你了解Java中的ForkJoin吗？

我就是我 2023-09-23 18:52 143阅读 0赞

### 什么是ForkJoin? ###

ForkJoin 从字面上看Fork是分岔的意思，Join是结合的意思，我们可以理解为将大任务拆分成小任务进行计算求解，最后将小任务的结果进行结合求出大任务的解，这些裂变出来的小任务，我们就可以交给不同的线程去进行计算，这也就是分布式计算的一种思想。这与大数据中的分布式离线计算MapReduce类似，对ForkJoin最经典的一个应用就是Java8中的Stream，我们知道Stream分为串行流和并行流，其中并行流parallelStream就是依赖于ForkJoin来实现并行处理的。

下面我们一起来看一下最为核心的ForkJoinTask和ForkJoinPool。

### ForkJoinTask 任务 ###

ForkJoinTask本身的依赖关系并不复杂，它与异步任务计算FutureTask一样均实现了Future接口，FutureTask我们在之前的文章中有讲到感兴趣的可以阅读一下——[从源码看异步任务计算FutureTask][FutureTask]。

![22c29b6febfbc9954f651a6d5ba8f3e2.png][]

下面我们就ForkJoinTask的核心源码来研究一下，该任务是如何通过分治法进行计算。

ForkJoinTask最核心的莫过于fork()和join()方法了。

*  fork()
    
     *  判断当前线程是不是ForkJoinWorkerThread线程
        
         *  是 直接将当前线程push到工作队列中
         *  否 调用ForkJoinPool 的externalPush方法
            
            在ForkJoinPool构建了一个静态的common对象，这里调用的就是common的externalPush()
 *  join()
    
     *  调用doJoin()方法，等待线程执行完成

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;
        }
    
        public final V join() {
            int s;
            if ((s = doJoin() & DONE_MASK) != NORMAL)
                reportException(s);
            return getRawResult();
        }
    
        private int doJoin() {
            int s; Thread t; ForkJoinWorkerThread wt; ForkJoinPool.WorkQueue w;
            return (s = status) < 0 ? s :
                ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) ?
                (w = (wt = (ForkJoinWorkerThread)t).workQueue).
                tryUnpush(this) && (s = doExec()) < 0 ? s :
                wt.pool.awaitJoin(w, this, 0L) :
                externalAwaitDone();
        }
    
    	// 获取结果的方法由子类实现
    	public abstract V getRawResult();	
    复制代码

RecursiveTask 是ForkJoinTask的一个子类主要对获取结果的方法进行了实现，通过泛型约束结果。我们如果需要自己创建任务，仍需要实现RecursiveTask，并去编写最为核心的计算方法compute()。

public abstract class RecursiveTask<V> extends ForkJoinTask<V> {
        private static final long serialVersionUID = 5232453952276485270L;
    
        V result;
    
        protected abstract V compute();
    
        public final V getRawResult() {
            return result;
        }
    
        protected final void setRawResult(V value) {
            result = value;
        }
        
        protected final boolean exec() {
            result = compute();
            return true;
        }
    
    }
    复制代码

### ForkJoinPool 线程池 ###

ForkJoinTask 中许多功能都依赖于ForkJoinPool线程池，所以说ForkJoinTask运行离不开ForkJoinPool，ForkJoinPool与ThreadPoolExecutor有许多相似之处，他是专门用来执行ForkJoinTask任务的线程池，我之前也有文章对线程池技术进行了介绍，感兴趣的可以进行阅读——[从源码分析线程池(池化技术)的实现原理][Link 1]。

ForkJoinPool与ThreadPoolExecutor的继承关系几乎是相同的，他们相当于兄弟关系。

![f944364e91a5fa6243ec01182f1a0f9c.png][]

#### 工作窃取算法 ####

ForkJoinPool中采取工作窃取算法，如果每次fork子任务如果都去创建新线程去处理的话，对系统资源的开销是巨大的，所以必须采取线程池。一般的线程池只有一个任务队列，但是对于ForkJoinPool来说，由于同一个任务Fork出的各个子任务是平行关系，为了提高效率，减少线程的竞争，需要将这些平行的任务放到不同的队列中，由于线程处理不同任务的速度不同，这样就可能存在某个线程先执行完了自己队列中的任务，这时为了提升效率，就可以让该线程去“**窃取**”其它任务队列中的任务，这就是所谓的“**工作窃取算法**”。

