before it is started. In run( ), the thread is put to sleep for a little bit. Once the threads are all started, the program terminates immediately, before any threads can print themselves, because there are no non-daemon threads (other than main( )) holding the program open. Thus, the program terminates without printing any output.

You can find out if a thread is a daemon by calling isDaemon( ). If a thread is a daemon, then any threads it creates will automatically be daemons, as the following example demonstrates:

//: c13:Daemons.java
// Daemon threads spawn other daemon threads.
import java.io.*;
import com.bruceeckel.simpletest.*;

class Daemon extends Thread {
  private Thread[] t = new Thread[10];
  public Daemon() {
    setDaemon(true);
    start();
  }
  public void run() {
    for(int i = 0; i < t.length; i++)
      t[i] = new DaemonSpawn(i);
    for(int i = 0; i < t.length; i++)
      System.out.println("t[" + i + "].isDaemon() = "
        + t[i].isDaemon());
    while(true)
      yield();
  }
}

class DaemonSpawn extends Thread {
  public DaemonSpawn(int i) {
    start();
    System.out.println("DaemonSpawn " + i + " started");
  }
  public void run() {
    while(true)
      yield();
  }
}

public class Daemons {
  private static Test monitor = new Test();
  public static void main(String[] args) throws Exception {
    Thread d = new Daemon();
    System.out.println("d.isDaemon() = " + d.isDaemon());
    // Allow the daemon threads to
    // finish their startup processes:
    Thread.sleep(1000);
    monitor.expect(new String[] {
      "d.isDaemon() = true",
      "DaemonSpawn 0 started",
      "DaemonSpawn 1 started",
      "DaemonSpawn 2 started",
      "DaemonSpawn 3 started",
      "DaemonSpawn 4 started",
      "DaemonSpawn 5 started",
      "DaemonSpawn 6 started",
      "DaemonSpawn 7 started",
      "DaemonSpawn 8 started",
      "DaemonSpawn 9 started",
      "t[0].isDaemon() = true",
      "t[1].isDaemon() = true",
      "t[2].isDaemon() = true",
      "t[3].isDaemon() = true",
      "t[4].isDaemon() = true",
      "t[5].isDaemon() = true",
      "t[6].isDaemon() = true",
      "t[7].isDaemon() = true",
      "t[8].isDaemon() = true",
      "t[9].isDaemon() = true"
    }, Test.IGNORE_ORDER + Test.WAIT);
  }
} ///:~

The Daemon thread sets its daemon flag to “true” and then spawns a bunch of other threads—which do not set themselves to daemon mode—to show that they are daemons anyway. Then it goes into an infinite loop that calls yield( ) to give up control to the other processes.

There’s nothing to keep the program from terminating once main( ) finishes its job, since there are nothing but daemon threads running. So that you can see the results of starting all the daemon threads, the main( ) thread is put to sleep for a second. Without this, you see only some of the results from the creation of the daemon threads. (Try sleep( ) calls of various lengths to see this behavior.)