java线程池(六)：ForkJoinPool源码分析之二(WorkQueue源码）

/**
 * Queues supporting work-stealing as well as external task
 * submission. See above for descriptions and algorithms.
 * Performance on most platforms is very sensitive to placement of
 * instances of both WorkQueues and their arrays -- we absolutely
 * do not want multiple WorkQueue instances or multiple queue
 * arrays sharing cache lines. The @Contended annotation alerts
 * JVMs to try to keep instances apart.
 */
@sun.misc.Contended
static final class WorkQueue {
}

/**
 * Capacity of work-stealing queue array upon initialization.
 * Must be a power of two; at least 4, but should be larger to
 * reduce or eliminate cacheline sharing among queues.
 * Currently, it is much larger, as a partial workaround for
 * the fact that JVMs often place arrays in locations that
 * share GC bookkeeping (especially cardmarks) such that
 * per-write accesses encounter serious memory contention.
 */
static final int INITIAL_QUEUE_CAPACITY = 1 << 13;

/**
 * Maximum size for queue arrays. Must be a power of two less
 * than or equal to 1 << (31 - width of array entry) to ensure
 * lack of wraparound of index calculations, but defined to a
 * value a bit less than this to help users trap runaway
 * programs before saturating systems.
 */
static final int MAXIMUM_QUEUE_CAPACITY = 1 << 26; // 64M

volatile int scanState;    // versioned, <0: inactive; odd:scanning
int stackPred;             // pool stack (ctl) predecessor
int nsteals;               // number of steals
int hint;                  // randomization and stealer index hint
int config;                // pool index and mode
volatile int qlock;        // 1: locked, < 0: terminate; else 0
volatile int base;         // index of next slot for poll
int top;                   // index of next slot for push
ForkJoinTask<?>[] array;   // the elements (initially unallocated)
final ForkJoinPool pool;   // the containing pool (may be null)
final ForkJoinWorkerThread owner; // owning thread or null if shared
volatile Thread parker;    // == owner during call to park; else null
volatile ForkJoinTask<?> currentJoin;  // task being joined in awaitJoin
volatile ForkJoinTask<?> currentSteal; // mainly used by helpStealer

 WorkQueue(ForkJoinPool pool, ForkJoinWorkerThread owner) {
        this.pool = pool;
        this.owner = owner;
        // Place indices in the center of array (that is not yet allocated)
        base = top = INITIAL_QUEUE_CAPACITY >>> 1;
}

/**
 * Pushes a task. Call only by owner in unshared queues.  (The
 * shared-queue version is embedded in method externalPush.)
 *
 * @param task the task. Caller must ensure non-null.
 * @throws RejectedExecutionException if array cannot be resized
 */
final void push(ForkJoinTask<?> task) {
    ForkJoinTask<?>[] a; ForkJoinPool p;
    int b = base, s = top, n;
    //判断array不为空
    if ((a = array) != null) {    // ignore if queue removed
        //m为最高为位置的index
        int m = a.length - 1;     // fenced write for task visibility
        //将task采用cas的方式，put到数组中的top+1的位置，下面代码的一大堆操作实际上是与cas相关的，需要计算类对象中的偏移量，如果我们不用usafe类，那么这个地方就会非常简单。
        U.putOrderedObject(a, ((m & s) << ASHIFT) + ABASE, task);
        //采用cas的方式，将top的指针加1
        U.putOrderedInt(this, QTOP, s + 1);
        //如果n小于等于1则 且poll不为空 则触发worker窃取或者产生新的worker
        if ((n = s - b) <= 1) {
            if ((p = pool) != null)
                p.signalWork(p.workQueues, this);
        }
        //如果n大于等于了m 则说明需要扩容了
        else if (n >= m)
            growArray();
    }
}

