ConcurrentHashMap

https://www.cnblogs.com/shihaiming/p/11399472.html
https://blog.csdn.net/programmer_at/article/details/79715177

数据结构

JDK1.8
结构与HashMap类似

transient volatile Node<K,V>[] table;
private transient volatile Node<K,V>[] nextTable; // 扩容时新的数组
static class Node<K,V> implements Map.Entry<K,V> {
        final int hash;
        final K key;
        volatile V val;
        volatile Node<K,V> next;
}

核心方法

put()

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;

    // 自旋
    for (Node<K,V>[] tab = table;;) {
        Node<K,V> f; int n, i, fh;

        // 懒加载，如果为空则初始化
        if (tab == null || (n = tab.length) == 0)
            tab = initTable();

        // 计算hash槽位，为空则直接使用CAS插入
        else if ((f = tabAt(tab, i = (n - 1) & hash)) == null) {
            if (casTabAt(tab, i, null,
                         new Node<K,V>(hash, key, value, null)))
                break;                   // no lock when adding to empty bin
        }

        // 如果在扩容，当前线程协助迁移一个桶的数据
        else if ((fh = f.hash) == MOVED)
            tab = helpTransfer(tab, f);
        else {

            // 存在hash碰撞
            V oldVal = null;

            // 同步锁对象为桶的第一个元素
            synchronized (f) {
                // 再次检查首元素是否有被修改，如果有则继续自旋
                if (tabAt(tab, i) == f) {
                    // firstHash>=0, 判断为链表
                    if (fh >= 0) {
                        binCount = 1;
                        // 遍历链表
                        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;
                            }
                        }
                    }

                   // 判断为红黑树
                    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) {
                // 红黑树转化
                if (binCount >= TREEIFY_THRESHOLD)
                    treeifyBin(tab, i);
                if (oldVal != null)
                    return oldVal;
                break;
            }
        }
    }

    // 尝试扩容
    addCount(1L, binCount);
    return null;
}

get()

与HashMap类似

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;
}

扩容

• 当链表超过8个时，会尝试树化
• 转换红黑树前，会检查table长度是否大于64
• 如果不是先扩容到table的两倍
• 创建新的数组，扩容时会尝试给原节点添加ForwardingNode代表已经处理过了