源码解析之ConcurrentHashmap

ConcurrentHashmap算是我看的集合源码里最难理解的了(当然ConcurrentLinkedList虽然代码少但理解起来也累)，在Java1.8版本中DougLea大师巧通过妙地代码把锁粒度已经将成桶级别了，不得不说非常厉害。本文暂时贴上代码，内容后续补充。

看ConcurrentHashmap之前要掌握的基础。

1、对Hashmap的原理了解。

2、Volatile关键字、CAS操作和Synchronized关键字要理解。

3、配合网上解析和并发的书一同食用，而且要看源码里的注释，看源码前先了解其运作过程。

推荐一篇源码解析：https://www.jianshu.com/p/487d00afe6ca

推荐的书《Java并发编程的艺术》 集合那章

正文

get操作

    public V get(Object key) {
        Node<K,V>[] tab; Node<K,V> e, p; int n, eh; K ek;
        int h = spread(key.hashCode());  //高16位与低6位散列
        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)  //eh小于0表示该table正在扩容，将旧table上的node移到新table上，被移过去的节点旧位置上标记一个hash<0的node
　　　　　　　　　　　　　　　　　　 //find就是用this找到本该在这里的节点，然后判断是否为null返回相应值。
                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;
    }

put操作

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 Nul lPointerException();
        int hash = spread(key.hashCode()); //(h ^ (h >>> 16)) & HASH_BITS
        int binCount = 0;
        for (Node<K,V>[] tab = table;;) {
            Node<K,V> f; int n, i, fh;
            if (tab == null || (n = tab.length) == 0)
                tab = initTable();  //初始化Node数组table
            else if ((f = tabAt(tab, i = (n - 1) & hash)) == null) {//数组不为空，分配到的地址中没有node
                if (casTabAt(tab, i, null,
                        new Node<K,V>(hash, key, value, null)))//cas操作把它加放入该地址
                    break;                   // no lock when adding to empty bin  数组每个位置上的第一个节点不需要获得锁
            }
            else if ((fh = f.hash) == MOVED)//？？ 好像是扩容时被put的操作
                tab = helpTransfer(tab, f);
            else {//数组被初始化了且地址不为空，非扩容时期的正常操作
                V oldVal = null;
                synchronized (f) {//拿到该位置第一个节点的对象锁
                    if (tabAt(tab, i) == f) {//再次确认头节点
                        if (fh >= 0) {//头节点hash>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)))) {//找到相同的key，更新值
                                    oldVal = e.val;
                                    if (!onlyIfAbsent)
                                        e.val = value;
                                    break;
                                }
                                Node<K,V> pred = e;
                                if ((e = e.next) == null) {//到尾部，添加到尾部
                                    pred.next = new Node<K,V>(hash, key,
                                            value, null);
                                    break;
                                }
                            }
                        }
                        else if (f instanceof TreeBin) { //f是tree节点，用红黑树方法
                            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)//链表大于阈值，树化
                        treeifyBin(tab, i);
                    if (oldVal != null)//如果是更新值，返回oldval
                        return oldVal;
                    break;
                }
            }
        }
        addCount(1L, binCount);
        return null;
    }

private final Node<K,V>[] initTable() {
        Node<K,V>[] tab; int sc;
        while ((tab = table) == null || tab.length == 0) {
            if ((sc = sizeCtl) < 0) //已被别人抢先初始化了（第一个初始化的线程将sizeCtl改为-1) 进入准备状态（等待被唤醒
                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; //sizeCtl大于0就使用它的大小，等于0默认容量大小
                        @SuppressWarnings("unchecked")
                        Node<K,V>[] nt = (Node<K,V>[])new Node<?,?>[n];//new个node数组
                        table = tab = nt;
                        sc = n - (n >>> 2);//sc=0.75n
                    }
                } finally {
                    sizeCtl = sc;//sizeCtl为0.75n  sizeCtl像是数组扩容阈值
                }
                break;
            }
        }
        return tab;
    }

final Node<K,V>[] helpTransfer(Node<K,V>[] tab, Node<K,V> f) { //与后面addcount（）函数代码相似，这里就不解释了，直接往后看。
        Node<K,V>[] nextTab; int sc;　　　　　　　　　　　　　　　　//函数大意就是 满足一定条件也进入transfer方法 帮助扩容。
        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;
    }

