JDK1.7
JDK1.8
//CHM初始化（一个16个节点的node数组）
private final Node<K,V>[] initTable() {
Node<K,V>[] tab; int sc;
while ((tab = table) == null || tab.length == 0) {
if ((sc = sizeCtl) < 0)
Thread.yield(); // 已经有线程把sizeCtl赋值为-1了，此线程通过Thread.yield();释放CPU时间片
else if (U.compareAndSwapInt(this, SIZECTL, sc, -1)) { //设置SIZECTL为-1
try {
if ((tab = table) == null || tab.length == 0) { //(tab = table) 把全局变量赋值给本地变量，为了提升性能
int n = (sc > 0) ? sc : DEFAULT_CAPACITY; //DEFAULT_CAPACITY=16
@SuppressWarnings("unchecked")
Node<K,V>[] nt = (Node<K,V>[])new Node<?,?>[n];
table = tab = nt;
sc = n - (n >>> 2);//无符合右移两位 扩容因子：16*0.75
}
} finally {
sizeCtl = sc;
}
break;
}
}
return tab;
}
put方法
//假设put("xiaofeng","架构师");
final V putVal(K key, V value, boolean onlyIfAbsent) {
if (key == null || value == null) throw new NullPointerException();
int hash = spread(key.hashCode());
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(); //1.第一步初始化node数组
else if ((f = tabAt(tab, i = (n - 1) & hash)) == null) { // 2,(n - 1) & hash) 位置计算，会得到一个下标值，下标值可能是0~15中任何一个值
if (casTabAt(tab, i, null, new Node<K,V>(hash, key, value, null))) //当下标i位置的值为null的时候，就替换成一个new Node<"xiaofeng","架构师"> 注意不要把node<k,v>当做一个hashmap,它只是hashmap中一个底层的数据节点
break; // no lock when adding to empty bin
}
else if ((fh = f.hash) == MOVED)
tab = helpTransfer(tab, f);
else {
V oldVal = null;
synchronized (f) { //当前要赋值的节点已经有值了，存在哈希冲突，此时要锁住节点f，因为有可能同时有其他数据也哈希到此节点上了，
if (tabAt(tab, i) == f) {
if (fh >= 0) { //fh在前面已经赋值了，表示f的hash值
binCount = 1;
for (Node<K,V> e = f;; ++binCount) {
K ek;
if (e.hash == hash &&
((ek = e.key) == key ||
(ek != null && key.equals(ek)))) { //这个if其实相同的Key,需要把value覆盖. [存在hash冲突的可能是同一个key]
oldVal = e.val;
if (!onlyIfAbsent)
e.val = value;
break;
}
Node<K,V> pred = e; //e表示头节点
if ((e = e.next) == null) { //建立一个链表关系【通过链式去解决hash冲突】
pred.next = new Node<K,V>(hash, key,
value, null);
break;
}
}
}
else if (f instanceof TreeBin) {
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)
treeifyBin(tab, i);
if (oldVal != null)
return oldVal;
break;
}
}
}
addCount(1L, binCount);
return null;
}
addCount 两个阶段： 1，初始化阶段 2，并发更新的阶段
private transient volatile long baseCount; //在没有竞争的情况下，通过cas操作更新元素格个数
private transient volatile CounterCell[] counterCells;//存在线程竞争的情况下，存储元素个数
@sun.misc.Contended static final class CounterCell {
volatile long value;
CounterCell(long x) { value = x; }
}
final long sumCount() {
CounterCell[] as = counterCells; CounterCell a;
long sum = baseCount;
if (as != null) {
for (int i = 0; i < as.length; ++i) {
if ((a = as[i]) != null)
sum += a.value;
}
}
return sum;
}
size()计算:
public int size() {
long n = sumCount();
return ((n < 0L) ? 0 :
(n > (long)Integer.MAX_VALUE) ? Integer.MAX_VALUE :
(int)n);
}
原理：baseCount + (遍历CounterCell[]数组中每个CounterCell中的value) = sum
1).直接去访问baseCount 累加元素个数
2).找到CousnterCell[] 随机的某个下标位置， value=v+x() -> 表示记录元素个数 如果
3).前面都是失败， 则进入到fullAndCount();
private final void addCount(long x, int check) {
CounterCell[] as; long b, s; //CounterCell[] as 定义局部变量，提升访问性能
if ((as = counterCells) != null ||
