ConcurrentHashMap浅析

简述

ConcurrentHashMap是针对HashMap非线程安全和HashTable低性能线程安全。它是线程安全的。JDK1.8和之前的版本不同。
JDK1.8前结构是一个Map中有多个Segment，一个Segment中多个数组。
JDK1.8中结构跟HashMap同。

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JDK1.8

总体来说1.8在很多地方使用cas来实现无锁的同步。只有在put、remove的时候在必须上锁的时候上锁。
put

public V put(K key, V value) {
        return putVal(key, value, false);
}
final V putVal(K key, V value, boolean onlyIfAbsent) {
        if (key == null || value == null) throw new NullPointerException();
        //求hash值
        int hash = spread(key.hashCode());
        //用来判断是否变树
        int binCount = 0;
        for (Node[] tab = table;;) {
            Node f; int n, i, fh;
            //table为空
            if (tab == null || (n = tab.length) == 0)
                //初始化table
                tab = initTable();
            //当前hash值取数组下标、数组该下标节点无数据
            else if ((f = tabAt(tab, i = (n - 1) & hash)) == null) {
                if (casTabAt(tab, i, null,
                             new Node(hash, key, value, null)))
                    break;                   // no lock when adding to empty bin
            }
            //该节点标记为移动
            else if ((fh = f.hash) == MOVED)
                tab = helpTransfer(tab, f);
            //正常放入节点
            else {
                V oldVal = null;
                //锁该下标处头节点
                synchronized (f) {
                    if (tabAt(tab, i) == f) {
                        if (fh >= 0) {
                            binCount = 1;
                            //链表方式
                            for (Node 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 pred = e;
                                if ((e = e.next) == null) {
                                    pred.next = new Node(hash, key,
                                                              value, null);
                                    break;
                                }
                            }
                        }
                        //树方式
                        else if (f instanceof TreeBin) {
                            Node p;
                            binCount = 2;
                            if ((p = ((TreeBin)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;
                }
            }
        }
        //计数
        addCount(1L, binCount);
        return null;
}
}

整个过程跟HashMap的方式类似

• 定位到位置不存在元素则通过cas来将元素放入
• 如果存在则锁住链头元素，下面过程与HashMap同

get

public V get(Object key) {
        Node[] tab; Node 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;
    }

整个get操作是无锁的，通过cas来定位来找到元素，并遍历链表来找到元素。

size

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[] 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();
            }
        }
    }

public int size() {
        long n = sumCount();
        return ((n < 0L) ? 0 :
                (n > (long)Integer.MAX_VALUE) ? Integer.MAX_VALUE :
                (int)n);
    }
 final long sumCount() {
        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;
    }

通过baseSize和CounterCell对象来实现计数。

• 在put操作或者remove操作的时候会调用addCount方法来创建CounterCell对象，并会统计CounterCell数组来统计数量。
• 在调用size方法的时候根据前面计算出来的静态数值和CountCell来计算动态的数值。(PS更新baseCount通过cas操作来实现)

如何扩容