Java Daemon Example, all credit to user Sreekesh Okky in this post http://stackoverflow.com/questions/2213340/what-is-daemon-thread-in-java

App.java

public class App{
    
    public static void main(String[] args){
        DaemonThread dt = new DaemonThread();
        
        dt.setDaemon(true); // must be set before thread starting;
        dt.start(); // spawning the thread
        
        try{
            Thread.sleep(3000); // wait for 3 seconds. This actually let the daemon thread to print out some messages.
        }catch(InterruptedException e){
            
        }
        
        System.out.println("Leaving main method");
    }
}

DaemonThread.java

public DaemonThread extends Thread{
    public void run(){
        System.out.println("Entering the run method");
        
        try {
            System.out.println("In run Method: currentThread() is" + Thread.currentThread());

            while (true) { // a daemon thread usually involves a infinite loop in the run method
                try {
                    Thread.sleep(500);
                } catch (InterruptedException x) {}
                // as long as the non-daemon thread is running (in this case, the main thread)
                // this thread prints out the following message every 0.5 second.
                System.out.println("In run method: woke up again");
            }
        } finally {
            // the message in the finally block won't be printed out 
            // since when the JVM halts, finally block in daemon thread is not executed.
            System.out.println("Leaving run Method");
        }
    }
}

OutputAnalysis

Entering main Method
Entering run method
In run Method: currentThread() isThread[Thread-0,5,main]
In run method: woke up again
In run method: woke up again
In run method: woke up again
In run method: woke up again
In run method: woke up again
In run method: woke up again
Leaving main method

===============================
As we can see here, while the main thread in sleep, the daemon thread performed 6 printing service. 
When the main thread wakes up, it prints out the leaving message and exits. 
At this point, there are no more non-daemon threads in the program, so the JVM halts.
That message in the finally block of the daemon thread never got printed.