ConcurrentHashMap（基于jdk1.8）

ConcurrentHashMap（基于jdk1.8）

之前写过关于HashMap和HashTable的区别

1. HashMap 无锁 ConcurrentHashMap是有锁的

ConcurrentHashMap结构介绍

/*Key-value entry.  This class is never exported out as a
user-mutable Map.Entry (i.e., one supporting setValue; see
MapEntry below), but can be used for read-only traversals used
in bulk tasks.  Subclasses of Node with a negative hash field
are special, and contain null keys and values (but are never
exported).  Otherwise, keys and vals are never null.*/
/*译：键值结构。这个类不可以导出成为一个用户可变的Map.Entry。但是可以在批量任务中做只读遍历。是一个具有负的hash值的特殊子类。并且包含null的值和键（但是不能导出），否则，键和值不可为空。*/
static class Node<K,V> implements Map.Entry<K,V> {
        final int hash;
        final K key;
        volatile V val;
        volatile Node<K,V> next;

        Node(int hash, K key, V val, Node<K,V> next) {
            this.hash = hash;
            this.key = key;
            this.val = val;
            this.next = next;
        }

        public final K getKey()       { return key; }
        public final V getValue()     { return val; }
        public final int hashCode()   { return key.hashCode() ^ val.hashCode(); }
        public final String toString(){ return key + "=" + val; }
        public final V setValue(V value) {
            throw new UnsupportedOperationException();
        }

        public final boolean equals(Object o) {
            Object k, v, u; Map.Entry<?,?> e;
            return ((o instanceof Map.Entry) &&
                    (k = (e = (Map.Entry<?,?>)o).getKey()) != null &&
                    (v = e.getValue()) != null &&
                    (k == key || k.equals(key)) &&
                    (v == (u = val) || v.equals(u)));
        }

        /**
         * Virtualized support for map.get(); overridden in subclasses.
         */
        Node<K,V> find(int h, Object k) {
            Node<K,V> e = this;
            if (k != null) {
                do {
                    K ek;
                    if (e.hash == h &&
                        ((ek = e.key) == k || (ek != null && k.equals(ek))))
                        return e;
                } while ((e = e.next) != null);
            }
            return null;
        }
    }

底层实现采用的是Node和红黑树（红黑树）以及cas实现的

• 首先来看一下get方法

/**
     * Returns the value to which the specified key is mapped,
     * or {@code null} if this map contains no mapping for the key.
     *
     * <p>More formally, if this map contains a mapping from a key
     * {@code k} to a value {@code v} such that {@code key.equals(k)},
     * then this method returns {@code v}; otherwise it returns
     * {@code null}.  (There can be at most one such mapping.)
     *
     * @throws NullPointerException if the specified key is null
     */
    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;
    }

看一下这个spread方法是干啥的？

在Java 8 中求hash的方法从hash改为了spread。Key的hashCode值与其高16位作异或并保证最高位为0（从而保证最终结果为正整数）。

(tab = table) != null && (n = tab.length) > 0 &&
            (e = tabAt(tab, (n - 1) & h)) != null

/**
     * The array of bins. Lazily initialized upon first insertion.
     * Size is always a power of two. Accessed directly by iterators.
     */
    transient volatile Node<K,V>[] table;

看一下这个比较条件

tab 局部变量的Node数组 table是一个全局的Node数组 被transient 修饰的属性如果实现的是Serializable接口则不被序列化 volatile 保证了 table的可见性，也就是任意一个线程对table做了修改他会马上同步到主存中

e=tabAt(tab, (n - 1) & h)) 作用是获取首节点的的Node元素

也就是说如果table不为null且 长度大于0 且首节点不为空，那么就继续判断

if ((eh = e.hash) == h) {
    if ((ek = e.key) == key || (ek != null && key.equals(ek)))
        return e.val;
}

这个的意思的如果获取到的e节点的hash值等于要获取值的hash值，那么就判断key值是否相同（hash冲突）如果满足则返回valu值

else if (eh < 0)
    return (p = e.find(h, key)) != null ? p.val : null;

hash值为负值表示正在扩容，这个时候查的是ForwardingNode的find方法来定位到nextTable来
eh=-1，说明该节点是一个ForwardingNode，正在迁移，此时调用ForwardingNode的find方法去nextTable里找。
eh=-2，说明该节点是一个TreeBin，此时调用TreeBin的find方法遍历红黑树，由于红黑树有可能正在旋转变色，所以find里会有读写锁。
eh>=0，说明该节点下挂的是一个链表，直接遍历该链表即可。

• put方法

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();//值和value不可以为null
        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)    //如果为空初始化table
                tab = initTable();
            else if ((f = tabAt(tab, i = (n - 1) & hash)) == null) {  //首节点为空
                if (casTabAt(tab, i, null,
                             new Node<K,V>(hash, key, value, null)))    //是否存在该节点不存在就添加
                    break;                   // no lock when adding to empty bin
            }
            else if ((fh = f.hash) == MOVED)     // 如果当前结点为forwarding则帮助扩容
                                                 
                tab = helpTransfer(tab, f);
            else {
                V oldVal = null;
                synchronized (f) {                      // 如果非forwarding且非null则加锁
                    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;
    }