源码分析（4）-ConcurrentHashMap（JDK1.8）

一、UML类图

ConcurrentHashMap键值不能为null；底层数据结构是数组+链表/红黑二叉树；采用CAS（比较并交换）和synchronized来保证并发安全。

CAS文章：https://blog.csdn.net/v123411739/article/details/79561458

JDK1.7ConcurrentHashMap源码：https://www.cnblogs.com/chengxiao/p/6842045.html

二、源码分析

重要的成员变量：

//数组，类似HashMap的table
transient volatile Node<K,V>[] table;
//扩容时使用，完成后置空
private transient volatile Node<K,V>[] nextTable;
//节点数，类似HashMap的size
private transient volatile long baseCount;

 2.1、插入

    final V putVal(K key, V value, boolean onlyIfAbsent) {
        if (key == null || value == null) throw new NullPointerException();
        int hash = spread(key.hashCode());//①计算(h ^ (h >>> 16)) & HASH_BITS;
        int binCount = 0;
        for (Node<K,V>[] tab = table;;) {//遍历数组元素；获取插入元素位置，插入元素
            Node<K,V> f; int n, i, fh;
            if (tab == null || (n = tab.length) == 0)
                tab = initTable();//②初始化数组
            else if ((f = tabAt(tab, i = (n - 1) & hash)) == null) {//Java并发
                if (casTabAt(tab, i, null, new Node<K,V>(hash, key, value, null)))//CAS机制，添加节点
                    break;                   // no lock when adding to empty bin
            }
            else if ((fh = f.hash) == MOVED)//元素不为空；检测到桶结点是ForwardingNode类型，协助扩容
                tab = helpTransfer(tab, f);
            else {//hash桶结点，锁定该桶头结点并在该链表尾部添加一个节点
                V oldVal = null;
                synchronized (f) {//同步锁
                    if (tabAt(tab, i) == f) {
                        if (fh >= 0) {//链表添加节点
                            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) {//红黑树节点
                            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;
                            }
                        }
                    }
                }
　　　　　　　　　　//binCount==0:新节点成为hash桶头节点
　　　　　　　　　　//binCount!=0:链表或红黑树成功添加或修改一个节点
                if (binCount != 0) {
                    if (binCount >= TREEIFY_THRESHOLD)//hash桶节点数量大于8，链表转化为红黑树
                        treeifyBin(tab, i);//转换红黑树
                    if (oldVal != null)
                        return oldVal;
                    break;
                }
            }
        }
        addCount(1L, binCount);//CAS机制更新baseCount并判断是否需要扩容
        return null;
    }

//初始化数组
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;
    }

//U是sun.misc.Unsafe类实例对象，Java并发操作底层实现核心类
//可以参考网上的博客进行了解
@SuppressWarnings("unchecked")
    static final <K,V> Node<K,V> tabAt(Node<K,V>[] tab, int i) {
        return (Node<K,V>)U.getObjectVolatile(tab, ((long)i << ASHIFT) + ABASE);//getObjectVolatile方法强制从主存中获取属性值；标记红色的代码作用是计算偏移量
    }

//查找节点位置
static final <K,V> boolean casTabAt(Node<K,V>[] tab, int i, Node<K,V> c, Node<K,V> v) {
        return U.compareAndSwapObject(tab, ((long)i << ASHIFT) + ABASE, c, v);
    }

//扩容
final Node<K,V>[] helpTransfer(Node<K,V>[] tab, Node<K,V> f) {
        Node<K,V>[] nextTab; int sc;
        if (tab != null && (f instanceof ForwardingNode) &&
            (nextTab = ((ForwardingNode<K,V>)f).nextTable) != null) {
            int rs = resizeStamp(tab.length);
            while (nextTab == nextTable && table == tab &&
                   (sc = sizeCtl) < 0) {
                if ((sc >>> RESIZE_STAMP_SHIFT) != rs || sc == rs + 1 ||
                    sc == rs + MAX_RESIZERS || transferIndex <= 0)
                    break;
                if (U.compareAndSwapInt(this, SIZECTL, sc, sc + 1)) {
                    transfer(tab, nextTab);
                    break;
                }
            }
            return nextTab;
        }
        return table;
    }

//CAS机制更新baseCount并判断是否需要扩容
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;
                    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();
            }
        }
    }

2.2、查询

//
public V get(Object key) {
        Node<K,V>[] tab; Node<K,V> e, p; int n, eh; K ek;
        int h = spread(key.hashCode());//(h ^ (h >>> 16)) & HASH_BITS
        if ((tab = table) != null && (n = tab.length) > 0 && (e = tabAt(tab, (n - 1) & h)) != null) {
            if ((eh = e.hash) == h) {//hash桶首节点
                if ((ek = e.key) == key || (ek != null && key.equals(ek)))
                    return e.val;
            }
            else if (eh < 0)//链表查询
                return (p = e.find(h, key)) != null ? p.val : null;
            while ((e = e.next) != null) {//红黑树查询
                if (e.hash == h && ((ek = e.key) == key || (ek != null && key.equals(ek))))
                    return e.val;
            }
        }
        return null;
    }

三、总结

ConcurrentHashMap线程安全实现使用CAS，关于CAS内容需要深入了解（是Java并发实现核心内容）。