Daemon Threads

Daemon threads are low priority threads, which are mainly used for background processing

How to make a thread Daemon

Syntax

public final void setDaemon(boolean on)

When you set the setDaemon to true for a thread, then it acts as a daemon thread.

How to know whether a thread is daemon or not

Syntax:

isDaemon()

returns true if this thread is a daemon thread; false otherwise.

Notable points

1. The Java Virtual Machine exits when the only threads running are all daemon threads.

Example

class MyThread implements Runnable{
 public void run(){
  while(true){
   System.out.println("My Name is " + Thread.currentThread().getName());
  }
 }

 public static void main(String args[]){
  MyThread task1 = new MyThread();
  Thread t1 = new Thread(task1);
  Thread t2 = new Thread(task1);
  Thread t3 = new Thread(task1);

  t1.setDaemon(true);
  t2.setDaemon(true);
  t3.setDaemon(true);

  t1.start();
  t2.start();
  t3.start();
 }
}

         

Sample Output      

 My Name is Thread-0

Observation

Above program has 3 threads. All the threads are set to daemon. If the threads are not set to daemon, then the program will run infinitely. In the above program all the threads are daemon, So once the main thread finishes its exection, JVM don't bother about the daemon threads. It simply quits.

2. SetDaemon() must be called before the thread is started. If you call setDaemon() after calling the Thread start method, setDaemon() throws IllegalThreadStateException.

Example

class MyThread implements Runnable{
 public void run(){
  try{
   System.out.println("I am feeling sleepy");
   Thread.currentThread().sleep(5000);
   System.out.println("Time to work");
  }
  catch(InterruptedException e){
  }
 }

 public static void main(String args[]){
  MyThread task1 = new MyThread();
  Thread t1 = new Thread(task1);

  t1.start();
  t1.setDaemon(true);

 }
}

   

Sample Output

I am feeling sleepyException in thread "main"
java.lang.IllegalThreadStateException
at java.lang.Thread.setDaemon(Unknown Source)
at MyThread.main(MyThread.java:22)
Time to work

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