ConcurrentHashMap 1.8源码解析
来源：互联网 发布：linux运维基础知识 编辑：程序博客网 时间：2024/06/05 02:42
网上介绍ConcurrentHashMap的文章很多，我就只讲我阅读的部分笔记记录一下。
public class ConcurrentHashMap<K, V> extends AbstractMap<K, V> implements ConcurrentMap<K, V>, Serializable {//put方法final V putVal(K key, V value, boolean onlyIfAbsent) { if (key == null || value == null) throw new NullPointerException(); //计算当前key的hash值 int hash = spread(key.hashCode()); int binCount = 0; for (Node<K, V>[] tab = table; ; ) { Node<K, V> f; int n, i, fh; // 1. 如果table为空，初始化； if (tab == null || (n = tab.length) == 0) tab = initTable(); else if ((f = tabAt(tab, i = (n - 1) & hash)) == null) { //2.根据hash值计算得到数组索引i，如果tab[i]为空，直接新建节点Node即可。 if (casTabAt(tab, i, null, new Node<K, V>(hash, key, value, null))) break; // no lock when adding to empty bin } //3.如果tab[i]不为空并且hash值为MOVED，说明该链表正在进行transfer操作，返回扩容完成后的table else if ((fh = f.hash) == MOVED) //帮助执行Transfer操作,并返回transfer后的table tab = helpTransfer(tab, f); else { //4.tab[i]不为空，可能是链表首节点 也可能是红黑树首节点 V oldVal = null; //对首个节点进行加锁操作 synchronized (f) { if (tabAt(tab, i) == f) { //再次校验节点是否发生变更，防止并发操作 //4.1 链表首节点 if (fh >= 0) { binCount = 1; for (Node<K, V> e = f; ; ++binCount) { K ek; // 4.1.1 当前节点为所需节点，直接设置e.val = value即可。 if (e.hash == hash && ((ek = e.key) == key || (ek != null && key.equals(ek)))) { oldVal = e.val; if (!onlyIfAbsent) //对onlyIfAbsent支持 e.val = value; break; } Node<K, V> pred = e; //4.1.2 遍历链表 if ((e = e.next) == null) { //未找到值为key的节点，直接新建Node并加入链表即可。 pred.next = new Node<K, V>(hash, key, value, null); break; } } } //4.2 如果首节点为TreeBin类型，说明为红黑树结构，执行putTreeVal操作。 else if (f instanceof TreeBin) { Node<K, V> p; binCount = 2; //如果当前位置已经为红黑树，则 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) //1.如果tab数组长度长度小于64，直接扩容数组，不再转红黑树。 //2.链表需转成红黑树， treeifyBin(tab, i); if (oldVal != null) return oldVal; break; } } } //检查当前容量是否需要进行扩容。 addCount(1L, binCount); return null; } //初始化table private final Node<K, V>[] initTable() { Node<K, V>[] tab; int sc; while ((tab = table) == null || tab.length == 0) { //其他线程已经在进行初始化，当前线程只需要让出cpu片刻 if ((sc = sizeCtl) < 0) Thread.yield(); // lost initialization race; just spin else if (U.compareAndSwapInt(this, SIZECTL, sc, -1)) { {//利用CAS方法把sizectl的值置为-1 表示本线程正在进行初始化 try { if ((tab = table) == null || tab.length == 0) { int n = (sc > 0) ? sc : DEFAULT_CAPACITY; //sizeCtl默认为0 @SuppressWarnings("unchecked") Node<K, V>[] nt = (Node<K, V>[]) new Node<?, ?>[n]; //初始化tab table = tab = nt; //下一次扩容的大小 sc = n - (n >>> 2);//相当于0.75*n 设置一个扩容的阈值 } } finally { sizeCtl = sc; } break; } } //返回tab return tab; } //帮助执行Transfer操作 final Node<K, V>[] helpTransfer(Node<K, V>[] tab, Node<K, V> f) { Node<K, V>[] nextTab; int sc; //需要帮助扩容 if (tab != null && (f instanceof ForwardingNode) && (nextTab = ((ForwardingNode<K, V>) f).nextTable) != null) { //扩容长度 int rs = resizeStamp(tab.length); while (nextTab == nextTable && table == tab && (sc = sizeCtl) < 0) { //扩容未完成 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);//调用扩容方法，直接进入复制阶段 break; } } return nextTab; } return table; }private final void addCount(long x, int check) { CounterCell[] as; long