private final void addCount(long x, int check) {
        CounterCell[] as; long b, s;
        if ((as = counterCells) != null ||   //countercells为2e幂，应该相当于数组长度  basecount应该是实时键值对数量
                !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) { //键值对数量超过了阈值，且小于最大值
                int rs = resizeStamp(n); //Integer.numberOfLeadingZeros(n) | (1 << (RESIZE_STAMP_BITS - 1));
                if (sc < 0) {//其他线程正在扩容
                    //第一个条件：因为第一个线程扩容后会将sc设为rs << RESIZE_STAMP_SHIFT) + 2)，它退回去会等于rs，如果
                    //不等于说明第一个线程还没开始扩容。
                    //第二、三个条件：未知
                    //第四个条件：新数组还没创建
                    if ((sc >>> RESIZE_STAMP_SHIFT) != rs || sc == rs + 1 ||
                            sc == rs + MAX_RESIZERS || (nt = nextTable) == null ||
                            transferIndex <= 0)
                        break; //不帮助扩容
                    if (U.compareAndSwapInt(this, SIZECTL, sc, sc + 1)) //帮助扩容 s=sc+1
                        transfer(tab, nt);
                }
                else if (U.compareAndSwapInt(this, SIZECTL, sc,
                        (rs << RESIZE_STAMP_SHIFT) + 2))  //初次扩容 将值设为很小的负数
                    transfer(tab, null);
                s = sumCount();
            }
        }
    }