/**
 * Initializes or doubles the capacity of array. Call either
 * by owner or with lock held -- it is OK for base, but not
 * top, to move while resizings are in progress.
 */
final ForkJoinTask<?>[] growArray() {
    //旧的数组 oldA
    ForkJoinTask<?>[] oldA = array;
    //如果oldA不为空，则size就为oldA的长度*2，反之说明数组没有被初始化，那么长度就应该为初始化的长度8192
    int size = oldA != null ? oldA.length << 1 : INITIAL_QUEUE_CAPACITY;
    //如果size比允许的最大容量还大，那么此时会抛出异常
    if (size > MAXIMUM_QUEUE_CAPACITY)
        throw new RejectedExecutionException("Queue capacity exceeded");
    int oldMask, t, b;
    //array a 为根据size new出来的一个新的数组
    ForkJoinTask<?>[] a = array = new ForkJoinTask<?>[size];
    //如果oldA不为空且其长度大于等于0为有效数组，且top-base大于0 说明不为空
    if (oldA != null && (oldMask = oldA.length - 1) >= 0 &&
        (t = top) - (b = base) > 0) {
        //按size定义掩码
        int mask = size - 1;
        //从旧的数组中poll全部task，然后push到新的array中
        do { // emulate poll from old array, push to new array
            ForkJoinTask<?> x;
            //采用unsafe操作
            int oldj = ((b & oldMask) << ASHIFT) + ABASE;
            int j    = ((b &    mask) << ASHIFT) + ABASE;
            //实际上直接进行的内存对象copy，这样效率比循环调用push和poll要高很多
            x = (ForkJoinTask<?>)U.getObjectVolatile(oldA, oldj);
            //判断  x不为空 则使用unsafe进行操作
            if (x != null &&
                U.compareAndSwapObject(oldA, oldj, x, null))
                U.putObjectVolatile(a, j, x);
        } while (++b != t);
    }
    //返回新的数组
    return a;
}

/**
 * Takes next task, if one exists, in LIFO order.  Call only
 * by owner in unshared queues.
 */
final ForkJoinTask<?> pop() {
    ForkJoinTask<?>[] a; ForkJoinTask<?> t; int m;
    //如果array不为空切长度大于0
    if ((a = array) != null && (m = a.length - 1) >= 0) {
        //循环，s为top的指针减1，即top减1之后要大于0 也就是说要存在task
        for (int s; (s = top - 1) - base >= 0;) {
            //计算unsafe的偏移量 得到s的位置
            long j = ((m & s) << ASHIFT) + ABASE;
            //如果这个索引处的对象为空，则退出
            if ((t = (ForkJoinTask<?>)U.getObject(a, j)) == null)
                break;
            //反之用usafe的方法将这个值取走，之后返回，并更新top的指针
            if (U.compareAndSwapObject(a, j, t, null)) {
                U.putOrderedInt(this, QTOP, s);
                return t;
            }
        }
    }
    return null;
}

/**
 * Takes next task, if one exists, in FIFO order.
 */
final ForkJoinTask<?> poll() {
    ForkJoinTask<?>[] a; int b; ForkJoinTask<?> t;
    //判断 base-top小于0说明存在task 切array不为空
    while ((b = base) - top < 0 && (a = array) != null) {
        //计算出unsafe操作的索引 实际上就是拿到b
        int j = (((a.length - 1) & b) << ASHIFT) + ABASE;
        //之后拿到这个task 用volatile的方式
        t = (ForkJoinTask<?>)U.getObjectVolatile(a, j);
        //之后如果base和b相等
        if (base == b) {
           //如果拿到的task不为空
            if (t != null) {
                //那么将这个位置的元素移除 base+1 然后返回t
                if (U.compareAndSwapObject(a, j, t, null)) {
                    base = b + 1;
                    return t;
                }
            }
            //在上述操作之后，如果base比top小1说明已经为空了 直接退出循环
            else if (b + 1 == top) // now empty
                break;
        }
    }
    //默认返回null
    return null;
}

/**
 * Takes a task in FIFO order if b is base of queue and a task
 * can be claimed without contention. Specialized versions
 * appear in ForkJoinPool methods scan and helpStealer.
 */
final ForkJoinTask<?> pollAt(int b) {
    ForkJoinTask<?> t; ForkJoinTask<?>[] a;
    //数组不为空
    if ((a = array) != null) {
        //计算索引b的位置
        int j = (((a.length - 1) & b) << ASHIFT) + ABASE;
        //如果此处的task不为空，则将此处置为null然后将对象task返回
        if ((t = (ForkJoinTask<?>)U.getObjectVolatile(a, j)) != null &&
            base == b && U.compareAndSwapObject(a, j, t, null)) {
            base = b + 1;
            return t;
        }
    }
    return null;
}

/**
 * Takes next task, if one exists, in order specified by mode.
 */
final ForkJoinTask<?> nextLocalTask() {
    return (config & FIFO_QUEUE) == 0 ? pop() : poll();
}

