ForkJoinPool

1. 为什么使用fork join框架？

ForkJoinPool是ExecutorSerice的一个补充，而不是替代品

并不适合所有场景；
特别适合用于“分而治之”，递归计算的算法；

JAVA8中CompeleteFuture、并发流等都是基于ForkJoinPool实现；

本文将讲述什么样的场景合适、什么样的场景不合适，并且深入剖析为啥某些场景不合适。

2. fork join框架使用简单实例
   使用RecursiveTask实现一个累加的功能，使用分而治之的思想，实现分段求和后汇总

public class SumTask extends RecursiveTask<Integer> {

    private Integer start = 0;
    private Integer end = 0;

    public SumTask(int start, int end) {
        this.start = start;
        this.end = end;
    }

    @Override
    protected Integer compute() {

        if (end - start < 100) {
            //小于100时直接返回结果
            int sumResult = 0;
            for (int i = start; i <= end; i++) {
                sumResult += i;
            }
            return sumResult;
        } else {
            //大于一百时进行分割
            int middle = (end + start) / 2;
            SumTask leftSum = new SumTask(this.start, middle);
            SumTask rightSum = new SumTask(middle, this.end);
            leftSum.fork();
            rightSum.fork();
            return leftSum.join() + rightSum.join();
        }
    }

    public static void main(String[] args) {
        SumTask sumTask = new SumTask(1, 999999);
        sumTask.fork();
        System.out.print("result:" + sumTask.join());
    }
} 

默认线程池有固定的线程数,会根据可用的availableProcessors来计算线程数量；

/**
     * Creates a {@code ForkJoinPool} with parallelism equal to {@link
     * java.lang.Runtime#availableProcessors}, using the {@linkplain
     * #defaultForkJoinWorkerThreadFactory default thread factory},
     * no UncaughtExceptionHandler, and non-async LIFO processing mode.
     *
     * @throws SecurityException if a security manager exists and
     *         the caller is not permitted to modify threads
     *         because it does not hold {@link
     *         java.lang.RuntimePermission}{@code ("modifyThread")}
     */
    public ForkJoinPool() {
        this(Math.min(MAX_CAP, Runtime.getRuntime().availableProcessors()),
             defaultForkJoinWorkerThreadFactory, null, false);
    }

如果想使用自定义线程池，比如100个线程的线程池，可以如下：
当然无论你初始化多少线程，都只会有和CPU数量相等的几个线程运行，大部分场景下并无异于这个框架的加速；

public static void main(String[] args) {
        ForkJoinPool forkJoinPool=new ForkJoinPool(100);
        SumTask sumTask = new SumTask(1, 999999);
        forkJoinPool.submit(sumTask);
        System.out.print("result:" + sumTask.join());
    }

我们来看看fork的时候做了些什么

 /**
     * Arranges to asynchronously execute this task in the pool the
     * current task is running in, if applicable, or using the {@link
     * ForkJoinPool#commonPool()} if not {@link #inForkJoinPool}.  While
     * it is not necessarily enforced, it is a usage error to fork a
     * task more than once unless it has completed and been
     * reinitialized.  Subsequent modifications to the state of this
     * task or any data it operates on are not necessarily
     * consistently observable by any thread other than the one
     * executing it unless preceded by a call to {@link #join} or
     * related methods, or a call to {@link #isDone} returning {@code
     * true}.
     *
     * @return {@code this}, to simplify usage
     */
    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;
    }

当前线程不是个ForkJoinWorkerThread的时候，则加入到ForkJoinPool线程池(基于ExecutorService实现);
如果当前线程已经是个ForkJoinWorkerThread了，则把这个任务加入到当前线程的workQueue;

大概的图
这是和普通线程池不同的地方，task并不是交给线程池中的queue，而是放到线程本地的workQueue

我们来看看ForkJoinPool中的task是如何运行的
a. 线程以LIFO先进后出方式从本地队列获取任务，执行，直到自己的队列为空；
b. 查看其他ForkJoinWorkerThread是否有未执行task，有的话通过work−stealing窃取,窃取方式为FIFO先进先出，减少竞争；优先看曾今从自己那里窃取任务的thread，如果有的话；
c. 任务运行完成时，返回结果；

3. java8并发流
   java8的并发流使用的也是ForkJoinPool

myList.parallelStream.map(obj -> longRunningOperation())

默认使用公用线程池，使用独立线程池的话如下

ForkJoinPool forkJoinPool = new ForkJoinPool(3);  
forkJoinPool.submit(() -> {  
    firstRange.parallelStream().forEach((number) -> {
        try {
            Thread.sleep(5);
        } catch (InterruptedException e) { }
    });
});
 
ForkJoinPool forkJoinPool2 = new ForkJoinPool(3);  
forkJoinPool2.submit(() -> {  
    secondRange.parallelStream().forEach((number) -> {
        try {
            Thread.sleep(5);
        } catch (InterruptedException e) {
        }
    });
});

4.为什么不适合执行有block比如有io的任务
线程block的时候，线程池会调度线程池队列中的其他未线程运行，这是ExecutorService的机制；
但是ForkJoinPool中的ForkJoinWorkerThread工作机制是不停执行本地workQueue中的task，task是一个个取的，顺序执行，没有塞回去的动作，并不会因为某个task引起block后而换个task继续执行；

参考：
Java Tip: When to use ForkJoinPool vs ExecutorService
http://blog.dyngr.com/blog/2016/09/15/java-forkjoinpool-internals/
https://www.jianshu.com/p/bd825cb89e00
https://www.cnblogs.com/richaaaard/p/6601445.html