ConcurrentHashMap源码剖析
- 数据结构
  - Node
  - ForwardingNode
  - TreeNode
  - TreeBin
- 核心成员
- 核心函数
  - ConcurrentHashMap(int initialCapacity)
  - initTable
  - put
  - get
  - treeifyBin
  - tryPresize
  - transfer
  - addCount

ConcurrentHashMap源码剖析

基于jdk1.8。

参考文章：
https://yq.aliyun.com/articles/36781
http://blog.csdn.net/u012834750/article/details/71536618

数据结构

仅列出最重要的代码片段

Node

static class Node<K,V> implements Map.Entry<K,V> {final int hash;final K key;volatile V val;volatile Node<K,V> next;/*** 子类中重写了这个方法，这里的find实现了在链表中查找hash值等于h且key等于k的节点*/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;}}

ForwardingNode

     /*** A node inserted at head of bins during transfer operations.*/// 并不是我们传统的包含key-value的节点，只是一个标志节点，并且指向nextTable，提供find方法而已。生命周期：仅存活于扩容操作且bin不为null时，一定会出现在每个bin的首位。static final class ForwardingNode<K,V> extends Node<K,V> {final Node<K,V>[] nextTable;ForwardingNode(Node<K,V>[] tab) {super(MOVED, null, null, null);this.nextTable = tab;}Node<K,V> find(int h, Object k) {// loop to avoid arbitrarily deep recursion on forwarding nodesouter: for (Node<K,V>[] tab = nextTable;;) {Node<K,V> e; int n;if (k == null || tab == null || (n = tab.length) == 0 ||(e = tabAt(tab, (n - 1) & h)) == null)// 头结点存在e中return null;for (;;) {// 检查头结点是否为要找的nodeint eh; K ek;if ((eh = e.hash) == h &&((ek = e.key) == k || (ek != null && k.equals(ek))))return e;// 如果头结点不是要找的节点if (eh < 0) {// 头结点hash值小于0// 如果头结点是ForwardingNode，那么继续下一个ForwardingNode的find逻辑if (e instanceof ForwardingNode) {tab = ((ForwardingNode<K,V>)e).nextTable;continue outer;}// 如果头结点不是ForwardingNode，就进行相应的find逻辑elsereturn e.find(h, k);}// 查找到尾部仍然没有找到对应的nodeif ((e = e.next) == null)return null;}}}}

TreeNode

红黑树中的节点类，值得注意的是：TreeNode可用于构造双向链表，Node包含next成员，同时，TreeNode加入了prev成员。

static final class TreeNode<K,V> extends Node<K,V> {TreeNode<K,V> parent;  // red-black tree linksTreeNode<K,V> left;TreeNode<K,V> right;TreeNode<K,V> prev;    // needed to unlink next upon deletionboolean red;TreeNode(int hash, K key, V val, Node<K,V> next,TreeNode<K,V> parent) {super(hash, key, val, next);this.parent = parent;}Node<K,V> find(int h, Object k) {return findTreeNode(h, k, null);}final TreeNode<K,V> findTreeNode(int h, Object k, Class<?> kc) {if (k != null) {TreeNode<K,V> p = this;do  {int ph, dir; K pk; TreeNode<K,V> q;TreeNode<K,V> pl = p.left, pr = p.right;if ((ph = p.hash) > h)p = pl;else if (ph < h)p = pr;else if ((pk = p.key) == k || (pk != null && k.equals(pk)))return p;// hash值相等，key不等，左子树不存在，搜索右子树else if (pl == null)p = pr;// hash值相等，key不等，右子树不存在，搜索左子树else if (pr == null)p = pl;/** comparableClassFor的作用是:* 如果k实现了Comparable接口，返回k的Class,* 否则返回null。* compareComparables的作用是：* 将k与pk做比较* 如果TreeNode的Key可以作比较，就可以继续在树中搜索*/else if ((kc != null ||(kc = comparableClassFor(k)) != null) &&(dir = compareComparables(kc, k, pk)) != 0)p = (dir < 0) ? pl : pr;// 由于hash相等，key无法做比较，因此先在右子树中找else if ((q = pr.findTreeNode(h, k, kc)) != null)return q;// 右子树没有找到，继续从当前的节点的左子树中找elsep = pl;} while (p != null);}return null;}}

