Java 并发编程之 ConcurrentHashMap 1.8 源码分析

Forwarding node

扩容时如果某个索引位置的节点迁移完毕，用ForwardingNode作为旧table的头节点。

get

spread方法保证获取到的hash码为正数，因为，负数用做了ForwardingNode和TreeBin的标志位。

(eh = e.hash) == h如果hash相同则直接替换value。

eh < 0如果是负数，就去新的table中找这个key，或者去树中找这个key。

while循环中是正常的链表中查找。

public V get(Object key) {
    Node<K,V>[] tab; Node<K,V> e, p; int n, eh; K ek;
    int h = spread(key.hashCode());
    if ((tab = table) != null && (n = tab.length) > 0 &&
        (e = tabAt(tab, (n - 1) & h)) != null) {
        if ((eh = e.hash) == h) {
            if ((ek = e.key) == key || (ek != null && key.equals(ek)))
                return e.val;
        }
        else if (eh < 0)
            return (p = e.find(h, key)) != null ? p.val : null;
        while ((e = e.next) != null) {
            if (e.hash == h &&
                ((ek = e.key) == key || (ek != null && key.equals(ek))))
                return e.val;
        }
    }
    return null;
}

put

f是链表头节点，fh是链表头节点的hash。

CAS初始化数组，初始化完成后再次进入for循环。

如果头节点为空，则CAS占据头节点，失败，说明已经有人抢占头节点了，则再进入for循环。

此时判断头节点是不是在扩容移动状态，如果是就帮助扩容，之后再次进入for循环。

如果当前不是处在扩容，头节点也已经有值了（即：hash冲突了，往链表或树中put）。使用synchronized锁住头节点。

tabAt(tab, i) == f：确认头节点没有发生扩容而移动。

fh >= 0：如果是普通节点，相同更新，不同追加到链表尾部。

f instanceof TreeBin：如果是红黑树，则放入红黑树。

synchronized释放后判断下链表大小是否需要转换为红黑树。

final V putVal(K key, V value, boolean onlyIfAbsent) {
    if (key == null || value == null) throw new NullPointerException();
    int hash = spread(key.hashCode());
    int binCount = 0;
    for (Node<K,V>[] tab = table;;) {
        Node<K,V> f; int n, i, fh;
        if (tab == null || (n = tab.length) == 0)
            // 创建数组，使用了 cas
            tab = initTable();
        else if ((f = tabAt(tab, i = (n - 1) & hash)) == null) {
            // 头节点为空，则使用 cas 创建
            if (casTabAt(tab, i, null,
                         new Node<K,V>(hash, key, value, null)))
                break;                   // no lock when adding to empty bin
        }
        else if ((fh = f.hash) == MOVED)
            // 帮忙扩容
            tab = helpTransfer(tab, f);
        else {
            V oldVal = null;
            synchronized (f) {
                if (tabAt(tab, i) == f) {
                    if (fh >= 0) {
                        binCount = 1;
                        for (Node<K,V> e = f;; ++binCount) {
                            K ek;
                            if (e.hash == hash &&
                                ((ek = e.key) == key ||
                                 (ek != null && key.equals(ek)))) {
                                oldVal = e.val;
                                if (!onlyIfAbsent)
                                    e.val = value;
                                break;
                            }
                            Node<K,V> pred = e;
                            if ((e = e.next) == null) {
                                pred.next = new Node<K,V>(hash, key,
                                                          value, null);
                                break;
                            }
                        }
                    }
                    else if (f instanceof TreeBin) {
                        Node<K,V> p;
                        binCount = 2;
                        if ((p = ((TreeBin<K,V>)f).putTreeVal(hash, key,
                                                       value)) != null) {
                            oldVal = p.val;
                            if (!onlyIfAbsent)
                                p.val = value;
                        }
                    }
                }
            }
            if (binCount != 0) {
                if (binCount >= TREEIFY_THRESHOLD)
                    treeifyBin(tab, i);
                if (oldVal != null)
                    return oldVal;
                break;
            }
        }
    }
    addCount(1L, binCount);
    return null;
}

sizeCtl

初始化为-1

扩容时为-(1+ 扩容线程数)