b, s; //利用CAS方法更新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(); } //校验是否需要扩容 if (check >= 0) { Node<K, V>[] tab, nt; int n, sc; while (s >= (long) (sc = sizeCtl) //数组实际容量大于阀值 && (tab = table) != null && //tab数组不为空 (n = tab.length) < MAXIMUM_CAPACITY) { //小于最大容量 int rs = resizeStamp(n); //计算阀值 if (sc < 0) { //扩容是否已经结束，扩容线程数达到最大数量 if ((sc >>> RESIZE_STAMP_SHIFT) != rs || sc == rs + 1 || sc == rs + MAX_RESIZERS || (nt = nextTable) == null || transferIndex <= 0) break; //已经其他线程在扩容 if (U.compareAndSwapInt(this, SIZECTL, sc, sc + 1)) transfer(tab, nt); //协助扩容 } else if (U.compareAndSwapInt(this, SIZECTL, sc,(rs << RESIZE_STAMP_SHIFT) + 2)) //这里sizeCtl的初始值是一个负值=(rs<<RESIZE_STAMP_SHIFT)+2， //每当一个线程参与进来执行迁移工作时，则该值进行CAS自增， //该线程的任务执行完毕要退出时对该值进行CAS自减操作， //所以当sizeCtl的值等于上述初值则说明了此时未有其他线程还在执行迁移工作，可以去执行收尾工作了 //Demo: //int rs = resizeStamp(16); => rs =32795 //int sc = (rs << RESIZE_STAMP_SHIFT) + 2; => rs =-2145714174 //int sc2 = sc >>> RESIZE_STAMP_SHIFT; => sc2 =32795 transfer(tab, null); //实际扩容*2 s = sumCount(); //获取实际容量 } } }private final void tryPresize(int size) { //计算需扩容容量 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; if (tab == null || (n = tab.length) == 0) { //初始化tab n = (sc > c) ? sc : c; //确定容量 if (U.compareAndSwapInt(this, SIZECTL, sc, -1)) { //标示位：正在初始化 try { if (table == tab) { @SuppressWarnings("unchecked") Node<K, V>[] nt = (Node<K, V>[]) new Node<?, ?>[n]; table = nt; sc = n - (n >>> 2);//下次扩容的阀值 } } finally { sizeCtl = sc; } } } else if (c <= sc || n >= MAXIMUM_CAPACITY) //未超过阀值，或 超出最大长度 break; else if (tab == table) { int rs = resizeStamp(n); 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); } } }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) //计算扩容处理单元数目。 //这里描述可能不是很清晰 stride = MIN_TRANSFER_STRIDE; // subdivide range //初始化nextTab if (nextTab == null) { // initiating try { //构造一个nextTable对象 它的容量是原来的两倍 @SuppressWarnings("unchecked") Node<K, V>[] nt = (Node<K, V>[]) new Node<?, ?>[n << 1]; nextTab = nt; } catch (Throwable ex) { // try to cope with OOME sizeCtl = Integer.MAX_VALUE; return; } nextTable = nextTab; transferIndex = n; //老table长度 } int nextn = nextTab.length; //构造一个连节点指针 用于标志位 ForwardingNode<K, V> fwd = new ForwardingNode<K, V>(nextTab); //并发扩容的关键属性 如果等于true 说明这个节点已经处理过 boolean advance = true; boolean finishing = false; // to ensure sweep before committing nextTab //i表示当前处理位置 //bound表示本次处理界限 for (int i = 0, bound = 0; ; ) { Node<K, V> f; int fh; //这个while循环体的作用就是在控制i-- 通过i--可以依次遍历原hash表中的节点 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; //位置前移一位 advance = false; //当前节点未处理 } } if (i < 0 || i >= n || i + n >= nextn) { int sc; //如果所有的节点都已经完成复制工作 就把nextTable赋值给table 清空临时对象nextTable if (finishing) { nextTable = null; table = nextTab; //扩容阈值设置为原来容量的1.5倍 依然相当于现在容量的0.75倍 sizeCtl = (n << 1) - (n >>> 1); return; } //利用CAS方法更新这个扩容阈值，在这里面sizectl值减一，说明当前线程参与扩容结束 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 } //如果遍历到的节点为空 则放入ForwardingNode指针 } else if ((f = tabAt(tab, i)) == null) advance = casTabAt(tab, i, null, fwd); //如果遍历到ForwardingNode节点 说明这个点已经被处理过了 直接跳过 这里是控制并发扩容的核心 else if ((fh = f.hash) == MOVED) advance = true; // already processed else { //节点上锁 synchronized (f) { if (tabAt(tab, i) == f) { Node<K, V> ln, hn; if (fh >= 0) { //如果fh>=0 证明这是一个Node节点 int runBit = fh & n; //n为原数组的长度。