对于一般的队列来说，入队元素都是在队尾，出队元素在队首，要满足“工作窃取”的需求，任务队列应该支持从“队尾”出队元素，这样可以减少与其它工作线程的冲突（因为其它工作线程会从队首获取自己任务队列中的任务），这时就需要使用双端阻塞队列来解决。

#### 构造方法 ####

首先我们来看ForkJoinPool线程池的构造方法，他为我们提供了三种形式的构造，其中最为复杂的是四个入参的构造，下面我们看一下它四个入参都代表什么？

*  int parallelism 可并行级别（不代表最多存在的线程数量）
 *  ForkJoinWorkerThreadFactory factory 线程创建工厂
 *  UncaughtExceptionHandler handler 异常捕获处理器
 *  boolean asyncMode 先进先出的工作模式 或者 后进先出的工作模式

public ForkJoinPool() {
            this(Math.min(MAX_CAP, Runtime.getRuntime().availableProcessors()),
                 defaultForkJoinWorkerThreadFactory, null, false);
        }
    
    	public ForkJoinPool(int parallelism) {
            this(parallelism, defaultForkJoinWorkerThreadFactory, null, false);
        }
    
    	public ForkJoinPool(int parallelism,
                            ForkJoinWorkerThreadFactory factory,
                            UncaughtExceptionHandler handler,
                            boolean asyncMode) {
            this(checkParallelism(parallelism),
                 checkFactory(factory),
                 handler,
                 asyncMode ? FIFO_QUEUE : LIFO_QUEUE,
                 "ForkJoinPool-" + nextPoolId() + "-worker-");
            checkPermission();
        }
    复制代码

#### 提交方法 ####

下面我们看一下提交任务的方法

externalPush这个方法我们很眼熟，它正是在fork的时候如果当前线程不是ForkJoinWorkerThread，新提交任务也是会通过这个方法去执行任务。由此可见，fork就是新建一个子任务进行提交。

externalSubmit是最为核心的一个方法，它可以首次向池提交第一个任务，并执行二次初始化。它还可以检测外部线程的首次提交，并创建一个新的共享队列。

signalWork(ws, q)是发送工作信号，让工作队列进行运转。

public ForkJoinTask<?> submit(Runnable task) {
            if (task == null)
                throw new NullPointerException();
            ForkJoinTask<?> job;
            if (task instanceof ForkJoinTask<?>) // avoid re-wrap
                job = (ForkJoinTask<?>) task;
            else
                job = new ForkJoinTask.AdaptedRunnableAction(task);
            externalPush(job);
            return job;
        }
    
        final void externalPush(ForkJoinTask<?> task) {
            WorkQueue[] ws; WorkQueue q; int m;
            int r = ThreadLocalRandom.getProbe();
            int rs = runState;
            if ((ws = workQueues) != null && (m = (ws.length - 1)) >= 0 &&
                (q = ws[m & r & SQMASK]) != null && r != 0 && rs > 0 &&
                U.compareAndSwapInt(q, QLOCK, 0, 1)) {
                ForkJoinTask<?>[] a; int am, n, s;
                if ((a = q.array) != null &&
                    (am = a.length - 1) > (n = (s = q.top) - q.base)) {
                    int j = ((am & s) << ASHIFT) + ABASE;
                    U.putOrderedObject(a, j, task);
                    U.putOrderedInt(q, QTOP, s + 1);
                    U.putOrderedInt(q, QLOCK, 0);
                    if (n <= 1)
                        signalWork(ws, q);
                    return;
                }
                U.compareAndSwapInt(q, QLOCK, 1, 0);
            }
            externalSubmit(task);
        }
    