TreeBin

TreeBin封装了红黑树的逻辑，有关红黑树, 可以参考的资料有《Algorithm》网站以及中文翻译

也可以试玩Red/Black Tree Visualization 。

附文章中提到的红黑树旋转的动图与TreeBin中的rotateLeft、rotateRight代码片段帮助理解。

左旋：
rotateLeft

对应代码

    static <K,V> TreeNode<K,V> rotateLeft(TreeNode<K,V> root,TreeNode<K,V> p) {TreeNode<K,V> r, pp, rl;// p是图中的E节点，r是图中的S节点if (p != null && (r = p.right) != null) {if ((rl = p.right = r.left) != null)rl.parent = p;// p是根节点，则根节点需要变化if ((pp = r.parent = p.parent) == null)(root = r).red = false;// p不是根节点，如果p是pp的左节点，就更新pp的leftelse if (pp.left == p)pp.left = r;elsepp.right = r;// 把p放在左子树中r.left = p;p.parent = r;}return root;}

右旋：
rotateRight

对应代码

static <K,V> TreeNode<K,V> rotateRight(TreeNode<K,V> root,TreeNode<K,V> p) {TreeNode<K,V> l, pp, lr;// p是途中的S，l是图中的Eif (p != null && (l = p.left) != null) {if ((lr = p.left = l.right) != null)lr.parent = p;// p是根节点，则根节点需要变化if ((pp = l.parent = p.parent) == null)(root = l).red = false;else if (pp.right == p)pp.right = l;elsepp.left = l;l.right = p;p.parent = l;}return root;}

