Demonstrate Java Parallel streams fall back to main thread in the event that all workers in common fork join pool are saturated.

ParallelStreamSaturationTest.java

package com.simplymeasured.gus.controller;

import java.util.Arrays;

/**
 * Created by kenny on 9/20/16.
 *
 * Demonstrate Java Parallel streams fall back to main thread in the event that all workers in common fork join pool
 * are saturated.
 */
public class ParallelStreamSaturationTest {

    public static void main(final String[] args) {
        // start a thread that will execute a parallel stream AFTER the common fork join pool has been saturated.
        new Thread(() -> {
            try {
                System.out.println("Sleeping until ForkJoinPool is saturated, Thread Name: " + Thread.currentThread().getName());
                Thread.sleep(3_000L);
            } catch (final InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println("Starting another parallel stream, Thread Name: " + Thread.currentThread().getName());
            Arrays.asList(1, 2, 3, 4, 5)
                  .parallelStream()
                  .forEach(id -> System.out.println("Second stream: " + id + " Thread Name: " + Thread.currentThread().getName()));

        }).start();

        // saturate common fork join pool
        Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
              .parallelStream()
            .forEach(ParallelStreamSaturationTest::run);
    }

    private static void run(final int id) {
        if (Thread.currentThread().getName().toLowerCase().contains("fork")) {
            try {
                System.out.println("Blocking " + Thread.currentThread().getName());
                Thread.sleep(10_000_000L);
            } catch (final InterruptedException e) {
                e.printStackTrace();
            }
        } else {
            System.out.println("Main thread: " + id + " Thread Name: " + Thread.currentThread().getName());
        }
    }

}

In code, ForkJoinPoo.ForkJoinTask this can also be seen:

    public final ForkJoinTask<V> fork() {
        Thread t;
        if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread)
            ((ForkJoinWorkerThread)t).workQueue.push(this);
        else
            ForkJoinPool.common.externalPush(this);
        return this;
    }

Output of program:

Sleeping until ForkJoinPool is saturated, Thread Name: Thread-0
Blocking ForkJoinPool.commonPool-worker-3
Blocking ForkJoinPool.commonPool-worker-1
Blocking ForkJoinPool.commonPool-worker-2
Main thread: 7 Thread Name: main
Blocking ForkJoinPool.commonPool-worker-6
Main thread: 4 Thread Name: main
Main thread: 10 Thread Name: main
Blocking ForkJoinPool.commonPool-worker-4
Blocking ForkJoinPool.commonPool-worker-5
Blocking ForkJoinPool.commonPool-worker-7
Starting another parallel stream, Thread Name: Thread-0
Second stream: 3 Thread Name: Thread-0
Second stream: 5 Thread Name: Thread-0
Second stream: 4 Thread Name: Thread-0
Second stream: 2 Thread Name: Thread-0
Second stream: 1 Thread Name: Thread-0