        private void externalSubmit(ForkJoinTask<?> task) {
            int r;                                    // initialize caller's probe
            if ((r = ThreadLocalRandom.getProbe()) == 0) {
                ThreadLocalRandom.localInit();
                r = ThreadLocalRandom.getProbe();
            }
            for (;;) {
                WorkQueue[] ws; WorkQueue q; int rs, m, k;
                boolean move = false;
                if ((rs = runState) < 0) {
                    tryTerminate(false, false);     // help terminate
                    throw new RejectedExecutionException();
                }
                else if ((rs & STARTED) == 0 ||     // initialize
                         ((ws = workQueues) == null || (m = ws.length - 1) < 0)) {
                    int ns = 0;
                    rs = lockRunState();
                    try {
                        if ((rs & STARTED) == 0) {
                            U.compareAndSwapObject(this, STEALCOUNTER, null,
                                                   new AtomicLong());
                            // create workQueues array with size a power of two
                            int p = config & SMASK; // ensure at least 2 slots
                            int n = (p > 1) ? p - 1 : 1;
                            n |= n >>> 1; n |= n >>> 2;  n |= n >>> 4;
                            n |= n >>> 8; n |= n >>> 16; n = (n + 1) << 1;
                            workQueues = new WorkQueue[n];
                            ns = STARTED;
                        }
                    } finally {
                        unlockRunState(rs, (rs & ~RSLOCK) | ns);
                    }
                }
                else if ((q = ws[k = r & m & SQMASK]) != null) {
                    if (q.qlock == 0 && U.compareAndSwapInt(q, QLOCK, 0, 1)) {
                        ForkJoinTask<?>[] a = q.array;
                        int s = q.top;
                        boolean submitted = false; // initial submission or resizing
                        try {                      // locked version of push
                            if ((a != null && a.length > s + 1 - q.base) ||
                                (a = q.growArray()) != null) {
                                int j = (((a.length - 1) & s) << ASHIFT) + ABASE;
                                U.putOrderedObject(a, j, task);
                                U.putOrderedInt(q, QTOP, s + 1);
                                submitted = true;
                            }
                        } finally {
                            U.compareAndSwapInt(q, QLOCK, 1, 0);
                        }
                        if (submitted) {
                            signalWork(ws, q);
                            return;
                        }
                    }
                    move = true;                   // move on failure
                }
                else if (((rs = runState) & RSLOCK) == 0) { // create new queue
                    q = new WorkQueue(this, null);
                    q.hint = r;
                    q.config = k | SHARED_QUEUE;
                    q.scanState = INACTIVE;
                    rs = lockRunState();           // publish index
                    if (rs > 0 &&  (ws = workQueues) != null &&
                        k < ws.length && ws[k] == null)
                        ws[k] = q;                 // else terminated
                    unlockRunState(rs, rs & ~RSLOCK);
                }
                else
                    move = true;                   // move if busy
                if (move)
                    r = ThreadLocalRandom.advanceProbe(r);
            }
        }
    复制代码

#### 创建工人（线程） ####

提交任务后，通过signalWork(ws, q)方法，发送工作信号，当符合没有执行完毕，且没有出现异常的条件下，循环执行任务，根据控制变量尝试添加工人（线程），通过线程工厂，生成线程，并且启动线程，也控制着工人（线程）的下岗。

final void signalWork(WorkQueue[] ws, WorkQueue q) {
            long c; int sp, i; WorkQueue v; Thread p;
            while ((c = ctl) < 0L) {                       // too few active
                if ((sp = (int)c) == 0) {                  // no idle workers
                    if ((c & ADD_WORKER) != 0L)            // too few workers
                        tryAddWorker(c);
                    break;
                }
                if (ws == null)                            // unstarted/terminated
                    break;
                if (ws.length <= (i = sp & SMASK))         // terminated
                    break;
                if ((v = ws[i]) == null)                   // terminating
                    break;
                int vs = (sp + SS_SEQ) & ~INACTIVE;        // next scanState
                int d = sp - v.scanState;                  // screen CAS
                long nc = (UC_MASK & (c + AC_UNIT)) | (SP_MASK & v.stackPred);
                if (d == 0 && U.compareAndSwapLong(this, CTL, c, nc)) {
                    v.scanState = vs;                      // activate v
                    if ((p = v.parker) != null)
                        U.unpark(p);
                    break;
                }
                if (q != null && q.base == q.top)          // no more work
                    break;
            }
        }
    
        private void tryAddWorker(long c) {
            boolean add = false;
            do {
                long nc = ((AC_MASK & (c + AC_UNIT)) |
                           (TC_MASK & (c + TC_UNIT)));
                if (ctl == c) {
                    int rs, stop;                 // check if terminating
                    if ((stop = (rs = lockRunState()) & STOP) == 0)
                        add = U.compareAndSwapLong(this, CTL, c, nc);
                    unlockRunState(rs, rs & ~RSLOCK);
                    if (stop != 0)
                        break;
                    if (add) {
                        createWorker();
                        break;
                    }
                }
            } while (((c = ctl) & ADD_WORKER) != 0L && (int)c == 0);
        }
    