仅列出Treebin数据成员以及部分方法：

// 维护了一个红黑树
static final class TreeBin<K,V> extends Node<K,V> {TreeNode<K,V> root;// 链表头结点，每次都将新节点插入到链表的头部，成为新的头结点// 因此该链表中节点的顺序与插入顺序相反volatile TreeNode<K,V> first;volatile Thread waiter;volatile int lockState;/*** 返回匹配的node或者没有匹配的就返回null. 在树中从根节点开始比较，* 当锁不可用的时候进行线性搜索*/final Node<K,V> find(int h, Object k) {if (k != null) {for (Node<K,V> e = first; e != null; ) {int s; K ek;// 锁不可用，lockState包含了WAITER或者WRITER标志位if (((s = lockState) & (WAITER|WRITER)) != 0) {if (e.hash == h &&((ek = e.key) == k || (ek != null && k.equals(ek))))return e;e = e.next;}// 锁可用，当前对象设置为READER状态else if (U.compareAndSwapInt(this, LOCKSTATE, s,s + READER)) {TreeNode<K,V> r, p;try {// 在树中查找匹配的节点p = ((r = root) == null ? null :r.findTreeNode(h, k, null));} finally {Thread w;// 取消当前锁的READER状态if (U.getAndAddInt(this, LOCKSTATE, -READER) ==(READER|WAITER) && (w = waiter) != null)LockSupport.unpark(w);}return p;}}}return null;}// 寻找或者添加一个节点final TreeNode<K,V> putTreeVal(int h, K k, V v) {Class<?> kc = null;boolean searched = false;for (TreeNode<K,V> p = root;;) {int dir, ph; K pk;//  红黑树是空，直接插入到根节点if (p == null) {first = root = new TreeNode<K,V>(h, k, v, null, null);break;}// 根据hash值设置标记位else if ((ph = p.hash) > h)dir = -1;else if (ph < h)dir = 1;// hash值相同，并且k与pk相等（equals），直接返回else if ((pk = p.key) == k || (pk != null && k.equals(pk)))return p;// hash相同，p与pk不equals，但是按照比较接口发现p与pk相等else if ((kc == null &&(kc = comparableClassFor(k)) == null) ||(dir = compareComparables(kc, k, pk)) == 0) {if (!searched) {TreeNode<K,V> q, ch;searched = true;if (((ch = p.left) != null &&(q = ch.findTreeNode(h, k, kc)) != null) ||((ch = p.right) != null &&(q = ch.findTreeNode(h, k, kc)) != null))return q;}// 根据一种确定的规则来进行比较，至于规则本身具体是什么病不重要dir = tieBreakOrder(k, pk);}// 程序运行到这里，说明当前节点不匹配，但子树中可能会有匹配的NodeTreeNode<K,V> xp = p;// 根据大小关系移动p到左子树或者右子树// 如果满足p为null，则说明树中没有节点能与之匹配，应当在p位置插入新节点，然后维护红黑树的性质if ((p = (dir <= 0) ? p.left : p.right) == null) {TreeNode<K,V> x, f = first;first = x = new TreeNode<K,V>(h, k, v, f, xp);if (f != null)f.prev = x;if (dir <= 0)xp.left = x;elsexp.right = x;// 优先将新节点染为红色if (!xp.red)x.red = true;else {lockRoot();try {root = balanceInsertion(root, x);} finally {unlockRoot();}}break;}}assert checkInvariants(root);return null;}
}// 红黑树的平衡插入
static <K,V> TreeNode<K,V> balanceInsertion(TreeNode<K,V> root,TreeNode<K,V> x) {x.red = true; // 将x染成红色for (TreeNode<K,V> xp, xpp, xppl, xppr;;) {// 根节点必须是黑色if ((xp = x.parent) == null) {x.red = false;return x;}// 父节点是黑色或者父节点是根节点// 总之父节点是黑色，那么不会违反红黑树性质// 不需要调整结构，直接返回根节点即可else if (!xp.red || (xpp = xp.parent) == null)return root;// 父节点是红色（需要调整），且在祖父节点的左子树中if (xp == (xppl = xpp.left)) {// 因为父节点为红色，所以xppr必须是红色或空，不可能是黑色// 祖父节点的右节点为红色if ((xppr = xpp.right) != null && xppr.red) {/***     黑                  红*    /  \    （染色后）    / \*   红   红    ->        黑  黑*  /                   /* 红                  红* * 可见通过调整颜色后，子树不需要旋转就可以满足红黑树的性质* 但由于xpp变成了红色，有可能违反红黑树性质，仍然需要向上调整*/xppr.red = false;xp.red = false;xpp.red = true;x = xpp;}// xppr是空else {/***      黑*     /*    红 *      \*       红*/if (x == xp.right) {/*** 进行左旋操作，变为以下形式，* 可以看出此时任然违反红黑树的性质，* 然而x仍然指向了最下面冲突的红色节点，* 此处仅仅调整了树的形状**      黑*     /*    红*   /*  红*/root = rotateLeft(root, x = xp);xpp = (xp = x.parent) == null ? null : xp.parent;}/** 由于调整了树的形状，因此此时树一定长成这个样子* *      黑*     /*    红*   /*  红* * 在染色并右旋之后，变为* *    黑*   /  \* 红     红*/if (xp != null) {xp.red = false;if (xpp != null) {xpp.red = true;root = rotateRight(root, xpp);}}}}// x在祖父节点的右子树中，这种情况与x在祖父节点左子树中类似，因此不多作解释，不明白的话类比即可。else {/***   黑                    红*  /  \     (染色后)      / \* 红    红   ->         黑   黑*        \                    \*         红                   红色*/if (xppl != null && xppl.red) {xppl.red = false;xp.red = false;xpp.red = true;x = xpp;}else {if (x == xp.left) {root = rotateRight(root, x = xp);xpp = (xp = x.parent) == null ? null : xp.parent;}if (xp != null) {xp.red = false;if (xpp != null) {xpp.red = true;root = rotateLeft(root, xpp);}}}}}}