!U.compareAndSwapLong(this, BASECOUNT, b = baseCount, s = b + x)) { //修改b=b+x 添加失败走下面逻辑
CounterCell a; long v; int m;
boolean uncontended = true;
//ThreadLocalRandom.getProbe() & m ，随机函数与上m(数组长度)得到一个数组下标。
//
if (as == null || (m = as.length - 1) < 0 ||
(a = as[ThreadLocalRandom.getProbe() & m]) == null ||
!(uncontended = U.compareAndSwapLong(a, CELLVALUE, v = a.value, v + x))) { //找到CousnterCell[] 随机的某个下标位置， value=v+x() [表示记录元素个数]
fullAddCount(x, uncontended); //如果前面都是失败， 则进入到fullAndCount();
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) {
int rs = resizeStamp(n);
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);
}
else if (U.compareAndSwapInt(this, SIZECTL, sc,
(rs << RESIZE_STAMP_SHIFT) + 2))
transfer(tab, null);
s = sumCount();
}
}
}
//复制fullAndCount部分代码
for (;;) {
CounterCell[] as; CounterCell a; int n; long v;
//已经初始化后自旋后进入第一个if
if ((as = counterCells) != null && (n = as.length) > 0) {
if ((a = as[(n - 1) & h]) == null) {
if (cellsBusy == 0) {
CounterCell r = new CounterCell(x);
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;
}
else if (!wasUncontended) // CAS already known to fail
wasUncontended = true; // Continue after rehash
else if (U.compareAndSwapLong(a, CELLVALUE, v = a.value, v + x)) //如果某下标元素不为空，就修改v=v+x
break;
else if (counterCells != as || n >= NCPU)
collide = false; // At max size or stale
else if (!collide)
collide = true;
else if (cellsBusy == 0 &&
U.compareAndSwapInt(this, CELLSBUSY, 0, 1)) {
try {
//存在竞争的情况下扩容
if (counterCells == as) {// Expand table unless stale
CounterCell[] rs = new CounterCell[n << 1]; //<<1 扩容一倍[比如2变成4]
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);
}
//进到这里面的时候counterCells为null了，cellsBusy=0表示当前没有线程来操作，接下来就是进行一个占位[把cellsBusy改为1]
else if (cellsBusy == 0 && counterCells == as &&
U.compareAndSwapInt(this, CELLSBUSY, 0, 1)) {
//进入到下面逻辑表示当前线程获得了初始化的资格
boolean init = false;
try { // Initialize table
if (counterCells == as) {
CounterCell[] rs = new CounterCell[2]; //初始化一个长度为2的数组
rs[h & 1] = new CounterCell(x); //h & 1 hash值与上1还是计算下标的
counterCells = rs;
init = true;
}
} finally {
cellsBusy = 0; //相当于释放，有点锁的概念
}
if (init)
break;
}
else if (U.compareAndSwapLong(this, BASECOUNT, v = baseCount, v + x)) //
break; // Fall back on using base
}
(addCount图)
扩容
(当元素个数大于阈值的时候) 如果此时正在扩容， 在扩容阶段进来的线程会协助扩容
addCount方法中代码：
if (check >= 0) { //表示要去做扩容
Node<K,V>[] tab, nt; int n, sc;
while (s >= (long)(sc = sizeCtl) && (tab = table) != null &&
(n = tab.length) < MAXIMUM_CAPACITY) { //需要扩容了
int rs = resizeStamp(n);
//表示已经有线程正在扩容
if (sc < 0) {
if ((sc >>> RESIZE_STAMP_SHIFT) != rs || sc == rs + 1 ||
sc == rs + MAX_RESIZERS || (nt = nextTable) == null || transferIndex <= 0) //if(true)表示不需要扩容
break;
if (U.compareAndSwapInt(this, SIZECTL, sc, sc + 1))
transfer(tab, nt);//协助扩容
}
//表示当前没有线程在扩容
//rs << RESIZE_STAMP_SHIFT) + 2 ： 第一次扩容, + 2
else if (U.compareAndSwapInt(this, SIZECTL, sc,
(rs << RESIZE_STAMP_SHIFT) + 2))
transfer(tab, null);
s = sumCount();
}
}
static final int resizeStamp(int n) {
return Integer.numberOfLeadingZeros(n) | (1 << (RESIZE_STAMP_BITS - 1));
}
