JUC 一 ConcurrentHashMap

java.util.concurrent

ConcurrentHashMap是一个支持并发检索和并发更新的线程安全的HashMap（但不允许空key或value）。

JDK8以CAS+synchronized来保证并发安全。

ConcurrentHashMap、HashMap和HashTable

效率：

1. 当期望许多线程访问一个给定collection时,ConcurrentHashMap通常优于同步的HashMap，ConcurrentSkipListMap通常优于同步的TreeMap
2. 当期望的读数和遍历远远大于列表的更新数时,CopyOnWriteArrayList优于同步的ArrayList

ConcurrentHashMap、HashMap和HashTable的区别：

1. HashMap 是非线程安全的哈希表，常用于单线程程序中。
2. Hashtable 是线程安全的哈希表，由于是通过内置锁 synchronized 来保证线程安全，在资源争用比较高的环境下，Hashtable 的效率比较低。
3. ConcurrentHashMap 是一个支持并发操作的线程安全的HashMap，但是他不允许存储空key或value。使用CAS+synchronized来保证并发安全（在JDK 7之前是通过Lock和Segment（分段锁）实现并发安全），在并发访问时不需要阻塞线程，所以效率是比Hashtable 要高的。

结构

put(K, V)

public V put(K key, V value) {
    return putVal(key, value, false);
}

/** Implementation for put and putIfAbsent */
final V putVal(K key, V value, boolean onlyIfAbsent) {
    if (key == null || value == null) throw new NullPointerException();
    //计算hash值
    int hash = spread(key.hashCode());
    int binCount = 0;
    for (Node<K,V>[] tab = table;;) {//自旋
        //f:索引节点; n:tab.length; i:新节点索引 (n - 1) & hash; fh:f.hash
        Node<K,V> f; int n, i, fh;
        if (tab == null || (n = tab.length) == 0)
            //初始化
            tab = initTable();
        else if ((f = tabAt(tab, i = (n - 1) & hash)) == null) {//索引i节点为空，直接插入
            //cas插入节点,成功则跳出循环
            if (casTabAt(tab, i, null,
                         new Node<K,V>(hash, key, value, null)))
                break;                   // no lock when adding to empty bin
        }
        //当前节点处于移动状态-其他线程正在进行节点转移操作
        else if ((fh = f.hash) == MOVED)
            //帮助转移
            tab = helpTransfer(tab, f);
        else {
            V oldVal = null;
            synchronized (f) {
                if (tabAt(tab, i) == f) {//check stable
                    //f.hash>=0,说明f是链表的头结点
                    if (fh >= 0) {
                        binCount = 1;//记录链表节点数，用于后面是否转换为红黑树做判断
                        for (Node<K,V> e = f;; ++binCount) {
                            K ek;
                            //key相同 修改
                            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;
                            //到这里说明已经是链表尾，把当前值作为新的节点插入到队尾
                            if ((e = e.next) == null) {
                                pred.next = new Node<K,V>(hash, key,
                                                          value, null);
                                break;
                            }
                        }
                    }
                    //红黑树节点操作
                    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) {
                //如果链表中节点数binCount >= TREEIFY_THRESHOLD(默认是8)，则把链表转化为红黑树结构
                if (binCount >= TREEIFY_THRESHOLD)
                    treeifyBin(tab, i);
                if (oldVal != null)
                    return oldVal;
                break;
            }
        }
    }
    //更新新元素个数
    addCount(1L, binCount);
    return null;
}

get(Object key)

    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;
    }

remove(Object key)

    public V remove(Object key) {
        return replaceNode(key, null, null);
    }

    final V replaceNode(Object key, V value, Object cv) {
        int hash = spread(key.hashCode());
        for (Node<K,V>[] tab = table;;) {
            Node<K,V> f; int n, i, fh;
            if (tab == null || (n = tab.length) == 0 ||
                (f = tabAt(tab, i = (n - 1) & hash)) == null)
                break;
            else if ((fh = f.hash) == MOVED)
                tab = helpTransfer(tab, f);
            else {
                V oldVal = null;
                boolean validated = false;
                synchronized (f) {
                    if (tabAt(tab, i) == f) {
                        if (fh >= 0) {
                            validated = true;
                            for (Node<K,V> e = f, pred = null;;) {
                                K ek;
                                if (e.hash == hash &&
                                    ((ek = e.key) == key ||
                                     (ek != null && key.equals(ek)))) {
                                    V ev = e.val;
                                    if (cv == null || cv == ev ||
                                        (ev != null && cv.equals(ev))) {
                                        oldVal = ev;
                                        if (value != null)
                                            e.val = value;
                                        else if (pred != null)
                                            pred.next = e.next;
                                        else
                                            setTabAt(tab, i, e.next);
                                    }
                                    break;
                                }
                                pred = e;
                                if ((e = e.next) == null)
                                    break;
                            }
                        }
                        else if (f instanceof TreeBin) {
                            validated = true;
                            TreeBin<K,V> t = (TreeBin<K,V>)f;
                            TreeNode<K,V> r, p;
                            if ((r = t.root) != null &&
                                (p = r.findTreeNode(hash, key, null)) != null) {
                                V pv = p.val;
                                if (cv == null || cv == pv ||
                                    (pv != null && cv.equals(pv))) {
                                    oldVal = pv;
                                    if (value != null)
                                        p.val = value;
                                    else if (t.removeTreeNode(p))
                                        setTabAt(tab, i, untreeify(t.first));
                                }
                            }
                        }
                        else if (f instanceof ReservationNode)
                            throw new IllegalStateException("Recursive update");
                    }
                }
                if (validated) {
                    if (oldVal != null) {
                        if (value == null)
                            addCount(-1L, -1);
                        return oldVal;
                    }
                    break;
                }
            }
        }
        return null;
    }