/**
 * Returns next task, if one exists, in order specified by mode.
 */
final ForkJoinTask<?> peek() {
    ForkJoinTask<?>[] a = array; int m;
    //判断数组的合法性
    if (a == null || (m = a.length - 1) < 0)
        return null;
    //根据mode决定从top还是base处获得task
    int i = (config & FIFO_QUEUE) == 0 ? top - 1 : base;
    int j = ((i & m) << ASHIFT) + ABASE;
    //返回获得的task
    return (ForkJoinTask<?>)U.getObjectVolatile(a, j);
}

/**
 * Pops the given task only if it is at the current top.
 * (A shared version is available only via FJP.tryExternalUnpush)
*/
final boolean tryUnpush(ForkJoinTask<?> t) {
    ForkJoinTask<?>[] a; int s;
    //判断数组的合法性
    if ((a = array) != null && (s = top) != base &&
        //将top位置的task与t比较，如果相等则将其改为null
        U.compareAndSwapObject
        (a, (((a.length - 1) & --s) << ASHIFT) + ABASE, t, null)) {
        //将top减1
        U.putOrderedInt(this, QTOP, s);
        //返回操作成功
        return true;
    }
    //默认返回失败
    return false;
}

/**
 * Executes the given task and any remaining local tasks.
 */
final void runTask(ForkJoinTask<?> task) {
    //task不为空
    if (task != null) {
        //扫描状态标记为busy 那么说明当前的worker正在处理本地任务   此时这个操作会将scanState改为0
        scanState &= ~SCANNING; // mark as busy
        //执行这个task
        (currentSteal = task).doExec();
        //释放已执行任务的内存
        U.putOrderedObject(this, QCURRENTSTEAL, null); // release for GC
        //执行其他本地的task
        execLocalTasks();
        ForkJoinWorkerThread thread = owner;
        //增加增加steals的次数
        if (++nsteals < 0)      // collect on overflow
            transferStealCount(pool);
        //将scanState改为1 这样就变得活跃可以被其他worker scan
        scanState |= SCANNING;
        //如果thread不为null说明为worker线程 则调用后续的exec方法
        if (thread != null)
            thread.afterTopLevelExec();
    }
}

/**
 * Removes and executes all local tasks. If LIFO, invokes
 * pollAndExecAll. Otherwise implements a specialized pop loop
 * to exec until empty.
 */
final void execLocalTasks() {
    int b = base, m, s;
    //拿到数组
    ForkJoinTask<?>[] a = array;
    //如果b-s小于0说明存在task，a不为空，切a的长度大于0 这均是检测方法的合法性
    if (b - (s = top - 1) <= 0 && a != null &&
        (m = a.length - 1) >= 0) {
        //如果没有采用FIFO的mode  那么一定是LIFO 则从top处开始
        if ((config & FIFO_QUEUE) == 0) {
           //开始循环
            for (ForkJoinTask<?> t;;) {
               //从top开始取出task
                if ((t = (ForkJoinTask<?>)U.getAndSetObject
                     (a, ((m & s) << ASHIFT) + ABASE, null)) == null)
                    break;
                //修改top
                U.putOrderedInt(this, QTOP, s);
                //执行task
                t.doExec();
                //如果没有任务的了 则退出
                if (base - (s = top - 1) > 0)
                    break;
            }
        }
        else
           //FIFO的方式调用pollAndExecAll
            pollAndExecAll();
    }
}

  final void pollAndExecAll() {
        for (ForkJoinTask<?> t; (t = poll()) != null;)
            t.doExec();
    }

