Java中的Fork/Join框架及呼叫方法

什麼是ForkJoin?

ForkJoin 從字面上看Fork是分岔的意思，Join是結合的意思，我們可以理解為將大任務拆分成小任務進行計​​算求解，最後將小任務的結果進行結合求出大任務的解，這些裂變出來的小任務，我們就可以交給不同的線程去進行計算，這也就是分散式計算的一種思想。這與大數據中的分散式離線計算MapReduce類似，對ForkJoin最經典的一個應用就是Java8中的Stream，我們知道Stream分為串行流和平行流，其中並行流parallelStream就是依賴ForkJoin來實現並行處理的。

下面我們一起來看看最核心的ForkJoinTask和ForkJoinPool。

ForkJoinTask 任務

ForkJoinTask本身的依賴關係並不複雜，它與非同步任務計算FutureTask一樣均實作了Future介面

#下面我們就ForkJoinTask的核心原始碼來研究一下，該任務是如何透過分治法進行計算。

ForkJoinTask最核心的莫過於fork()和join()方法了。

fork()

• 判斷目前執行緒是不是ForkJoinWorkerThread執行緒​​

• 是直接將目前執行緒push到工作佇列中

• #否呼叫ForkJoinPool 的externalPush方法

#在

ForkJoinPool建構了一個靜態的common對象，這裡呼叫的就是common的externalPush()

join()

• 呼叫doJoin()方法，等待執行緒執行完成

    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();	

RecursiveTask 是ForkJoinTask的一個子類別主要對取得結果的方法進行了實現，透過泛型約束結果。我們如果需要自己建立任務，仍需要實作RecursiveTask，並且去寫最核心的計算方法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;
    }

}

ForkJoinPool 執行緒池

ForkJoinTask 中許多功能都依賴ForkJoinPool執行緒池，所以說ForkJoinTask運行離不開ForkJoinPool，ForkJoinPool與ThreadPoolExecutor有許多相似之處，他是專門用相似之處執行ForkJoinTask任務的線程池，我之前也有文章對線程池技術進行了介紹，感興趣的可以進行閱讀——從java源碼分析線程池(池化技術)的實現原理

#ForkJoinPool與ThreadPoolExecutor的繼承關係幾乎是相同的，他們相當於兄弟關係。

工作竊取演算法

ForkJoinPool中採取工作竊取演算法，如果每次fork子任務如果都去建立新執行緒去處理的話，對系統資源的開銷是巨大的，所以必須採取執行緒池。一般的執行緒池只有一個任務佇列，但是對於ForkJoinPool來說，由於同一個任務Fork出的各個子任務是平行關係，為了提高效率，減少執行緒的競爭，需要將這些平行的任務放到不同的佇列中，由於線程處理不同任務的速度不同，這樣就可能存在某個線程先執行完了自己隊列中的任務，這時為了提升效率，就可以讓該線程去“

竊取”其它任務隊列中的任務，這就是所謂的「工作竊取演算法」。

對於一般的隊列來說，入隊元素都是在隊尾，出隊元素在隊首，要滿足「工作竊取」的需求，任務隊列應該支援從「隊尾」出隊元素，這樣可以減少與其它工作線程的衝突（因為其它工作線程會從隊首獲取自己任務隊列中的任務），這時就需要使用雙端阻塞隊列來解決。

建構方法

首先我們來看ForkJoinPool執行緒池的構造方法，他為我們提供了三種形式的構造，其中最為複雜的是四個入參的構造，下面我們看一下它四個入參都代表什麼？

• int parallelism 可並行層級（不代表最多存在的執行緒數量）

• ForkJoinWorkerThreadFactory factory 執行緒建立工廠

• #UncaughtExceptionHandler handler 異常捕捉處理器

• boolean asyncMode 先進先出的工作模式或後進先出的工作模式

    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();
    }

提交方法

下面我們看一下提交任務的方法：

externalPush這個方法我們很眼熟，它正是在fork的時候如果目前執行緒不是ForkJoinWorkerThread，新提交任務也是會透過這個方法去執行任務。由此可見，fork就是新建一個子任務進行提交。

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&#39;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);
        }
    }

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&#39;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;
    }

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;
    }
}

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。

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