        private boolean createWorker() {
            ForkJoinWorkerThreadFactory fac = factory;
            Throwable ex = null;
            ForkJoinWorkerThread wt = null;
            try {
                if (fac != null && (wt = fac.newThread(this)) != null) {
                    wt.start();
                    return true;
                }
            } catch (Throwable rex) {
                ex = rex;
            }
            deregisterWorker(wt, ex);
            return false;
        }
    
       final void deregisterWorker(ForkJoinWorkerThread wt, Throwable ex) {
            WorkQueue w = null;
            if (wt != null && (w = wt.workQueue) != null) {
                WorkQueue[] ws;                           // remove index from array
                int idx = w.config & SMASK;
                int rs = lockRunState();
                if ((ws = workQueues) != null && ws.length > idx && ws[idx] == w)
                    ws[idx] = null;
                unlockRunState(rs, rs & ~RSLOCK);
            }
            long c;                                       // decrement counts
            do {} while (!U.compareAndSwapLong
                         (this, CTL, c = ctl, ((AC_MASK & (c - AC_UNIT)) |
                                               (TC_MASK & (c - TC_UNIT)) |
                                               (SP_MASK & c))));
            if (w != null) {
                w.qlock = -1;                             // ensure set
                w.transferStealCount(this);
                w.cancelAll();                            // cancel remaining tasks
            }
            for (;;) {                                    // possibly replace
                WorkQueue[] ws; int m, sp;
                if (tryTerminate(false, false) || w == null || w.array == null ||
                    (runState & STOP) != 0 || (ws = workQueues) == null ||
                    (m = ws.length - 1) < 0)              // already terminating
                    break;
                if ((sp = (int)(c = ctl)) != 0) {         // wake up replacement
                    if (tryRelease(c, ws[sp & m], AC_UNIT))
                        break;
                }
                else if (ex != null && (c & ADD_WORKER) != 0L) {
                    tryAddWorker(c);                      // create replacement
                    break;
                }
                else                                      // don't need replacement
                    break;
            }
            if (ex == null)                               // help clean on way out
                ForkJoinTask.helpExpungeStaleExceptions();
            else                                          // rethrow
                ForkJoinTask.rethrow(ex);
        }
    
        public static interface ForkJoinWorkerThreadFactory {
            public ForkJoinWorkerThread newThread(ForkJoinPool pool);
        }
        static final class DefaultForkJoinWorkerThreadFactory
            implements ForkJoinWorkerThreadFactory {
            public final ForkJoinWorkerThread newThread(ForkJoinPool pool) {
                return new ForkJoinWorkerThread(pool);
            }
        }
        protected ForkJoinWorkerThread(ForkJoinPool pool) {
            // Use a placeholder until a useful name can be set in registerWorker
            super("aForkJoinWorkerThread");
            this.pool = pool;
            this.workQueue = pool.registerWorker(this);
        }
    
        final WorkQueue registerWorker(ForkJoinWorkerThread wt) {
            UncaughtExceptionHandler handler;
            wt.setDaemon(true);                           // configure thread
            if ((handler = ueh) != null)
                wt.setUncaughtExceptionHandler(handler);
            WorkQueue w = new WorkQueue(this, wt);
            int i = 0;                                    // assign a pool index
            int mode = config & MODE_MASK;
            int rs = lockRunState();
            try {
                WorkQueue[] ws; int n;                    // skip if no array
                if ((ws = workQueues) != null && (n = ws.length) > 0) {
                    int s = indexSeed += SEED_INCREMENT;  // unlikely to collide
                    int m = n - 1;
                    i = ((s << 1) | 1) & m;               // odd-numbered indices
                    if (ws[i] != null) {                  // collision
                        int probes = 0;                   // step by approx half n
                        int step = (n <= 4) ? 2 : ((n >>> 1) & EVENMASK) + 2;
                        while (ws[i = (i + step) & m] != null) {
                            if (++probes >= n) {
                                workQueues = ws = Arrays.copyOf(ws, n <<= 1);
                                m = n - 1;
                                probes = 0;
                            }
                        }
                    }
                    w.hint = s;                           // use as random seed
                    w.config = i | mode;
                    w.scanState = i;                      // publication fence
                    ws[i] = w;
                }
            } finally {
                unlockRunState(rs, rs & ~RSLOCK);
            }
            wt.setName(workerNamePrefix.concat(Integer.toString(i >>> 1)));
            return w;
        }
    复制代码

