1.哪些地方使用了commonPool？

• CompletableFuture
• Parallel Streams。

2.为什么要引入commonPool？

为了避免任何并行操作都引入一个线程池，最坏情况会导致在单个JVM上创建了太多的池线程，降低效率。

3.commonPool线程池是怎么创建和使用的？

ForkJoinTask一定会运行在一个ForkJoinPool中，如果没有显式地交它提交到ForkJoinPool，会使用一个common池（全进程共享）来执行任务。

3.1 parallelStream调用流程

以parallelStream为例，其ReduceTask会调用invoke：

        @Override
        public <P_IN> R evaluateParallel(PipelineHelper<T> helper,
                                         Spliterator<P_IN> spliterator) {
            return new ReduceTask<>(this, helper, spliterator).invoke().get();
        }

调用的是ForkJoinTask的invoke：

    public final V invoke() {
        int s;
        if ((s = doInvoke() & DONE_MASK) != NORMAL)
            reportException(s);
        return getRawResult();
    }

    private int doInvoke() {
        int s; Thread t; ForkJoinWorkerThread wt;
        return (s = doExec()) < 0 ? s :
            ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) ?
            (wt = (ForkJoinWorkerThread)t).pool.
            awaitJoin(wt.workQueue, this, 0L) :
            externalAwaitDone();
    }

    final int doExec() {
        int s; boolean completed;
        if ((s = status) >= 0) {
            try {
                completed = exec();
            } catch (Throwable rex) {
                return setExceptionalCompletion(rex);
            }
            if (completed)
                s = setCompletion(NORMAL);
        }
        return s;
    }

调用CountedCompleter的exec()：

    /**
     * Implements execution conventions for CountedCompleters.
     */
    protected final boolean exec() {
        compute();
        return false;
    }

调用stream包下面的AbstractTask.compute：

    @Override
    public void compute() {
        Spliterator<P_IN> rs = spliterator, ls; // right, left spliterators
        long sizeEstimate = rs.estimateSize();
        long sizeThreshold = getTargetSize(sizeEstimate);
        boolean forkRight = false;
        @SuppressWarnings("unchecked") K task = (K) this;
        while (sizeEstimate > sizeThreshold && (ls = rs.trySplit()) != null) {
            K leftChild, rightChild, taskToFork;
            task.leftChild  = leftChild = task.makeChild(ls);
            task.rightChild = rightChild = task.makeChild(rs);
            task.setPendingCount(1);
            if (forkRight) {
                forkRight = false;
                rs = ls;
                task = leftChild;
                taskToFork = rightChild;
            }
            else {
                forkRight = true;
                task = rightChild;
                taskToFork = leftChild;
            }
            taskToFork.fork();
            sizeEstimate = rs.estimateSize();
        }
        task.setLocalResult(task.doLeaf());
        task.tryComplete();
    }

调用ForkJoinTask的fork：

    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;
    }

3.2 common初始化

    static {
        // initialize field offsets for CAS etc
        try {
            U = sun.misc.Unsafe.getUnsafe();
            Class<?> k = ForkJoinPool.class;
            CTL = U.objectFieldOffset
                (k.getDeclaredField("ctl"));
            RUNSTATE = U.objectFieldOffset
                (k.getDeclaredField("runState"));
            STEALCOUNTER = U.objectFieldOffset
                (k.getDeclaredField("stealCounter"));
            Class<?> tk = Thread.class;
            PARKBLOCKER = U.objectFieldOffset
                (tk.getDeclaredField("parkBlocker"));
            Class<?> wk = WorkQueue.class;
            QTOP = U.objectFieldOffset
                (wk.getDeclaredField("top"));
            QLOCK = U.objectFieldOffset
                (wk.getDeclaredField("qlock"));
            QSCANSTATE = U.objectFieldOffset
                (wk.getDeclaredField("scanState"));
            QPARKER = U.objectFieldOffset
                (wk.getDeclaredField("parker"));
            QCURRENTSTEAL = U.objectFieldOffset
                (wk.getDeclaredField("currentSteal"));
            QCURRENTJOIN = U.objectFieldOffset
                (wk.getDeclaredField("currentJoin"));
            Class<?> ak = ForkJoinTask[].class;
            ABASE = U.arrayBaseOffset(ak);
            int scale = U.arrayIndexScale(ak);
            if ((scale & (scale - 1)) != 0)
                throw new Error("data type scale not a power of two");
            ASHIFT = 31 - Integer.numberOfLeadingZeros(scale);
        } catch (Exception e) {
            throw new Error(e);
        }

        commonMaxSpares = DEFAULT_COMMON_MAX_SPARES;
        defaultForkJoinWorkerThreadFactory =
            new DefaultForkJoinWorkerThreadFactory();
        modifyThreadPermission = new RuntimePermission("modifyThread");

        common = java.security.AccessController.doPrivileged
            (new java.security.PrivilegedAction<ForkJoinPool>() {
                public ForkJoinPool run() { return makeCommonPool(); }});
        int par = common.config & SMASK; // report 1 even if threads disabled
        commonParallelism = par > 0 ? par : 1;
    }

    /**
     * Creates and returns the common pool, respecting user settings
     * specified via system properties.
     */
    private static ForkJoinPool makeCommonPool() {
        int parallelism = -1;
        ForkJoinWorkerThreadFactory factory = null;
        UncaughtExceptionHandler handler = null;
        try {  // ignore exceptions in accessing/parsing properties
            String pp = System.getProperty
                ("java.util.concurrent.ForkJoinPool.common.parallelism");
            String fp = System.getProperty
                ("java.util.concurrent.ForkJoinPool.common.threadFactory");
            String hp = System.getProperty
                ("java.util.concurrent.ForkJoinPool.common.exceptionHandler");
            if (pp != null)
                parallelism = Integer.parseInt(pp);
            if (fp != null)
                factory = ((ForkJoinWorkerThreadFactory)ClassLoader.
                           getSystemClassLoader().loadClass(fp).newInstance());
            if (hp != null)
                handler = ((UncaughtExceptionHandler)ClassLoader.
                           getSystemClassLoader().loadClass(hp).newInstance());
        } catch (Exception ignore) {
        }
        if (factory == null) {
            if (System.getSecurityManager() == null)
                factory = defaultForkJoinWorkerThreadFactory;
            else // use security-managed default
                factory = new InnocuousForkJoinWorkerThreadFactory();
        }
        if (parallelism < 0 && // default 1 less than #cores
            (parallelism = Runtime.getRuntime().availableProcessors() - 1) <= 0)
            parallelism = 1;
        if (parallelism > MAX_CAP)
            parallelism = MAX_CAP;
        return new ForkJoinPool(parallelism, factory, handler, LIFO_QUEUE,
                                "ForkJoinPool.commonPool-worker-");
    }

使用common pool的优点就是我们可以通过指定系统参数的方式定义“并行度、线程工厂和异常处理类”。

参考

• Be Aware of ForkJoinPool#commonPool()