What is a daemon thread in Java?

Can anybody tell me what daemon threads are in Java?

A daemon thread is a thread that does not prevent the JVM from exiting when the program finishes but the thread is still running. An example for a daemon thread is the garbage collection.

You can use the setDaemon(boolean) method to change the Thread daemon properties before the thread starts.

A few more points (Reference: Java Concurrency in Practice)

• When a new thread is created it inherits the daemon status of its parent.

• When all non-daemon threads finish, the JVM halts, and any remaining daemon threads are abandoned:

  • finally blocks are not executed,
  • stacks are not unwound - the JVM just exits.
    
    

  Due to this reason daemon threads should be used sparingly, and it is dangerous to use them for tasks that might perform any sort of I/O.

All the above answers are good. Here's a simple little code snippet, to illustrate the difference. Try it with each of the values of true and false in setDaemon.

public class DaemonTest {
    
    public static void main(String[] args) {
        new WorkerThread().start();

        try {
            Thread.sleep(7500);
        } catch (InterruptedException e) {
            // handle here exception
        }

        System.out.println("Main Thread ending") ;
    }

}

class WorkerThread extends Thread {
    
    public WorkerThread() {
        // When false, (i.e. when it's a non daemon thread),
        // the WorkerThread continues to run.
        // When true, (i.e. when it's a daemon thread),
        // the WorkerThread terminates when the main 
        // thread or/and user defined thread(non daemon) terminates.
        setDaemon(true); 
    }
    
    public void run() {
        int count = 0;

        while (true) {
            System.out.println("Hello from Worker "+count++);

            try {
                sleep(5000);
            } catch (InterruptedException e) {
                // handle exception here
            }
        }
    }
}

Traditionally daemon processes in UNIX were those that were constantly running in background, much like services in Windows.

A daemon thread in Java is one that doesn't prevent the JVM from exiting. Specifically the JVM will exit when only daemon threads remain. You create one by calling the setDaemon() method on Thread.

Have a read of Daemon threads.