初始化完成或扩容完成时为，下一次扩容的阈值

initTable

将sizeCtl从正数改为-1，

(sc = sizeCtl) < 0：如果sizeCtl小于0，则让出CPU执行权。继续while循环直到初始化数组完成。

finally中的sizeCtl = sc是赋值新阈值。

private final Node<K,V>[] initTable() {
    Node<K,V>[] tab; int sc;
    while ((tab = table) == null || tab.length == 0) {
        if ((sc = sizeCtl) < 0)
            Thread.yield(); // lost initialization race; just spin
        else if (U.compareAndSwapInt(this, SIZECTL, sc, -1)) {
            try {
                if ((tab = table) == null || tab.length == 0) {
                    int n = (sc > 0) ? sc : DEFAULT_CAPACITY;
                    @SuppressWarnings("unchecked")
                    Node<K,V>[] nt = (Node<K,V>[])new Node<?,?>[n];
                    table = tab = nt;
                    sc = n - (n >>> 2);
                }
            } finally {
                sizeCtl = sc;
            }
            break;
        }
    }
    return tab;
}

addCount

与LongAdder中实现差不多，都是使用累加单元和base单元。

当check >= 0时需要扩容，

U.compareAndSwapInt(this, SIZECTL, sc, (rs << RESIZE_STAMP_SHIFT) + 2)：将sizeCtl改为负数，表示进入扩容状态。

U.compareAndSwapInt(this, SIZECTL, sc, sc + 1)：帮忙扩容

private final void addCount(long x, int check) {
    CounterCell[] as; long b, s;
    if ((as = counterCells) != null ||
        !U.compareAndSwapLong(this, BASECOUNT, b = baseCount, s = b + x)) {
        CounterCell a; long v; int m;
        boolean uncontended = true;
        if (as == null || (m = as.length - 1) < 0 ||
            (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;
                // new table 已经创建，帮忙扩容
                if (U.compareAndSwapInt(this, SIZECTL, sc, sc + 1))
                    transfer(tab, nt);
            }
            // 需要扩容，未创建 new table
            else if (U.compareAndSwapInt(this, SIZECTL, sc,
                                         (rs << RESIZE_STAMP_SHIFT) + 2))
                transfer(tab, null);
            s = sumCount();
        }
    }
}

transfer

(f = tabAt(tab, i)) == null：如果链表头已经处理完了，就将链表头赋值为ForwardingNode。

(fh = f.hash) == MOVED：如果已经是ForwardingNode了，表示已经移动完成。

如果是有元素的就synchronized锁住头节点进行处理。

private final void transfer(Node<K,V>[] tab, Node<K,V>[] nextTab) {
    int n = tab.length, stride;
    if ((stride = (NCPU > 1) ? (n >>> 3) / NCPU : n) < MIN_TRANSFER_STRIDE)
        stride = MIN_TRANSFER_STRIDE; // subdivide range
    if (nextTab == null) {            // initiating
        try {
            @SuppressWarnings("unchecked")
            Node<K,V>[] nt = (Node<K,V>[])new Node<?,?>[n << 1];
            nextTab = nt;
        } catch (Throwable ex) {      // try to cope with OOME
            sizeCtl = Integer.MAX_VALUE;
            return;
        }
        nextTable = nextTab;
        transferIndex = n;
    }
    int nextn = nextTab.length;
    ForwardingNode<K,V> fwd = new ForwardingNode<K,V>(nextTab);
    boolean advance = true;
    boolean finishing = false; // to ensure sweep before committing nextTab
    for (int i = 0, bound = 0;;) {
        Node<K,V> f; int fh;
        // ...
        if (i < 0 || i >= n || i + n >= nextn) {
            // ...
        }
        else if ((f = tabAt(tab, i)) == null)
            advance = casTabAt(tab, i, null, fwd);
        else if ((fh = f.hash) == MOVED)
            advance = true; // already processed
        else {
            synchronized (f) {
                // ...
            }
        }
    }
}