### 例：ForkJoinTask实现归并排序 ###

这里我们就用经典的归并排序为例，构建一个我们自己的ForkJoinTask，按照归并排序的思路，重写其核心的compute()方法，通过ForkJoinPool.submit(task)提交任务，通过get()同步获取任务执行结果。

package com.zhj.interview;
    
    import java.util.*;
    import java.util.concurrent.ExecutionException;
    import java.util.concurrent.ForkJoinPool;
    import java.util.concurrent.RecursiveTask;
    
    public class Test16 {
    
        public static void main(String[] args) throws ExecutionException, InterruptedException {
            int[] bigArr = new int[10000000];
            for (int i = 0; i < 10000000; i++) {
                bigArr[i] = (int) (Math.random() * 10000000);
            }
            ForkJoinPool forkJoinPool = new ForkJoinPool();
            MyForkJoinTask task = new MyForkJoinTask(bigArr);
            long start = System.currentTimeMillis();
            forkJoinPool.submit(task).get();
            long end = System.currentTimeMillis();
            System.out.println("耗时：" + (end-start));
    	}
    
    }
    class MyForkJoinTask extends RecursiveTask<int[]> {
    
        private int source[];
    
        public MyForkJoinTask(int source[]) {
            if (source == null) {
                throw new RuntimeException("参数有误！！！");
            }
            this.source = source;
        }
    
        @Override
        protected int[] compute() {
            int l = source.length;
            if (l < 2) {
                return Arrays.copyOf(source, l);
            }
            if (l == 2) {
                if (source[0] > source[1]) {
                    int[] tar = new int[2];
                    tar[0] = source[1];
                    tar[1] = source[0];
                    return tar;
                } else {
                    return Arrays.copyOf(source, l);
                }
            }
            if (l > 2) {
                int mid = l / 2;
                MyForkJoinTask task1 = new MyForkJoinTask(Arrays.copyOf(source, mid));
                task1.fork();
                MyForkJoinTask task2 = new MyForkJoinTask(Arrays.copyOfRange(source, mid, l));
                task2.fork();
                int[] res1 = task1.join();
                int[] res2 = task2.join();
                int tar[] = merge(res1, res2);
                return tar;
            }
            return null;
        }
    	// 合并数组
        private int[] merge(int[] res1, int[] res2) {
            int l1 = res1.length;
            int l2 = res2.length;
            int l = l1 + l2;
            int tar[] = new int[l];
            for (int i = 0, i1 = 0, i2 = 0; i < l; i++) {
                int v1 = i1 >= l1 ? Integer.MAX_VALUE : res1[i1];
                int v2 = i2 >= l2 ? Integer.MAX_VALUE : res2[i2];
                // 如果条件成立，说明应该取数组array1中的值
                if(v1 < v2) {
                    tar[i] = v1;
                    i1++;
                } else {
                    tar[i] = v2;
                    i2++;
                }
            }
            return tar;
        }
    }
    复制代码

### ForkJoin计算流程 ###

通过ForkJoinPool提交任务，获取结果流程如下，拆分子任务不一定是二分的形式，可参照MapReduce的模式，也可以按照具体需求进行灵活的设计。

![7a11adb2009edbc3afcb8670787194d8.png][]

[FutureTask]: https://juejin.cn/post/7062980018208669709
[22c29b6febfbc9954f651a6d5ba8f3e2.png]: https://img-blog.csdnimg.cn/img_convert/22c29b6febfbc9954f651a6d5ba8f3e2.png
[Link 1]: https://juejin.cn/post/7063379162437779469
[f944364e91a5fa6243ec01182f1a0f9c.png]: https://img-blog.csdnimg.cn/img_convert/f944364e91a5fa6243ec01182f1a0f9c.png
[7a11adb2009edbc3afcb8670787194d8.png]: https://img-blog.csdnimg.cn/img_convert/7a11adb2009edbc3afcb8670787194d8.png