初始化容量不小于concurrencyLevel(estimated threads).
public ConcurrentHashMap(int initialCapacity,
float loadFactor, int concurrencyLevel) {
if (!(loadFactor > 0.0f) || initialCapacity < 0 || concurrencyLevel <= 0)
throw new IllegalArgumentException();
if (initialCapacity < concurrencyLevel) // Use at least as many bins
initialCapacity = concurrencyLevel; // as estimated threads
long size = (long)(1.0 + (long)initialCapacity / loadFactor);
int cap = (size >= (long)MAXIMUM_CAPACITY) ?
MAXIMUM_CAPACITY : tableSizeFor((int)size);
this.sizeCtl = cap;
}
init方法
private final Node<K,V>[] initTable() {
Node<K,V>[] tab; int sc;
while ((tab = table) == null || tab.length == 0) {
//sizeCtl由volatile修饰, 如果为负, 意味其他线程正在扩容, 所以该线程请往后稍稍
if ((sc = sizeCtl) < 0)
Thread.yield(); // lost initialization race; just spin
//CAS修改,四个参数为: this, valueOffset, expect, update);
//即通过(对象, 偏移量)来获取修改处位置, 通过expect, update进行修改
else if (U.compareAndSwapInt(this, SIZECTL, sc, -1)) {
try {
if ((tab = table) == null || tab.length == 0) {
int n = (sc > 0) ? sc : DEFAULT_CAPACITY;
@SuppressWarnings("unchecked")
Node<K,V>[] nt = (Node<K,V>[])new Node<?,?>[n];
table = tab = nt;
//求0.75n的高效算法
sc = n - (n >>> 2);
}
} finally {
//下次扩容的阈值
sizeCtl = sc;
}
break;
}
}
return tab;
}
treeifyBin方法
private final void treeifyBin(Node<K,V>[] tab, int index) {
Node<K,V> b; int n, sc;
if (tab != null) {
//如果容量小于64, 则只扩容, 不转化
if ((n = tab.length) < MIN_TREEIFY_CAPACITY)
tryPresize(n << 1);
//如果此处有值
else if ((b = tabAt(tab, index)) != null && b.hash >= 0) {
synchronized (b) {
//双重确认
if (tabAt(tab, index) == b) {
TreeNode<K,V> hd = null, tl = null;\
//从b开始, 依次转换
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;
}
setTabAt(tab, index, new TreeBin<K,V>(hd));
}
}
}
}
}
get方法
public V get(Object key) {
Node<K,V>[] tab; Node<K,V> e, p; int n, eh; K ek;
int h = spread(key.hashCode());
//如果tab有数据
if ((tab = table) != null && (n = tab.length) > 0 &&
//通过内存位移找到对应的e不为空
(e = tabAt(tab, (n - 1) & h)) != null) {
if ((eh = e.hash) == h) {
//直接找到
if ((ek = e.key) == key || (ek != null && key.equals(ek)))
return e.val;
}
//find的实现有好几种, 与e的种类有关
//eh=-1, 说明该节点是ForwardingNode, 正在迁移
//eh=-2, 说明该节点是TreeBin, 会加锁读值
//eh>=0, 说明该节点下挂链表, 直接遍历
else 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;
}
分别来看看Node, TreeBin, ForwardingNode节点的find方法
/**
* Virtualized support for map.get(); overridden in subclasses.
*/
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;
}
TreeBin的find方法
final Node<K,V> find(int h, Object k) {
if (k != null) {
for (Node<K,V> e = first; e != null; ) {
int s; K ek;
//如果lockState=Waiter 或者 Writer, 就使用链表形式
//
if (((s = lockState) & (WAITER|WRITER)) != 0) {
if (e.hash == h &&
((ek = e.key) == k || (ek != null && k.equals(ek))))
return e;
e = e.next;
}
//WRITER: 001, WAITER: 010, READER: 100
//将LOCKSTATE增加READER状态
else if (U.compareAndSwapInt(this, LOCKSTATE, s,
s + READER)) {
TreeNode<K,V> r, p;
try {
p = ((r = root) == null ? null :
r.findTreeNode(h, k, null));
} finally {
Thread w;
// 如果当前线程是最后一个读线程，且有写线程因为读锁而阻塞，则写线程，告诉它可以尝试获取写锁了
if (U.getAndAddInt(this, LOCKSTATE, -READER) ==
(READER|WAITER) && (w = waiter) != null)
LockSupport.unpark(w);
}
return p;
}
}
}
return null;
}
ForwardingNode节点的find方法
//A node inserted at head of bins during transfer operations.
static final class ForwardingNode<K,V> extends Node<K,V> {
final Node<K,V>[] nextTable;
ForwardingNode(Node<K,V>[] tab) {
//父类的构造器, hash为-1, key,val,next均为null
super(MOVED, null, null, null);
this.nextTable = tab;
}
//重写了find方法, h: hash, k: key
Node<K,V> find(int h, Object k) {
// loop to avoid arbitrarily(任意) deep recursion on forwarding nodes
//多年不见的类似goto的语句
outer: for (Node<K,V>[] tab = nextTable;;) {
Node<K,V> e; int n;
//经典的连环灵魂拷问: 你key是null么, 你tab是null么, 你e是null?
