ConcurrentHashMap怎么用

小编给大家分享一下ConcurrentHashMap怎么用，希望大家阅读完这篇文章之后都有所收获，下面让我们一起去探讨吧！

首先看一下putVal方法，

if (tab == null || (n = tab.length) == 0)    tab = initTable();

如果还没有table的话，就要先初始化table

private final Node[] initTable() {    Node[] 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[] nt = (Node[])new Node[n];                    table = tab = nt;                    // size 控制在  n的0.75                    sc = n - (n >>> 2);                }            } finally {                sizeCtl = sc;            }            break;        }    }    return tab;}

这一段代码相对简单，这里的sizeCtl是整个过程中的一个非常重要的属性，在扩容，初始化等过程过程中会多次遇到。在这里也是充当了一个排他锁的作用，当它为-1的时候，其它线程等待。

else if ((f = tabAt(tab, i = (n - 1) & hash)) == null) {    if (casTabAt(tab, i, null,                 new Node(hash, key, value, null)))        break;                   // no lock when adding to empty bin}

如果要插入的槽是空的，那么直接插入就可以了。

else if ((fh = f.hash) == MOVED)    tab = helpTransfer(tab, f);

那么如果要插入的hash值为moved状态即-1的时候，那么就要执行helpTransfer方法了，对，就是先让帮助扩容。这里就要扯出来比较多的东西了，我们一点点来进行分析。

首先看看什么时候一个node的hash值变成了-1，一路看下去，只有

static final class ForwardingNode extends Node

这个类使用到了，它里面有一个属性

final Node[] nextTable;

从这里也大概就能看出来，这个时候ConcurrentHashmap处于扩容状态，通过ForwardingNode就可以找到扩容后的table。

接着来看helpTransfer

final Node[] helpTransfer(Node[] tab, Node f) {    Node[] nextTab; int sc;    // 这里还要再次检查 当前节点是不是 ForwardingNode 因为如果不是的话，没有办法找到nextTable，也就没有办法帮助扩容了    if (tab != null && (f instanceof ForwardingNode) &&        (nextTab = ((ForwardingNode)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;            // 进来一个线程，则对sizeCtl+1,用以标记参与扩容的线程数            if (U.compareAndSwapInt(this, SIZECTL, sc, sc + 1)) {                // 进行扩容操作                transfer(tab, nextTab);                break;            }        }        return nextTab;    }    return table;}

下来看看transfer扩容操作是如何执行的，这里感觉是ConcurrentHashmap的一个精华点，叹为观止。

private final void transfer(Node[] tab, Node[] nextTab) {    int n = tab.length, stride;    // 首先进行分段，既每个线程每次处理的node数量，最小16    if ((stride = (NCPU > 1) ? (n >>> 3) / NCPU : n) < MIN_TRANSFER_STRIDE)        stride = MIN_TRANSFER_STRIDE; // subdivide range    if (nextTab == null) {            // initiating        try {            @SuppressWarnings("unchecked")            Node[] nt = (Node[])new Node[n << 1];            nextTab = nt;        } catch (Throwable ex) {      // try to cope with OOME            sizeCtl = Integer.MAX_VALUE;            return;        }        nextTable = nextTab;        transferIndex = n;    }    int nextn = nextTab.length;    // 在这里创建了ForwardingNode    ForwardingNode fwd = new ForwardingNode(nextTab);    boolean advance = true;    boolean finishing = false; // to ensure sweep before committing nextTab    for (int i = 0, bound = 0;;) {        Node f; int fh;        // 判断是否要继续        while (advance) {            int nextIndex, nextBound;            // 如果已经结束了或者当前已经到了边界            if (--i >= bound || finishing)                advance = false;            // 扩容时用的指针已经小于0，则结束            else if ((nextIndex = transferIndex) <= 0) {                i = -1;                advance = false;            }            // 扩容的指针，从大向小移动，从大向小移动，每次减小stride            else if (U.compareAndSwapInt                     (this, TRANSFERINDEX, nextIndex,                      nextBound = (nextIndex > stride ?                                   nextIndex - stride : 0))) {                bound = nextBound;                i = nextIndex - 1;                advance = false;            }        }        // i 小于 0 ，已经结束了        if (i < 0 || i >= n || i + n >= nextn) {            int sc;            // 如果已经结束了，那么把table设置为nextTable            if (finishing) {                nextTable = null;                table = nextTab;                sizeCtl = (n << 1) - (n >>> 1);                return;            }            // 说明当前的线程已经工作结束，sizeCtl - 1            if (U.compareAndSwapInt(this, SIZECTL, sc = sizeCtl, sc - 1)) {                if ((sc - 2) != resizeStamp(n) << RESIZE_STAMP_SHIFT)                    return;                finishing = advance = true;                i = n; // recheck before commit            }        }        //如果节点为空，设置该节点为fwd        else if ((f = tabAt(tab, i)) == null)            advance = casTabAt(tab, i, null, fwd);        else if ((fh = f.hash) == MOVED)            advance = true; // already processed        else {            synchronized (f) {                if (tabAt(tab, i) == f) {                    Node ln, hn;                    if (fh >= 0) {                        // 这里为什么是 fh & n 做 & 运算 因为 15 的二进制是 1111  16是10000 31是 11111                        // 所以，扩容前和扩容后只有第一位 & 运算后会变，其它位都不变，所以与 table.length & 就可以了                        int runBit = fh & n;                        Node lastRun = f;                        // 先遍历一遍，确定 ni -> n rehash相等的一段，这样下一次重新分配槽的时候这一段就不再遍历                        for (Node 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 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(ph, pk, pv, ln);                            else                                hn = new Node(ph, pk, pv, hn);                        }                        setTabAt(nextTab, i, ln);                        setTabAt(nextTab, i + n, hn);                        // 把已完成的节点标记为fwd                        setTabAt(tab, i, fwd);                        advance = true;                    }                    else if (f instanceof TreeBin) {                        TreeBin t = (TreeBin)f;                        TreeNode lo = null, loTail = null;                        TreeNode hi = null, hiTail = null;                        int lc = 0, hc = 0;                        for (Node e = t.first; e != null; e = e.next) {                            int h = e.hash;                            TreeNode p = new TreeNode                                (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(lo) : t;                        hn = (hc <= UNTREEIFY_THRESHOLD) ? untreeify(hi) :                            (lc != 0) ? new TreeBin(hi) : t;                        setTabAt(nextTab, i, ln);                        setTabAt(nextTab, i + n, hn);                        setTabAt(tab, i, fwd);                        advance = true;                    }                }            }        }    }}

