Got it right but don't understand small point

Question about the task

Under discussion

I initially failed: The AmigoThreadFactory factory must create threads that are not daemons. The AmigoThreadFactory factory must create threads with a normal priority level. Until I added:

thread.setDaemon(false);
thread.setPriority(5);

But aren't these the default values for any new Thread?

Comments (7)

Popular
New
Old

You must be signed in to leave a comment

Guadalupe Gagnon

Level 37 , Tampa, United States

22 April 2021, 14:35

1# Default values and the initializer testing that specific methods are called mean two different things. #2 the default for new threads: Daemon is "user", which is the same as a daemon thread. The program will stop if only daemon threads are running. Setting it to false means the program will not end as it keeps running if any daemon threads are still running. Priority is defaulted to the same priority of the thread that created the new thread. So any threads that are spawned from a priority 10 thread will also be priority 10 and the same for any other priority.

Anthony Chalk

Level 30 , London, United Kingdom

22 April 2021, 14:39

If I create a new Thread, and do not call the setDaemon method, I assume it will not be a Daemon thread?

Guadalupe Gagnon

Level 37 , Tampa, United States

22 April 2021, 14:40

Here is a small but informative article: Understanding Thread Priorities and Daemon Thread in Java

Anthony Chalk

Level 30 , London, United Kingdom

23 April 2021, 11:41

Thank you. I'm still unclear though, especially regarding daemon threads: You don't have to call the setDaemon method(). That being the case, I assume Threads have a default value for isDaemon(). Based on previous tasks, I'm also assuming that that value is false. If so, when would you need setDaemon(false) unless (off the top of my head): a) it was previously set to true (not sure how that would work, as you can't setDaemon after calling start()) b) it's parent Thread was set to true (this task the parent Thread was main? which is not a daemon thread)

Guadalupe Gagnon

Level 37 , Tampa, United States

23 April 2021, 14:11

I think I did say that originally backwards, here is the exact wording: "Java defines two types of thread: user thread (normal thread) and daemon thread. By default, when you create a new thread it is user thread. The Java Virtual Machine (JVM) won’t terminate if there are still user threads running. But it will exit if there are only daemon threads running." Also an example to run to see the difference:

public static void main(String[] args) throws Exception {
   Thread myThread = new Thread(new Runnable() {
      @Override
      public void run() {
         while(true){
            try {
               for(int i = 0; i < 4; i++){
                  System.out.print("Hot, ");
                  Thread.sleep(500);
               }
               System.out.print("Staying Alive, ");
               Thread.sleep(1000);
               System.out.println("Staying Alive!!");
               Thread.sleep(1500);
            } catch (Exception e){}
         }
      }
   });
   //myThread.setDaemon(true);  // <-- uncomment this line
   myThread.start();
   Thread.sleep(5000);
   return;
}

Anthony Chalk

Level 30 , London, United Kingdom

23 April 2021, 14:38

So I thought that was the case; back to the task - was it actually necessary to write setDaemon(false)?

Guadalupe Gagnon

Level 37 , Tampa, United States

23 April 2021, 15:12

In this small task, no... but that brings me back to point #1. If the task says specifically "set the thread to daemon to false", it most likely is going to check if your code physically calls the method. The ThreadFactory class could be returning threads that have their default Daemon property as true if the calling thread also has that property as true. In this case, if you do not set the Daemon to false, the behavior of the program changes. A more pertinent example:

public static void main(String[] args) {
   Runnable r = () -> {
      while(true){
         try {
            for(int i = 0; i < 4; i++){
            System.out.print("Hot, ");
            Thread.sleep(500);
         }
            System.out.print("Staying Alive, ");
            Thread.sleep(1000);
            System.out.println("Staying Alive!!");
            Thread.sleep(1500);
         } catch (Exception e){}
      }
   };

   Runnable t = new Runnable() {
      @Override
      public void run() {
         newThread(r).start();
         newThread(r).start();
         newThread(r).start();
      }
   };
   Thread mainSpawn = new Thread(t);
   //mainSpawn.setDaemon(true); //<-try with this on/off
   mainSpawn.start();
}

private static Thread newThread(Runnable r) {
   Thread t = new Thread(r);
   /*if (t.isDaemon())
      t.setDaemon(false);*/ // <- try with this on/off
   if (t.getPriority() != Thread.NORM_PRIORITY)
      t.setPriority(Thread.NORM_PRIORITY);
   return t;
}

Try with the mainSpawn.setDaemon(true) commented on/off. Then try both of those with the t.setDaemon(false) commented on/off.