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)

Next TopicMultithreading Java

← prev next →