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; }