runBit=0 或 runBit=n（n=2^x） //以下的部分在完成的工作是构造两个链表：一个顺序链表，一个开头反序结尾顺序的列表 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); } //在nextTable的i位置上插入一个链表 setTabAt(nextTab, i, ln); //在nextTable的i+n的位置上插入另一个链表 setTabAt(nextTab, i + n, hn); //在table的i位置上插入forwardNode节点 表示已经处理过该节点 setTabAt(tab, i, fwd); //设置advance为true 返回到上面的while循环中 就可以执行i--操作 advance = true; } else if (f instanceof TreeBin) {//对TreeBin对象进行处理 与上面的过程类似 TreeBin<K, V> t = (TreeBin<K, V>) f; TreeNode<K, V> lo = null, loTail = null; //第x为0 TreeNode<K, V> hi = null, hiTail = null; //第x为1 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; } } //如果扩容后已经不再需要tree的结构 反向转换为链表结构 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; //在nextTable的i位置上插入一个链表 setTabAt(nextTab, i, ln); //在nextTable的i+n的位置上插入另一个链表 setTabAt(nextTab, i + n, hn); //在table的i位置上插入forwardNode节点 表示已经处理过该节点 setTabAt(tab, i, fwd); //设置advance为true 返回到上面的while循环中 就可以执行i--操作 advance = true; } } } } } }//当前方法主要是判断链表是否需要转成红黑树。及构造红黑树 private final void treeifyBin(Node<K, V>[] tab, int index) { Node<K, V> b; int n, sc; if (tab != null) { //1.如果tab数组长度长度小于64，直接扩容数组，不再转红黑树。 if ((n = tab.length) < MIN_TREEIFY_CAPACITY) tryPresize(n << 1); //2.链表需转成红黑树 else if ((b = tabAt(tab, index)) != null && b.hash >= 0) { synchronized (b) { if (tabAt(tab, index) == b) { //防止并发，判断当前节点是否发生改变 TreeNode<K, V> hd = null, tl = null; //遍历链表,得到一个由树节点组成的链表 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; else tl.next = p; tl = p; } //将当前链表转成红黑树对象TreeBin，存放在tab的位置i上 setTabAt(tab, index, new TreeBin<K, V>(hd)); //树的根节点 } } } } }//查找方法，比较简单 public V get(Object key) { Node<K, V>[] tab; Node<K, V> e, p; int n, eh; K ek; int h = spread(key.hashCode()); //计算h if ((tab = table) != null && (n = tab.length) > 0 && (e = tabAt(tab, (n - 1) & h)) != null) { if ((eh = e.hash) == h) { //首节点为所找节点 if ((ek = e.key) == key || (ek != null && key.equals(ek))) return e.val; } else if (eh < 0) //红黑树节点，或正在扩容中的中转节点 return (p = e.find(h, key)) != null ? p.val : null; while ((e = e.next) != null) {//遍历链表，查找所需key if (e.hash == h && ((ek = e.key) == key || (ek != null && key.equals(ek)))) return e.val; } } return null; }}阅读全文
0 0
- ConcurrentHashMap 1.8源码解析
- ConcurrentHashMap源码解析（JDK 1.8）
- ConcurrentHashMap源码解析
- ConcurrentHashMap源码解析
- 源码解析ConcurrentHashMap
- ConcurrentHashMap源码解析
- ConcurrentHashMap源码解析
- java concurrentHashMap 源码解析
- 源码解析-集合-ConcurrentHashMap
- ConcurrentHashMap源码解析
- ConcurrentHashMap源码解析
- ConcurrentHashMap源码解析
- ConcurrentHashMap源码解析
- hashmap与concurrenthashmap源码解析
- jdk8之ConcurrentHashMap源码解析
- ConcurrentHashMap源码解析（JDK8）
- ConcurrentHashMap源码分析1.8
- JDK 源码解析 —— ConcurrentHashMap
- 题解
- 【离散数学】1.4集合的运算定律
- 前端用户行为统计
- Ubuntu 简单配置
- 关于通用雷达信号的时频分析与图像绘制（Matlab）
- ConcurrentHashMap 1.8源码解析
- Java Web 项目配置 环境搭建 从零到HelloWorld
- PAT 1069. 微博转发抽奖(20)
- 阿里云服务器Java+Mysql+Tomcat环境搭建(Mac OS下配置)
- 远程桌面无法使用剪切板的解决方法
- unity滚动层dotween移动到指定索引
- leetcode解题方案--062--Unique Paths
- 实验三 二进制补码加法器
- kmeans++