/**
 * If present, removes from queue and executes the given task,
 * or any other cancelled task. Used only by awaitJoin.
 *
 * @return true if queue empty and task not known to be done
 */
final boolean tryRemoveAndExec(ForkJoinTask<?> task) {
    ForkJoinTask<?>[] a; int m, s, b, n;
    //判断数组的合法性 task不能为空
    if ((a = array) != null && (m = a.length - 1) >= 0 &&
        task != null) {
        //循环  n为task的数量，必须大于0
        while ((n = (s = top) - (b = base)) > 0) {
            //死循环 从top遍历到base
            for (ForkJoinTask<?> t;;) {      // traverse from s to b
                long j = ((--s & m) << ASHIFT) + ABASE;
                if ((t = (ForkJoinTask<?>)U.getObject(a, j)) == null)
                    return s + 1 == top;     // shorter than expected
                //如果task处于top位置
                else if (t == task) {
                    boolean removed = false;
                    if (s + 1 == top) {      // pop
                        //pop的方式获取task  然后替换为null
                        if (U.compareAndSwapObject(a, j, task, null)) {
                            U.putOrderedInt(this, QTOP, s);
                            removed = true;
                        }
                    }
                    //用emptytask代替
                    else if (base == b)      // replace with proxy
                        removed = U.compareAndSwapObject(
                            a, j, task, new EmptyTask());
                    //如果remove成功 则执行这个task
                    if (removed)
                        task.doExec();
                    break;
                }
                //如果task的status为负数 切 top=s=1
                else if (t.status < 0 && s + 1 == top) {
                    //移除
                    if (U.compareAndSwapObject(a, j, t, null))
                        U.putOrderedInt(this, QTOP, s);
                    break;                  // was cancelled
                }
                if (--n == 0)
                    return false;
            }
            if (task.status < 0)
                return false;
        }
    }
    return true;
}

/**
 * Pops task if in the same CC computation as the given task,
 * in either shared or owned mode. Used only by helpComplete.
 */
final CountedCompleter<?> popCC(CountedCompleter<?> task, int mode) {
    int s; ForkJoinTask<?>[] a; Object o;
    //判断队列数组合法性
    if (base - (s = top) < 0 && (a = array) != null) {
        //从top处开始
        long j = (((a.length - 1) & (s - 1)) << ASHIFT) + ABASE;
        //如果获的的task不为null
        if ((o = U.getObjectVolatile(a, j)) != null &&
            //且为CountedCompleter对象
            (o instanceof CountedCompleter)) {
            //转换为CountedCompleter
            CountedCompleter<?> t = (CountedCompleter<?>)o;
            //死循环
            for (CountedCompleter<?> r = t;;) {
                //如果task与获得的r相等为同一对象
                if (r == task) {
                    //如果mode小于0 
                    if (mode < 0) { // must lock
                         //cas的方式加锁
                        if (U.compareAndSwapInt(this, QLOCK, 0, 1)) {
                            //将这个对象清除 并修改top后解锁
                            if (top == s && array == a &&
                                U.compareAndSwapObject(a, j, t, null)) {
                                U.putOrderedInt(this, QTOP, s - 1);
                                U.putOrderedInt(this, QLOCK, 0);
                                //返回t
                                return t;
                            }
                            //解锁
                            U.compareAndSwapInt(this, QLOCK, 1, 0);
                        }
                    }
                    else if (U.compareAndSwapObject(a, j, t, null)) {
                        U.putOrderedInt(this, QTOP, s - 1);
                        return t;
                    }
                    break;
                }
                else if ((r = r.completer) == null) // try parent
                    break;
            }
        }
    }
    return null;
}

/**
 * Steals and runs a task in the same CC computation as the
 * given task if one exists and can be taken without
 * contention. Otherwise returns a checksum/control value for
 * use by method helpComplete.
 *
 * @return 1 if successful, 2 if retryable (lost to another
 * stealer), -1 if non-empty but no matching task found, else
 * the base index, forced negative.
 */
final int pollAndExecCC(CountedCompleter<?> task) {
    int b, h; ForkJoinTask<?>[] a; Object o;
    //判断array的合法性
    if ((b = base) - top >= 0 || (a = array) == null)
        h = b | Integer.MIN_VALUE;  // to sense movement on re-poll
    else {
        //从base开始获得task
        long j = (((a.length - 1) & b) << ASHIFT) + ABASE;
        if ((o = U.getObjectVolatile(a, j)) == null)
            h = 2;                  // retryable
        else if (!(o instanceof CountedCompleter))
            h = -1;                 // unmatchable
        else {
            CountedCompleter<?> t = (CountedCompleter<?>)o;
            //死循环
            for (CountedCompleter<?> r = t;;) {
                if (r == task) {
                    if (base == b &&
                        U.compareAndSwapObject(a, j, t, null)) {
                        base = b + 1;
                        t.doExec();
                        h = 1;      // success
                    }
                    else
                        h = 2;      // lost CAS
                    break;
                }
                else if ((r = r.completer) == null) {
                    h = -1;         // unmatched
                    break;
                }
            }
        }
    }
    return h;
}

 int m = a.length - 1;
 U.putOrderedObject(a, ((m & s) << ASHIFT) + ABASE, task);