再来看一下对于元素总数的统计实现。

private final void addCount(long x, int check)

首先我们遇到了CounterCell这个类，结构很简单，只有一个long value，它是存储数量的最小单元。

先看第一次的判断条件，如果conterCells已经不为空，说明之前已经出现了并发增加baseCount，否则counterCell不会被初始化。

if ((as = counterCells) != null ||    !U.compareAndSwapLong(this, BASECOUNT, b = baseCount, s = b + x))

或者在改变baseCount的时候出现了冲突，执行下面代码。

CounterCell a; long v; int m;boolean uncontended = true;// 如果 counterCell未初始化，或者长度为0  亦或者没有这个对应的槽   再或者更新对应槽的时候出现冲突// 这个时候说明要么 counterCell未初始化，要么说明又出现了对于同一个槽冲突，所以需要 fullAddCount来解决冲突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();

再来看看fullAddCount做了什么

private final void fullAddCount(long x, boolean wasUncontended) {    int h;    // 如果ThreadLocalRandom还没有被初始化过，说明还没有发生过碰撞    if ((h = ThreadLocalRandom.getProbe()) == 0) {        ThreadLocalRandom.localInit();      // force initialization        h = ThreadLocalRandom.getProbe();        wasUncontended = true;    }    boolean collide = false;                // True if last slot nonempty    for (;;) {        CounterCell[] as; CounterCell a; int n; long v;        // 如果数组已经被初始化        if ((as = counterCells) != null && (n = as.length) > 0) {            // 随机选取的槽还未被初始化            if ((a = as[(n - 1) & h]) == null) {                // 获取锁                if (cellsBusy == 0) {            // Try to attach new Cell                    CounterCell r = new CounterCell(x); // Optimistic create                    // U.compareAndSwapInt(this, CELLSBUSY, 0, 1)通过cas操作来获取锁                    if (cellsBusy == 0 &&                        U.compareAndSwapInt(this, CELLSBUSY, 0, 1)) {                        boolean created = false;                        try {               // Recheck under lock                            CounterCell[] rs; int m, j;                            if ((rs = counterCells) != null &&                                (m = rs.length) > 0 &&                                rs[j = (m - 1) & h] == null) {                                rs[j] = r;                                created = true;                            }                        } finally {                            cellsBusy = 0;                        }                        if (created)                            break;                        continue;           // Slot is now non-empty                    }                }                collide = false;            }            // 有竞争的            else if (!wasUncontended)       // CAS already known to fail                wasUncontended = true;      // Continue after rehash            else if (U.compareAndSwapLong(a, CELLVALUE, v = a.value, v + x))                break;            // 如果槽的数量已经超过了cpu个数，就不会碰撞了            else if (counterCells != as || n >= NCPU)                collide = false;            // At max size or stale            else if (!collide)                collide = true;            // 获取锁，并对CounterCell进行扩容操作            else if (cellsBusy == 0 &&                     U.compareAndSwapInt(this, CELLSBUSY, 0, 1)) {                try {                    if (counterCells == as) {// Expand table unless stale                        CounterCell[] rs = new CounterCell[n << 1];                        for (int i = 0; i < n; ++i)                            rs[i] = as[i];                        counterCells = rs;                    }                } finally {                    cellsBusy = 0;                }                collide = false;                continue;                   // Retry with expanded table            }            h = ThreadLocalRandom.advanceProbe(h);        }        // counter cell 没有初始化的情况        else if (cellsBusy == 0 && counterCells == as &&                 U.compareAndSwapInt(this, CELLSBUSY, 0, 1)) {            boolean init = false;            try {                           // Initialize table                if (counterCells == as) {                    // 进行初始化                    CounterCell[] rs = new CounterCell[2];                    rs[h & 1] = new CounterCell(x);                    counterCells = rs;                    init = true;                }            } finally {                cellsBusy = 0;            }            if (init)                break;        }        else if (U.compareAndSwapLong(this, BASECOUNT, v = baseCount, v + x))            break;                          // Fall back on using base    }}

我们前面讲了扩容的机制，那么扩容的发起者肯定就是在addCount中了

while (s >= (long)(sc = sizeCtl) && (tab = table) != null &&       (n = tab.length) < MAXIMUM_CAPACITY)

这里有判断 s 为 sumCount 即 baseCount加上各个节点的和为总数。如果s大于sizeCtl或者table不为空而且没有到达最大值，则进行扩容操作。

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