ConcurrentHashMap的size方法源码分析

/**
 * Returns the number of mappings. This method should be used
 * instead of {@link #size} because a ConcurrentHashMap may
 * contain more mappings than can be represented as an int. The
 * value returned is an estimate; the actual count may differ if
 * there are concurrent insertions or removals.
 *
 * @return the number of mappings
 * @since 1.8
 */
public long mappingCount() {
    long n = sumCount();
    return (n < 0L) ? 0L : n; // ignore transient negative values
}

/**
 * map长度超过Integer.MAX_VALUE?  惊人
 * {@inheritDoc}
 */
public int size() {
    long n = sumCount();
    return ((n < 0L) ? 0 :
            (n > (long)Integer.MAX_VALUE) ? Integer.MAX_VALUE :
            (int)n);
}

final long sumCount() {
  /* 取base+counterCells */
  CounterCell[] as = counterCells; CounterCell a;
  long sum = baseCount;
  if (as != null) {
      for (int i = 0; i < as.length; ++i) {
          if ((a = as[i]) != null)
              sum += a.value;
      }
  }
  return sum;
}

/**
 * Adds to count, and if table is too small and not already
 * resizing, initiates transfer. If already resizing, helps
 * perform transfer if work is available.  Rechecks occupancy
 * after a transfer to see if another resize is already needed
 * because resizings are lagging additions.
 *
 * @param x the count to add
 * @param check if <0, don't check resize, if <= 1 only check if uncontended
 */
private final void addCount(long x, int check) {
    /* CounterCell改造自Striped64，用以在计数过程中降低并发冲突，因为是数组，内存位置相邻，所以使用jdk1.8中的@sun.misc.Contended 进行填充防止伪共享 */
    CounterCell[] as; long b, s;
    /* 已经有counterCells直接使用，否则操作baseCount，如果失败则进入if */
    if ((as = counterCells) != null ||
        /* 先读到baseCount到本地变量，防止ABA */
        !U.compareAndSwapLong(this, BASECOUNT, b = baseCount, s = b + x)) {
        CounterCell a; long v; int m;
        /* 有没有在CounterCell上发生竞争 */
        boolean uncontended = true;
        if (as == null || (m = as.length - 1) < 0 ||
            /* 对当前线程的probe用CounterCell[]长度取模得到CounterCell[]的一项，如果ThreadLocalRandom未初始化，则取第一项，如果此项为null，则认为找到的 位置未被使用，进入fullAddCount，否则在此存在的cell上进行CAS，如果CAS发生冲突，则进入fullAddCount, 继续进行size的count计数。 */
            (a = as[ThreadLocalRandom.getProbe() & m]) == null ||
            !(uncontended =
              U.compareAndSwapLong(a, CELLVALUE, v = a.value, v + x))) {
            fullAddCount(x, uncontended);
            return;
        }
        if (check <= 1)
            return;
        s = sumCount();
    }
    if (check >= 0) {
        Node<K,V>[] tab, nt; int n, sc;
        while (s >= (long)(sc = sizeCtl) && (tab = table) != null &&
               (n = tab.length) < MAXIMUM_CAPACITY) {
            int rs = resizeStamp(n);
            if (sc < 0) {
                if ((sc >>> RESIZE_STAMP_SHIFT) != rs || sc == rs + 1 ||
                    sc == rs + MAX_RESIZERS || (nt = nextTable) == null ||
                    transferIndex <= 0)
                    break;
                if (U.compareAndSwapInt(this, SIZECTL, sc, sc + 1))
                    transfer(tab, nt);
            }
            else if (U.compareAndSwapInt(this, SIZECTL, sc,
                                         (rs << RESIZE_STAMP_SHIFT) + 2))
                transfer(tab, null);
            s = sumCount();
        }
    }
}

/**
 * 
 */
private final void fullAddCount(long x, boolean wasUncontended) {
    int h;
    if ((h = ThreadLocalRandom.getProbe()) == 0) {
        /* 确定是初始化后产生的随机数，用随机数对CounterCell[]长度取模(hash)来减少碰撞 */
        ThreadLocalRandom.localInit();      // force initialization
        h = ThreadLocalRandom.getProbe();
        wasUncontended = true;
    }
    boolean collide = false;                // True if last slot nonempty
    for (;;) {
        CounterCell[] as; CounterCell a; int n; long v;
        if ((as = counterCells) != null && (n = as.length) > 0) {
            if ((a = as[(n - 1) & h]) == null) {
                if (cellsBusy == 0) {            // Try to attach new Cell
                    CounterCell r = new CounterCell(x); // Optimistic create
                     /* 此处的LoadLoad屏障保证其它线程创建的CounterCell[]元素可见性 */
                    if (cellsBusy == 0 &&
                        /*使用CAS volatile的CELLSBUSY的方式加锁操作CounterCell[], 失败则用for自旋*/
                        U.compareAndSwapInt(this, CELLSBUSY, 0, 1)) {
                        boolean created = false;
                        try {               // Recheck under lock
                            CounterCell[] rs; int m, j;
                            if ((rs = counterCells) != null &&
                                (m = rs.length) > 0 &&
                                rs[j = (m - 1) & h] == null) {
                                rs[j] = r;
                                created = true;
                            }
                        } finally {
                            /* 此处的StoreStore屏障保证数组中的新CounterCell引用可见性 */
                            cellsBusy = 0;
                        }
                        if (created)
                            break;
                        continue;           // Slot is now non-empty
                    }
                }
                collide = false;
            }
            else if (!wasUncontended)       // CAS already known to fail
                wasUncontended = true;      // Continue after rehash
            else if (U.compareAndSwapLong(a, CELLVALUE, v = a.value, v + x))
                break;
            else if (counterCells != as || n >= NCPU)
                collide = false;            // At max size or stale
            else if (!collide)
                collide = true;
            else if (cellsBusy == 0 &&
                     U.compareAndSwapInt(this, CELLSBUSY, 0, 1)) {
                /* 冲突太厉害，就扩容cells */
                try {
                    if (counterCells == as) {// Expand table unless stale
                        CounterCell[] rs = new CounterCell[n << 1];
                        for (int i = 0; i < n; ++i)
                            rs[i] = as[i];
                        counterCells = rs;
                    }
                } finally {
                    cellsBusy = 0;
                }
                collide = false;
                continue;                   // Retry with expanded table
            }
            /* 冲突严重换个位置 */
            h = ThreadLocalRandom.advanceProbe(h);
        }
        else if (cellsBusy == 0 && counterCells == as &&
                 U.compareAndSwapInt(this, CELLSBUSY, 0, 1)) {
            boolean init = false;
            try {                           // Initialize table
                if (counterCells == as) {
                    /* 初始容量为2 */
                    CounterCell[] rs = new CounterCell[2];
                    rs[h & 1] = new CounterCell(x);
                    counterCells = rs;
                    init = true;
                }
            } finally {
                cellsBusy = 0;
            }
            if (init)
                break;
        }
        /* 初始化有竞争时，再试下baseCount */
        else if (U.compareAndSwapLong(this, BASECOUNT, v = baseCount, v + x))
            break;                          // Fall back on using base
    }
}