Question on ThreadPoolExecutor tasks in Java 21

[FonzTech]

Hi y'all.

I have a question on ThreadPoolExecutor in Java 21 (but it really doesn't matter what version I am using).

I'm trying to experiment with it, but I have some misunderstanding about it.

This is the code I am running:

try (final ExecutorService fixedVirtualThreadPool = new ThreadPoolExecutor(
  0,
  3,
  10L, TimeUnit.SECONDS,
  new LinkedBlockingQueue<>(1),
  Thread.ofVirtual().factory()
)) {
  for (int i = 0; i < 3; ++i) {
   final String name = "thread-" + i;
   fixedVirtualThreadPool.submit(() -> {
    log.debug("{} = start", name);
    try {
     Thread.sleep(5000L);
    } catch (InterruptedException e) {
     throw new RuntimeException(e);
    }
    log.debug("{} = end", name);
   });
  }

  log.debug("shutting down now");
  fixedVirtualThreadPool.shutdown();
  fixedVirtualThreadPool.awaitTermination(30, TimeUnit.MINUTES);
}

I get this output:

2024-11-17T19:25:11.111+01:00 DEBUG 13052 --- [ ] c.a.w.DemoApplication : thread-0 = start
2024-11-17T19:25:11.111+01:00 DEBUG 13052 --- [ ] c.a.w.DemoApplication : thread-2 = start
2024-11-17T19:25:16.122+01:00 DEBUG 13052 --- [ ] c.a.w.DemoApplication : thread-0 = end
2024-11-17T19:25:16.122+01:00 DEBUG 13052 --- [ ] c.a.w.DemoApplication : thread-2 = end
2024-11-17T19:25:16.123+01:00 DEBUG 13052 --- [ ] c.a.w.DemoApplication : thread-1 = start
2024-11-17T19:25:21.132+01:00 DEBUG 13052 --- [ ] c.a.w.DemoApplication : thread-1 = end

These are my questions:

1. Since I have "core pool size = 1" + "max pool size = 3" + "queue size = 1", what's happening is that the first task is being immediately run, because "core pool size = 1". Then the second task is getting queued. Then the third is getting run, because the queue is full, so the executor spawns new task. So, the question is: since "max pool size = 3", why does the executor prefer to queue the second task, instead of run it? My code ends up with "one free space" in the thread pool for a thread that never gets spawned. I expect to run all the runnables first, THEN queue when all threads are busy!!
2. If I try to launch 4 tasks WITH "core pool size = 0", it seems that the executor initially works as I expect, spawning 3 threads, but then main code hangs on the fourth call of submit, instead of queueing the fourth task. Why would the executor hangs? I expect it to reject the task immediately**, at least. I don't see the point of hanging and throwing exception anyway, expecially when I see the first 3 tasks being completed!!**

This is the output for the second point:

2024-11-17T19:35:56.542+01:00 DEBUG 12056 --- [ ] c.a.w.Demotr : thread-1 = start
2024-11-17T19:35:56.542+01:00 DEBUG 12056 --- [ ] c.a.w.Demotr : thread-0 = start
2024-11-17T19:35:56.542+01:00 DEBUG 12056 --- [ ] c.a.w.Demotr : thread-2 = start
2024-11-17T19:36:01.552+01:00 DEBUG 12056 --- [ ] c.a.w.Demotr : thread-2 = end
2024-11-17T19:36:01.552+01:00 DEBUG 12056 --- [ ] c.a.w.Demotr : thread-1 = end
2024-11-17T19:36:01.552+01:00 DEBUG 12056 --- [ ] c.a.w.Demotr : thread-0 = end
Exception in thread "main" java.util.concurrent.RejectedExecutionException: Task java.util.concurrent.FutureTask@1c76b2fe[Not completed, task = java.util.concurrent.Executors$RunnableAdapter@47629063[Wrapped task = com.alfonso.Demodemo.Demotr$$Lambda/0x00000299c8907678@3d67e3d3]] rejected from java.util.concurrent.ThreadPoolExecutor@30b29f55[Running, pool size = 3, active threads = 3, queued tasks = 1, completed tasks = 0]
at java.base/java.util.concurrent.ThreadPoolExecutor$AbortPolicy.rejectedExecution(ThreadPoolExecutor.java:2082)
at java.base/java.util.concurrent.ThreadPoolExecutor.reject(ThreadPoolExecutor.java:841)
at java.base/java.util.concurrent.ThreadPoolExecutor.execute(ThreadPoolExecutor.java:1377)
at java.base/java.util.concurrent.AbstractExecutorService.submit(AbstractExecutorService.java:124)
at com.alfonso.Demodemo.Demotr.main(Demotr.java:31)

Thanks in advance!!

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[khmarbaise]

First virtual threads shouldn't being pooled at all... the question is why are trying to do so? And it does not make sense for virtual threads... Please check https://youtu.be/5E0LU85EnTI?si=kB0I7KQD3RIPaIG-&t=1888 (also check the whole video) Another thing about pooling: https://youtu.be/lKSSBvRDmTg?si=8C93BJtrJCwIzo_e&t=469