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Dexter
Dexter

Reputation: 49

Java Multithreading - State of an Idle thread

  1. I'm new to Java Multithreading. Curious to know the state of idle thread incase of ThreadPoolExecutor. Is it in RUNNABLE/WAITING?

  2. In case the idle threads are in RUNNABLE state, how are the new tasks attached to idle threads? AFAIK we assign a runnable/callable object to thread/pool. But my question is how does ThreadPoolExecutor assign queued runnable objects to idle thread??

Upvotes: 0

Views: 587

Answers (2)

Stephen C
Stephen C

Reputation: 718826

  1. Curious to know the state of idle thread in case of ThreadPoolExecutor. Is it in RUNNABLE/WAITING?

It will be WAITING. It is waiting (in a Queue.take() call) for a new task to appear on the work queue. In the current implementations this involves a mechanism similar to wait / notify.

Your second question is therefore moot.

However, it is worth noting the following:

  1. No "idle" thread will ever be RUNNABLE.

  2. In current generation HotSpot JVMs, the actual scheduling (deciding which threads get priority and assigning them a core to run on) is handled by the operating system.

  3. In Loom JVMs (Loom is still an Incubator project), light-weight virtual threads ("fibres") are scheduled (to a native thread) by the JVM rather than the OS.

Upvotes: 0

tgdavies
tgdavies

Reputation: 11421

It's easy enough to find out:

import java.util.concurrent.Executor;
import java.util.concurrent.Executors;
import java.io.IOException;

public class ThreadExample {

    public static void main(String[] args) throws IOException {
        Executor executor = Executors.newFixedThreadPool(5);
        // force the threads to be started
        for (int i = 0; i < 5; i++) {
            executor.execute(() -> {
                try {Thread.sleep(1000);} catch (InterruptedException e) {
                }
            });
        }
        // don't terminate
        System.in.read();
    }
}

Run it:

$ javac ThreadExample.java
$ java ThreadExample

In another console, having waited at least one second for the tasks to complete:

$ ps
  PID TTY           TIME CMD
 3640 ttys000    0:00.25 -bash
 5792 ttys000    0:00.15 java ThreadExample
 5842 ttys001    0:00.05 -bash
$ jstack 5792
...
"pool-1-thread-1" #12 prio=5 os_prio=31 cpu=1.77ms elapsed=13.37s tid=0x00007fe99f833800 nid=0xa203 waiting on condition  [0x00007000094b2000]
   java.lang.Thread.State: WAITING (parking)
    at jdk.internal.misc.Unsafe.park([email protected]/Native Method)
    - parking to wait for  <0x000000061ff9e998> (a java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject)
    at java.util.concurrent.locks.LockSupport.park([email protected]/LockSupport.java:194)
    at java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject.await([email protected]/AbstractQueuedSynchronizer.java:2081)
    at java.util.concurrent.LinkedBlockingQueue.take([email protected]/LinkedBlockingQueue.java:433)
    at java.util.concurrent.ThreadPoolExecutor.getTask([email protected]/ThreadPoolExecutor.java:1054)
    at java.util.concurrent.ThreadPoolExecutor.runWorker([email protected]/ThreadPoolExecutor.java:1114)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run([email protected]/ThreadPoolExecutor.java:628)
    at java.lang.Thread.run([email protected]/Thread.java:834)
...

All the pool threads are in that state.

Upvotes: 1

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