JAVA EE: Java Tutorial: Java Threads (What happens if we set thread as Daemon thread after thread started)

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DisplayThread.java

/*
 * public final void setDaemon(boolean on)
 * 
 * Parameters:
 * ----------- 
 * 
 * on - if true, marks this thread as a
 * daemon thread
 */

public class DisplayThread extends Thread
{

    public void run()
    {
        /*
         * Tests if this thread is a daemon thread.
         */
        if (Thread.currentThread().isDaemon())
        {
            System.out.println("Daemon thread = "
                    + Thread.currentThread().getName());
        }
        else
        {
            System.out.println("user thread = "
                    + Thread.currentThread().getName());
        }
    }

    public static void main(String[] args)
    {
        DisplayThread t1 = new DisplayThread();
        t1.setName("Thread 1");
        DisplayThread t2 = new DisplayThread();
        t2.setName("Thread 2");
        DisplayThread t3 = new DisplayThread();
        t3.setName("Thread 3");

        t1.start();

        /*
         * If you want to make a user thread as Daemon, it
         * must not be started otherwise it will throw
         * IllegalThreadStateException.
         * 
         * below line will throw
         * IllegalThreadStateException, because t1 thread is
         * started already.
         */
        t1.setDaemon(true);
        t2.start();
        t3.start();
    }
}

Output

Exception in thread "main" java.lang.IllegalThreadStateException
    at java.lang.Thread.setDaemon(Thread.java:1352)
    at DisplayThread.main(DisplayThread.java:51)
user thread = Thread 1