核心成员

    // ForwardingNode的hash值都是-1static final int MOVED     = -1; // Treebin的hash值是-1static final int TREEBIN   = -2; /*** 在第一次insert的时候才进行初始化(延迟初始化)* Size总是2的幂. 直接通过迭代器访问.*/transient volatile Node<K,V>[] table;// nextTable的用途：只有在扩容时是非空的private transient volatile Node<K,V>[] nextTable;/*** Base counter value, used mainly when there is no contention,* but also as a fallback during table initialization* races. Updated via CAS.*/private transient volatile long baseCount;/*** sizeCtl是控制标识符，不同的值表示不同的意义。* -1代表正在初始化； * -(1+有效扩容线程的数量)，比如，-N 表示有N-1个线程正在进行扩容操作；* 0 表示还未进行初始化* 正数代表初始化或下一次进行扩容的大小，类似于扩容阈值。它的值始终是当前ConcurrentHashMap容量的0.75倍，这与loadfactor是对应的。实际容量>=sizeCtl，则扩容。*/private transient volatile int sizeCtl;// 扩容的时候，next数组下标+1private transient volatile int transferIndex;/*** Spinlock (locked via CAS) used when resizing and/or creating CounterCells.*/private transient volatile int cellsBusy;/*** Table of counter cells. When non-null, size is a power of 2.*/private transient volatile CounterCell[] counterCells;// 视图private transient KeySetView<K,V> keySet;private transient ValuesView<K,V> values;private transient EntrySetView<K,V> entrySet;

核心函数

ConcurrentHashMap(int initialCapacity)

之所以列出这个函数，是因为这个函数初始化了sizeCtl，并且可以看出table在这里并没有被初始化，而是在插入元素的时候进行延迟初始化。
我们要注意的是table的长度始终是2的幂，sizeCtl的值为正数时表示扩容的最小阀值。

 // 需要注意的是，构造了一个能够容纳initialCapacity个元素的对象，// 但实际table的大小比1.5倍的initialCapacity还多public ConcurrentHashMap(int initialCapacity) {if (initialCapacity < 0)throw new IllegalArgumentException();// 保证cap是2的幂，其中tableSizeFor返回大于入参的最小的2的幂int cap = ((initialCapacity >= (MAXIMUM_CAPACITY >>> 1)) ?MAXIMUM_CAPACITY :tableSizeFor(initialCapacity + (initialCapacity >>> 1) + 1));this.sizeCtl = cap;}

initTable

     // 初始化table，使用sizeCtl记录table的容量// 为了保证并发访问不会出现冲突，使用了Unsafe的CAS操作private final Node<K,V>[] initTable() {Node<K,V>[] tab; int sc;// tab是空的while ((tab = table) == null || tab.length == 0) {// 如果已经初始化过if ((sc = sizeCtl) < 0)Thread.yield(); // 退出初始化数组的竞争; just spin// 如果没有线程在初始化，将sizeCtl设置为-1，表示正在初始化// CAS操作，由此可见sizeCtl维护table的并发访问else if (U.compareAndSwapInt(this, SIZECTL, sc, -1)) {try {// 再次检查table是否为空if ((tab = table) == null || tab.length == 0) {// 计算分配多少个Node// sc大于0的时候表示要分配的大小// 否则默认分配16个nodeint n = (sc > 0) ? sc : DEFAULT_CAPACITY;@SuppressWarnings("unchecked")Node<K,V>[] nt = (Node<K,V>[])new Node<?,?>[n];table = tab = nt;// 下次扩容的最小阀值0.75*n// 注意0.75 * n < n，而且它很可能不是2的幂，// 例如n = 16， 则sc = 12；// 因此这个阀值在后续扩容情况下实际上不会成为数组的容量值，但它可以用来能保证用户提供了容量大小时，能够容纳用户要求数目的元素。sc = n - (n >>> 2);}} finally {sizeCtl = sc;}break;}}return tab;}

put

put过程的描述：

为表述方便，用符号i 来表示 (n - 1) & hash，用newNode表示使用key,value创建的节点

loop:
{if table == null{初始化一个默认长度为16的数组}else table[i] == null{   table[i] = newNode}else hash == -1，table[i]是ForwardingNode{进行整合表的操作}else{if hash >= 0，table[i]不是特殊Node(链表中的Node){将newNode插入到链表中}else table[i]是TreeBin{newNode插入到TreeNode中}}addCount(1L, binCount);
}

