Java Thread setDaemon() Method with Examples

by Online Tutorials Library July 22, 2022

Java Thread setDaemon() method

The setDaemon() method of thread class is used to mark the thread either daemon thread or a user thread. Its life depends on the user threads i.e. when all user threads die, JVM terminates this thread automatically. It must be invoked before the thread is started.

If you call the setDaemon() method after stating of the thread, this method will throw IllegalThreadStateException.

Syntax

Parameter

on: If true, marks the thread as a daemon thread.

Return

This method will return true if the thread is daemon thread otherwise return false.

Exception

IllegalThreadStateException: If the thread is alive.

SecurityException: If the current thread cannot modify the thread.

Example 1

  public class JavaSetDaemonExp1 extends Thread  {        public void run()      {            //checking for daemon thread            if(Thread.currentThread().isDaemon())          {              System.out.println(“daemon thread work”);            }            else          {                System.out.println(“user thread work”);            }        }        public static void main(String[] args)      {            // creating three threads          JavaSetDaemonExp1 t1=new JavaSetDaemonExp1();           JavaSetDaemonExp1 t2=new JavaSetDaemonExp1();            JavaSetDaemonExp1 t3=new JavaSetDaemonExp1();            // set user thread t1 to daemon thread            t1.setDaemon(true);          //call run() method          t1.start();           // set user thread t2 to daemon thread            t2.setDaemon(true);          // start of threads           t2.start();            t3.start();        }    }    

Test it Now

Output:

daemon thread work  daemon thread work  user thread work

Example 2

When you call the setDaemon() method after starting of thread.

  public class JavaSetDaemonExp2 extends Thread  {        public void run()      {            System.out.println(“Name of thread: “+Thread.currentThread().getName());            // //checking for daemon thread            System.out.println(“Daemon: “+Thread.currentThread().isDaemon());      }        public static void main(String[] args)      {            // creating two threads           JavaSetDaemonExp2 t1=new JavaSetDaemonExp2();            JavaSetDaemonExp2 t2=new JavaSetDaemonExp2();            // call run() method           t1.start();           // this will throw exception here           t1.setDaemon(true);           // call run() method            t2.start();        }    }    

Test it Now

Output:

Name of thread: Thread-0  Daemon: false  Exception in thread "main" java.lang.IllegalThreadStateException  at java.lang.Thread.setDaemon(Thread.java:1359)  at JavaSetDaemonExp2.main(JavaSetDaemonExp2.java:17)

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