在嵌套的 Java 8 并行流操作中使用信号量可能会发生死锁。这是一个错误吗？

请考虑以下情况：我们使用 Java 8 并行流来执行并行 forEach 循环，例如：

IntStream.range(0,20).parallel().forEach(i -> { /* work done here */})

并行线程的数量由系统属性“java.util.concurrent.ForkJoinPool.common.parallelism”控制，通常等于处理器的数量。

现在假设我们想要限制特定工作的并行执行次数 - 例如，因为该部分是内存密集型的，内存约束意味着并行执行的限制。

限制并行执行的一种明显而优雅的方法是使用信号量（此处建议），例如，以下代码图片将并行执行次数限制为5：

        final Semaphore concurrentExecutions = new Semaphore(5);
        IntStream.range(0,20).parallel().forEach(i -> {

            concurrentExecutions.acquireUninterruptibly();

            try {
                /* WORK DONE HERE */
            }
            finally {
                concurrentExecutions.release();
            }
        });

这工作得很好！

但是：在工作程序（at ）内使用任何其他并行流可能会导致死锁。/* WORK DONE HERE */

对我来说，这是一个意想不到的行为。

说明： 由于 Java 流使用 ForkJoin 池，因此内部 forEach 正在分叉，并且联接似乎在等待永远。但是，此行为仍然是意外的。请注意，如果设置为 1，则并行流甚至可以工作。"java.util.concurrent.ForkJoinPool.common.parallelism"

另请注意，如果存在内部平行 forEach，则它可能不透明。

问：此行为是否符合 Java 8 规范（在这种情况下，这意味着禁止在并行流工作程序中使用信号量），还是这是一个错误？

为方便起见：下面是一个完整的测试用例。两个布尔值的任意组合都有效，但“true，true”除外，这会导致死锁。

澄清：为了明确这一点，让我强调一个方面：死锁不会发生在信号量处。请注意，代码由acquire

1. 获取信号量
2. 运行一些代码
3. 释放信号量

并且死锁发生在 2 处。如果该段代码正在使用另一个并行流。然后死锁发生在该 OTHER 流中。因此，似乎不允许同时使用嵌套的并行流和阻塞操作（如信号量）！

请注意，有文档记录表明并行流使用ForkJoinPool，并且ForkJoinPool和Semaphore属于同一个包 - （所以人们会期望它们很好地互操作）。java.util.concurrent

/*
 * (c) Copyright Christian P. Fries, Germany. All rights reserved. Contact: email@christian-fries.de.
 *
 * Created on 03.05.2014
 */
package net.finmath.experiments.concurrency;

import java.util.concurrent.Semaphore;
import java.util.stream.IntStream;

/**
 * This is a test of Java 8 parallel streams.
 * 
 * The idea behind this code is that the Semaphore concurrentExecutions
 * should limit the parallel executions of the outer forEach (which is an
 * <code>IntStream.range(0,numberOfTasks).parallel().forEach</code> (for example:
 * the parallel executions of the outer forEach should be limited due to a
 * memory constrain).
 * 
 * Inside the execution block of the outer forEach we use another parallel stream
 * to create an inner forEach. The number of concurrent
 * executions of the inner forEach is not limited by us (it is however limited by a
 * system property "java.util.concurrent.ForkJoinPool.common.parallelism").
 * 
 * Problem: If the semaphore is used AND the inner forEach is active, then
 * the execution will be DEADLOCKED.
 * 
 * Note: A practical application is the implementation of the parallel
 * LevenbergMarquardt optimizer in
 * {@link http://finmath.net/java/finmath-lib/apidocs/net/finmath/optimizer/LevenbergMarquardt.html}
 * In one application the number of tasks in the outer and inner loop is very large (>1000)
 * and due to memory limitation the outer loop should be limited to a small (5) number
 * of concurrent executions.
 * 
 * @author Christian Fries
 */
public class ForkJoinPoolTest {

    public static void main(String[] args) {

        // Any combination of the booleans works, except (true,true)
        final boolean isUseSemaphore    = true;
        final boolean isUseInnerStream  = true;

        final int       numberOfTasksInOuterLoop = 20;              // In real applications this can be a large number (e.g. > 1000).
        final int       numberOfTasksInInnerLoop = 100;             // In real applications this can be a large number (e.g. > 1000).
        final int       concurrentExecusionsLimitInOuterLoop = 5;
        final int       concurrentExecutionsLimitForStreams = 10;

        final Semaphore concurrentExecutions = new Semaphore(concurrentExecusionsLimitInOuterLoop);

        System.setProperty("java.util.concurrent.ForkJoinPool.common.parallelism",Integer.toString(concurrentExecutionsLimitForStreams));
        System.out.println("java.util.concurrent.ForkJoinPool.common.parallelism = " + System.getProperty("java.util.concurrent.ForkJoinPool.common.parallelism"));

        IntStream.range(0,numberOfTasksInOuterLoop).parallel().forEach(i -> {

            if(isUseSemaphore) {
                concurrentExecutions.acquireUninterruptibly();
            }

            try {
                System.out.println(i + "\t" + concurrentExecutions.availablePermits() + "\t" + Thread.currentThread());

                if(isUseInnerStream) {
                    runCodeWhichUsesParallelStream(numberOfTasksInInnerLoop);
                }
                else {
                    try {
                        Thread.sleep(10*numberOfTasksInInnerLoop);
                    } catch (Exception e) {
                    }
                }
            }
            finally {
                if(isUseSemaphore) {
                    concurrentExecutions.release();
                }
            }
        });

        System.out.println("D O N E");
    }

    /**
     * Runs code in a parallel forEach using streams.
     * 
     * @param numberOfTasksInInnerLoop Number of tasks to execute.
     */
    private static void runCodeWhichUsesParallelStream(int numberOfTasksInInnerLoop) {
        IntStream.range(0,numberOfTasksInInnerLoop).parallel().forEach(j -> {
            try {
                Thread.sleep(10);
            } catch (Exception e) {
            }
        });
    }
}