private final void transfer(Node<K,V>[] tab, Node<K,V>[] nextTab) {
        int n = tab.length, stride; //stride步长 切割迁移数组为小份进行转移，用来设置transferIndex
        if ((stride = (NCPU > 1) ? (n >>> 3) / NCPU : n) < MIN_TRANSFER_STRIDE)   //NCPU大于1则为  n/（8*NCPU） 否则为数组长度。但要保证大于16
            stride = MIN_TRANSFER_STRIDE; // subdivide range
        if (nextTab == null) {            // initiating  //初始化nextTab，只在扩容时不为null
            try {
                @SuppressWarnings("unchecked")
                Node<K,V>[] nt = (Node<K,V>[])new Node<?,?>[n << 1]; //数组长度翻倍
                nextTab = nt;
            } catch (Throwable ex) {      // try to cope with OOME
                sizeCtl = Integer.MAX_VALUE;  //翻倍失败因为它是最大值了
                return;
            }
            nextTable = nextTab;  //新数组
            transferIndex = n;    //转移指针开始为原数组长度
        }
        int nextn = nextTab.length;   //扩容数组长度
        ForwardingNode<K,V> fwd = new ForwardingNode<K,V>(nextTab); //已迁移节点 他的hash为-1
        boolean advance = true;
        boolean finishing = false; // to ensure sweep before committing nextTab
        for (int i = 0, bound = 0;;) {
            Node<K,V> f; int fh;
            while (advance) {
                int nextIndex, nextBound;//指针指向下一个准备转移节点 界限指向划分该线程任务的终节点
                if (--i >= bound || finishing)  //i >= bound 说明节点到达了界限，它的任务完成或 finish
                    advance = false;
                else if ((nextIndex = transferIndex) <= 0) {  //倒序转移全部转移完成了
                    i = -1;// 准备退出迁移
                    advance = false;
                }
                else if (U.compareAndSwapInt
                        (this, TRANSFERINDEX, nextIndex,
                                nextBound = (nextIndex > stride ?
                                        nextIndex - stride : 0))) { //将转移指针按步长递减（开始是原数组长度）
                    bound = nextBound;//界限=nextIndex（开始为原数组长度）-步长
                    i = nextIndex - 1;//i=nextIndex-1
                    advance = false;  //跳出
                }
            }
            if (i < 0 || i >= n || i + n >= nextn) {//bound为0,i<0 或 i>=数组长度  或i+原长度>=现长度
                int sc;                             //可能原因是原数组长度为0则i<0 ，或，或已经是最大值不能扩容？
                if (finishing) {  //如果完成了就将nextTable清除，
                    nextTable = null;
                    table = nextTab;  //将扩容后数组作为当前数组
                    sizeCtl = (n << 1) - (n >>> 1);  //sizeCtl 为1.5 倍
                    return;   //返回
                }
                if (U.compareAndSwapInt(this, SIZECTL, sc = sizeCtl, sc - 1)) { //将SCTL 折为 sc-1成功（帮助转移时+1）现在减回去
                    if ((sc - 2) != resizeStamp(n) << RESIZE_STAMP_SHIFT)  //之前正常状态 将SIZECTL 设为了 rs << RESIZE_STAMP_SHIFT) + 2
                        return;                                             //现在返回去，不等则说明其他线程还没转移完
                    finishing = advance = true;
                    i = n; // recheck before commit
                }
            }
            else if ((f = tabAt(tab, i)) == null) //如果原tab上的某地址首节点为null 则换为 fwd
                advance = casTabAt(tab, i, null, fwd);
            else if ((fh = f.hash) == MOVED) //如果它 hash为 -1（MOVED） 说明已被移动
                advance = true; // already processed
            else {
                synchronized (f)  {//首节点上锁
                    if (tabAt(tab, i) == f) { //再次确认首节点
                        Node<K,V> ln, hn;
                        if (fh >= 0) { //首节点hash大于0
                            int runBit = fh & n;  //截取hash确定位置 n是扩容前长度
                            Node<K,V> lastRun = f;
                            for (Node<K,V> p = f.next; p != null; p = p.next) {
                                int b = p.hash & n;
                                if (b != runBit) {
                                    runBit = b;
                                    lastRun = p;
                                }
                            }
                            if (runBit == 0) {
                                ln = lastRun;
                                hn = null;
                            }
                            else {
                                hn = lastRun;
                                ln = null;
                            }
                            for (Node<K,V> p = f; p != lastRun; p = p.next) {
                                int ph = p.hash; K pk = p.key; V pv = p.val;
                                if ((ph & n) == 0)
                                    ln = new Node<K,V>(ph, pk, pv, ln);
                                else
                                    hn = new Node<K,V>(ph, pk, pv, hn);
                            }
                            setTabAt(nextTab, i, ln);//将链好的lownode首节点放入新数组低位
                            setTabAt(nextTab, i + n, hn);//将链好的hinode首节点放入新数组高位
                            setTabAt(tab, i, fwd);//把旧数组位置上hash设为-1
                            advance = true;
                        }
                        else if (f instanceof TreeBin) { //treebin的方法
                            TreeBin<K,V> t = (TreeBin<K,V>)f;
                            TreeNode<K,V> lo = null, loTail = null;
                            TreeNode<K,V> hi = null, hiTail = null;
                            int lc = 0, hc = 0;
                            for (Node<K,V> e = t.first; e != null; e = e.next) {
                                int h = e.hash;
                                TreeNode<K,V> p = new TreeNode<K,V>
                                        (h, e.key, e.val, null, null);
                                if ((h & n) == 0) {
                                    if ((p.prev = loTail) == null)
                                        lo = p;
                                    else
                                        loTail.next = p;
                                    loTail = p;
                                    ++lc;
                                }
                                else {
                                    if ((p.prev = hiTail) == null)
                                        hi = p;
                                    else
                                        hiTail.next = p;
                                    hiTail = p;
                                    ++hc;
                                }
                            }
                            ln = (lc <= UNTREEIFY_THRESHOLD) ? untreeify(lo) :
                                    (hc != 0) ? new TreeBin<K,V>(lo) : t;
                            hn = (hc <= UNTREEIFY_THRESHOLD) ? untreeify(hi) :
                                    (lc != 0) ? new TreeBin<K,V>(hi) : t;
                            setTabAt(nextTab, i, ln);
                            setTabAt(nextTab, i + n, hn);
                            setTabAt(tab, i, fwd);
                            advance = true;
                        }
                    }
                }
            }
        }
    }

private final Node<K,V>[] initTable() {
    Node<K,V>[] tab; int sc;
    while ((tab = table) == null || tab.length == 0) {
        if ((sc = sizeCtl) < 0) //已被别人抢先初始化了（第一个初始化的线程将sizeCtl改为-1) 进入准备状态（等待被唤醒
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; //sizeCtl大于0就使用它的大小，等于0默认容量大小
@SuppressWarnings("unchecked")
                    Node<K,V>[] nt = (Node<K,V>[])new Node<?,?>[n];//new个node数组
table = tab = nt;
                    sc = n - (n >>> 2);//sc=0.75n
}
            } finally {
                sizeCtl = sc;//sizeCtl为0.75n  sizeCtl像是数组扩容阈值
}
            break;
        }
    }
    return tab;
}