通过研读代码，发现Doug Lea使用了一种有效且高效的技巧：
在循环里面嵌套使用CAS操作。这种技巧把临界区变得很小，因此比较高效。

put源码如下：

    public V put(K key, V value) {return putVal(key, value, false);}/** put和putIfAbsent都是通过调用putVal方法来实现的*/final V putVal(K key, V value, boolean onlyIfAbsent) {// ConcurrentHashMap不支持key和value是nullif (key == null || value == null) throw new NullPointerException();// 获取hash值int hash = spread(key.hashCode());int binCount = 0;for (Node<K,V>[] tab = table;;) {Node<K,V> f; int n, i, fh;// case 1：tab为null，需要初始化tabif (tab == null || (n = tab.length) == 0)tab = initTable();// case 2: 没有任何节点hash值与当前要插入的节点相同else if ((f = tabAt(tab, i = (n - 1) & hash)) == null) {if (casTabAt(tab, i, null,new Node<K,V>(hash, key, value, null)))break;                   // no lock when adding to empty bin}// case 3: 当遇到表连接点时，需要进行整合表的操作// 需要注意的是，遇到连接点的时候，并没有插入新节点，仅仅帮助扩容，因为当前线程迫切需要尽快插入新节点，只能等待扩容完毕才有可能插入新节点else if ((fh = f.hash) == MOVED)tab = helpTransfer(tab, f);// case 4: 找到对应于hash值的链表首节点，且该节点不是连接节点else {V oldVal = null;synchronized (f) {if (tabAt(tab, i) == f) {if (fh >= 0) {binCount = 1;for (Node<K,V> e = f;; ++binCount) {K ek;// 如果找到相同key的node，根据onlyIfAbsent来更新node的值if (e.hash == hash &&((ek = e.key) == key ||(ek != null && key.equals(ek)))) {oldVal = e.val;if (!onlyIfAbsent)e.val = value;break;}// 如果一直到链表的尾部都没有找到任何node的key与key相同，就插入到链表的尾部Node<K,V> pred = e;if ((e = e.next) == null) {pred.next = new Node<K,V>(hash, key,value, null);break;}}}// 如果该节点是TreeBin，就插入到TreeBin中else if (f instanceof TreeBin) {Node<K,V> p;binCount = 2;// 当存在相同的key时，putTreeVal不会修改那个TreeNode，而是返回给p，由onlyIfAbsent决定是否修改p.valif ((p = ((TreeBin<K,V>)f).putTreeVal(hash, key,value)) != null) {oldVal = p.val;if (!onlyIfAbsent)p.val = value;}}}}// 若链表长度不低于8，就将链表转换为树if (binCount != 0) {if (binCount >= TREEIFY_THRESHOLD)treeifyBin(tab, i);if (oldVal != null)return oldVal;break;}}}// 添加计数，如有需要，扩容addCount(1L, binCount);return null;}// 给tab[i]赋值// 如果tab[i]等于c,就将tab[i]与v交换数值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);}/*** 协助扩容方法。* 多线程下，当前线程检测到其他线程正进行扩容操作，则协助其一起扩容；*（只有这种情况会被调用）从某种程度上说，其“优先级”很高，* 只要检测到扩容，就会放下其他工作，先扩容。* 调用之前，nextTable一定已存在。*/final Node<K,V>[] helpTransfer(Node<K,V>[] tab, Node<K,V> f) {Node<K,V>[] nextTab; int sc;// 如果f是tab中的连接节点，并且它所连接的table非空if (tab != null && (f instanceof ForwardingNode) &&(nextTab = ((ForwardingNode<K,V>)f).nextTable) != null) {// 标志位int rs = resizeStamp(tab.length);// 当正在扩容时，帮助扩容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;}

