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Java - Daemon Thread

A Daemon thread is created to support the user threads. It generallty works in background and terminated once all the other threads are closed. Garbage collector is one of the example of Daemon thread.

• A Daemon thread is a low priority thread.

• A Daemon thread is a service provider thread and should not be used as user thread.

• JVM automatically closes the daemon thread(s) if no active thread is present and revives it if user threads are active again.

• A daemon thread cannot prevent JVM to exit if all user threads are done.

The following are the methods provided by the Thread class for Daemon Thread in Java -

• Thread.setDaemon() Method: Marks this thread as either a daemon thread or a user thread.

• Thread.isDaemon() Method: Checks if this thread is a daemon thread.

In this example, we've created a ThreadDemo class which extends Thread class. In main method, we've created three threads. As we're not setting any thread as daemon thread, no thread is marked as daemon thread.

package com.tutorialspoint;
class ThreadDemo extends Thread {
   ThreadDemo( ) {
   }
   public void run() {
      System.out.println("Thread  " + Thread.currentThread().getName() 
         + ", is Daemon: " + Thread.currentThread().isDaemon());
   }
   public void start () {     
      super.start();
   }
}
public class TestThread {
   public static void main(String args[]) {
	  ThreadDemo thread1 = new ThreadDemo();
	  ThreadDemo thread2 = new ThreadDemo();
	  ThreadDemo thread3 = new ThreadDemo();
	  thread1.start();
	  thread2.start();
	  thread3.start();
   }
}

Thread  Thread-1, is Daemon: false
Thread  Thread-0, is Daemon: false
Thread  Thread-2, is Daemon: false

In this example, we've created a ThreadDemo class which extends Thread class. In main method, we've created three threads. As we're setting one thread as daemon thread, one thread will be printed as daemon thread.

package com.tutorialspoint;
class ThreadDemo extends Thread {
   ThreadDemo( ) {
   }

   public void run() {
      System.out.println("Thread  " + Thread.currentThread().getName() 
         + ", is Daemon: " + Thread.currentThread().isDaemon());
   }
   public void start () {     
      super.start();
   }
}
public class TestThread {
   public static void main(String args[]) {
	  ThreadDemo thread1 = new ThreadDemo();
	  ThreadDemo thread2 = new ThreadDemo();
	  ThreadDemo thread3 = new ThreadDemo();
      thread3.setDaemon(true);
	  thread1.start();
	  thread2.start();
	  thread3.start();
   }
}

Thread  Thread-1, is Daemon: false
Thread  Thread-2, is Daemon: true
Thread  Thread-0, is Daemon: false

In this example, we've created a ThreadDemo class which extends Thread class. In main method, we've created three threads. Once a thread starts, it cannot be set as daemon thread. As we're setting one thread as daemon thread after it started, a runtime exception will be raised.

package com.tutorialspoint;
class ThreadDemo extends Thread {
   ThreadDemo( ) {
   }
   public void run() {
      System.out.println("Thread  " + Thread.currentThread().getName() 
         + ", is Daemon: " + Thread.currentThread().isDaemon());
   }
   public void start () {     
      super.start();
   }
}
public class TestThread {
   public static void main(String args[]) {
	   ThreadDemo thread1 = new ThreadDemo();
	   ThreadDemo thread2 = new ThreadDemo();
	   ThreadDemo thread3 = new ThreadDemo();
	   thread1.start();
	   thread2.start();
	   thread3.start();
      thread3.setDaemon(true);
   }
}

Exception in thread "main" Thread  Thread-1, is Daemon: false
Thread  Thread-2, is Daemon: false
Thread  Thread-0, is Daemon: false
java.lang.IllegalThreadStateException
	at java.lang.Thread.setDaemon(Unknown Source)
	at com.tutorialspoint.TestThread.main(TestThread.java:27)

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