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;
}