get

get方法比较简单，没有使用锁，而是用Unsafe来保证获取的头结点是volatile的

 public V get(Object key) {Node<K,V>[] tab; Node<K,V> e, p; int n, eh; K ek;// 获取hash值hint h = spread(key.hashCode());// tab只是保存了hash值相同的头结点if ((tab = table) != null && (n = tab.length) > 0 && // table里面有元素(e = tabAt(tab, (n - 1) & h)) != null) {// 根据h来获取头结点e// hash值相同，如果找到key，直接返回if ((eh = e.hash) == h) {if ((ek = e.key) == key || (ek != null && key.equals(ek)))return e.val;}// todo：看一下hash值什么时候小于0else 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;}//tableAt方法使用了Unsafe对象来获取数组中下标为i的对象
static final <K,V> Node<K,V> tabAt(Node<K,V>[] tab, int i) {// 第i个元素实际地址i * (2^ASHIFT) + ABASEreturn (Node<K,V>)U.getObjectVolatile(tab, ((long)i << ASHIFT) + ABASE);}

treeifyBin

     // 如果tab的长度很小，小于64个，就尝试进行扩容为两倍，// 否则就将以tab[index]开头的链表转换为Treebinprivate final void treeifyBin(Node<K,V>[] tab, int index) {Node<K,V> b; int n, sc;if (tab != null) {// tab的长度小于64，就尝试进行扩容if ((n = tab.length) < MIN_TREEIFY_CAPACITY)tryPresize(n << 1);else if ((b = tabAt(tab, index)) != null && b.hash >= 0) {synchronized (b) {if (tabAt(tab, index) == b) {TreeNode<K,V> hd = null, tl = null;// 这个循环建立了TreeNode中的双向链表，hd保存了双向链表的头结点for (Node<K,V> e = b; e != null; e = e.next) {TreeNode<K,V> p =new TreeNode<K,V>(e.hash, e.key, e.val,null, null);if ((p.prev = tl) == null)hd = p;elsetl.next = p;tl = p;}setTabAt(tab, index, new TreeBin<K,V>(hd));}}}}}

tryPresize

有关扩容，可以参考深入分析 ConcurrentHashMap 1.8 的扩容实现这篇文章。

    // 尝试扩容使它能放size个元素private final void tryPresize(int size) {// 计算扩容后的数量int c = (size >= (MAXIMUM_CAPACITY >>> 1)) ? MAXIMUM_CAPACITY :tableSizeFor(size + (size >>> 1) + 1);int sc;while ((sc = sizeCtl) >= 0) {Node<K,V>[] tab = table; int n;// 如果tab是空的，直接扩容if (tab == null || (n = tab.length) == 0) {// 计算扩容后的容量n = (sc > c) ? sc : c;if (U.compareAndSwapInt(this, SIZECTL, sc, -1)) {try {if (table == tab) {@SuppressWarnings("unchecked")Node<K,V>[] nt = (Node<K,V>[])new Node<?,?>[n];table = nt;// 下次扩容的容量阀值是0.75 * nsc = n - (n >>> 2);}} finally {sizeCtl = sc;}}}// 容量已经够用，不需要进行扩容；或者容量太大，无法进行扩容。else if (c <= sc || n >= MAXIMUM_CAPACITY)break;// 仍然需要扩容else if (tab == table) {int rs = resizeStamp(n);// todo：不是很懂为什么会出现 sc < 0 ？先看一下transfer的实现if (sc < 0) {Node<K,V>[] nt;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);}}}

transfer

伪代码：n = table.lengthnextTable = new Node[2 * n]forwardingNode = new ForwardingNodeforwardingNode.nextTable = nextTable;for(table[i] : table)
{for(p = table[i]; p != null ; p = p.next){if(p.hash & n == 0)将p放入nextTable[i]的数据集合中else将p放入nextTable[i+n]的数据集合中}table[i] = forwardingNode;
}table = nextTable;nextTable = null;

数学公式：

已知：n = 2 ^ k ， hash & (n-1) = i，显而易见：
（1）若 hash & n = 0，则 hash &(2n - 1) = i ；
（2）若 hash & n != 0, 则 hash&(2n - 1) = i + n。

源代码在此：

     // 把table中所有的Node放入新的table中private final void transfer(Node<K,V>[] tab, Node<K,V>[] nextTab) {int n = tab.length, stride;if ((stride = (NCPU > 1) ? (n >>> 3) / NCPU : n) < MIN_TRANSFER_STRIDE)stride = MIN_TRANSFER_STRIDE; // subdivide rangeif (nextTab == null) {            // initiatingtry {@SuppressWarnings("unchecked")Node<K,V>[] nt = (Node<K,V>[])new Node<?,?>[n << 1];nextTab = nt;} catch (Throwable ex) {      // try to cope with OOMEsizeCtl = Integer.MAX_VALUE;return;}nextTable = nextTab;transferIndex = n;}int nextn = nextTab.length;ForwardingNode<K,V> fwd = new ForwardingNode<K,V>(nextTab);boolean advance = true;boolean finishing = false; // to ensure sweep before committing nextTabfor (int i = 0, bound = 0;;) {Node<K,V> f; int fh;while (advance) {int nextIndex, nextBound;if (--i >= bound || finishing)advance = false;else if ((nextIndex = transferIndex) <= 0) {i = -1;advance = false;}else if (U.compareAndSwapInt(this, TRANSFERINDEX, nextIndex,nextBound = (nextIndex > stride ?nextIndex - stride : 0))) {bound = nextBound;i = nextIndex - 1;advance = false;}}if (i < 0 || i >= n || i + n >= nextn) {int sc;if (finishing) {nextTable = null;table = nextTab;sizeCtl = (n << 1) - (n >>> 1);return;}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}}else if ((f = tabAt(tab, i)) == null)advance = casTabAt(tab, i, null, fwd);else if ((fh = f.hash) == MOVED)advance = true; // already processedelse {synchronized (f) {if (tabAt(tab, i) == f) {Node<K,V> ln, hn;if (fh >= 0) {int runBit = fh & n;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);elsehn = new Node<K,V>(ph, pk, pv, hn);}setTabAt(nextTab, i, ln);setTabAt(nextTab, i + n, hn);setTabAt(tab, i, fwd);advance = true;}else if (f instanceof TreeBin) {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;elseloTail.next = p;loTail = p;++lc;}else {if ((p.prev = hiTail) == null)hi = p;elsehiTail.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;}}}}}}

addCount

    /*** Adds to count, and if table is too small and not already* resizing, initiates transfer. If already resizing, helps* perform transfer if work is available.  Rechecks occupancy* after a transfer to see if another resize is already needed* because resizings are lagging additions.** @param x the count to add* @param check if <0, don't check resize, if <= 1 only check if uncontended*/// 添加计数，如果table太小且table没有在扩容，就进行扩容private final void addCount(long x, int check) {CounterCell[] as; long b, s;// 利用CAS快速更新baseCount的值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();}// 当之前检查的节点个数大于等于0时，才考虑扩容if (check >= 0) {Node<K,V>[] tab, nt; int n, sc;while (s >= (long)(sc = sizeCtl) && (tab = table) != null &&(n = tab.length) < MAXIMUM_CAPACITY) {// 为当前的n保留一个数，不同的数组n（这里n=2^k）得到的结果必然不同，可类比时间戳int rs = resizeStamp(n);// 如果有线程正在扩容，就帮助其扩容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);}// 没有线程在扩容，直接扩容else if (U.compareAndSwapInt(this, SIZECTL, sc,(rs << RESIZE_STAMP_SHIFT) + 2))transfer(tab, null);s = sumCount();}}}

转载于:https://www.cnblogs.com/mcai/p/9628512.html