if (k == null || tab == null || (n = tab.length) == 0 ||
(e = tabAt(tab, (n - 1) & h)) == null)
return null;
for (;;) {
int eh; K ek;
//开始在nextTable上找key, hash相等并且值相等(先判断hash,效率高, 如果hash不等, 值必然不等. 如果hash相等, 值大概率相等,需要验证)
//值相等有两种情况, 一种是绝对相等, 一种是equeals
if ((eh = e.hash) == h &&
((ek = e.key) == k || (ek != null && k.equals(ek))))
return e;
//如果eh是-1或者-2
if (eh < 0) {
//ForwardingNode接连ForwardingNode
if (e instanceof ForwardingNode) {
tab = ((ForwardingNode<K,V>)e).nextTable;
continue outer;
}
else
return e.find(h, k);
}
if ((e = e.next) == null)
return null;
}
}
}
}
put方法
/** Implementation for put and putIfAbsent */
final V putVal(K key, V value, boolean onlyIfAbsent) {
//不能为null
if (key == null || value == null) throw new NullPointerException();
int hash = spread(key.hashCode());
//目前在bin链上第几个
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();
else if ((f = tabAt(tab, i = (n - 1) & hash)) == null) {
//通过CAS更新值
if (casTabAt(tab, i, null,
new Node<K,V>(hash, key, value, null)))
break; // no lock when adding to empty bin
}
//MOVED: -1, hash for forwarding nodes
//说明正在扩容
else if ((fh = f.hash) == MOVED)
//更新tab
tab = helpTransfer(tab, f);
else {
//fh可能为-2, 或>=0, 即TreeBin或下挂链表
V oldVal = null;
//对该Node加锁, 其他线程无法改变该Node
synchronized (f) {
//双重确认
if (tabAt(tab, i) == f) {
//如果是链表
if (fh >= 0) {
binCount = 1;
for (Node<K,V> e = f;; ++binCount) {
K ek;
if (e.hash == hash &&
//key的hash匹配上的话
((ek = e.key) == key ||
(ek != null && key.equals(ek)))) {
oldVal = e.val;
if (!onlyIfAbsent)
e.val = value;
break;
}
Node<K,V> pred = e;
//巧妙得将e前进一个Node
if ((e = e.next) == null) {
pred.next = new Node<K,V>(hash, key,
value, null);
break;
}
}
}
//如果是TreeBin
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;
}
Cmap的计数原理
public int size() {
long n = sumCount();
return ((n < 0L) ? 0 :
(n > (long)Integer.MAX_VALUE) ? Integer.MAX_VALUE :
(int)n);
}
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;
}
private final void addCount(long x, int check) {
CounterCell[] as; long b, s;
//如果as为null 或者 在basecount上更新失败
if ((as = counterCells) != null ||
!U.compareAndSwapLong(this, BASECOUNT, b = baseCount, s = b + x)) {
CounterCell a; long v; int m;
boolean uncontended = true;
//如果 1. as为空 2. 从as中随机取出的值为空 3. 修改as中随机取出的值失败,
//就fullAddCount. ThreadLocalRandom.getProbe()方法可以避免性能问题及可能
//产生相同的随机数的问题.
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();
}
//判断是否扩容
if (check >= 0) {
Node<K,V>[] tab, nt; int n, sc;
//s为元素总数,
while (s >= (long)(sc = sizeCtl) && (tab = table) != null &&
(n = tab.length) < MAXIMUM_CAPACITY) {
//计算数组长度盖戳标记值
int rs = resizeStamp(n);
//sizeCtl: 0 初始值,意味还没数组初始化, -1: 线程开始初始化, -n: 有n个线程
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();
}
}
}
counterCells的原理: 为了统计总数, 构造了包含baseCount和counterCells的数据结构, sum = baseCount+sum of(counterCells), 执行add时, 先更新sum, 如果更新失败则随机更新counterCells中一个元素的值, 将CAS分散在counterCells数组中, 避免在高并发下同时CASbaseCount带来的性能问题.
关于transfer, Cmap定义了最小转移跨度为16,即每个线程至少要转移16个Node.
但每个cpu能操作多少线程呢, 计算方法为:1个cpu核心数运行8个线程并发执行扩容.
private static final int MIN_TRANSFER_STRIDE = 16;
tryPresize如何扩容:
/** 尝试对table数组进行扩容,tryPresize在putAll以及treeifyBin中调用
* Tries to presize table to accommodate the given number of elements.
*
* @param size number of elements (doesn't need to be perfectly accurate)
*/
private final void tryPresize(int size) {
// 给定的容量若>=MAXIMUM_CAPACITY的一半，直接扩容到允许的最大值，否则调用函数扩容
int c = (size >= (MAXIMUM_CAPACITY >>> 1)) ? MAXIMUM_CAPACITY :
tableSizeFor(size + (size >>> 1) + 1);
int sc;
while ((sc = sizeCtl) >= 0) { //没有正在初始化或扩容，或者说表还没有被初始化
Node<K,V>[] tab = table; int n;
//CASE 1: table还未初始化，则先进行初始化
if (tab == null || (n = tab.length) == 0) {
n = (sc > c) ? sc : c; // 扩容阀值取较大者
// 期间没有其他线程对表操作，则CAS将SIZECTL状态置为-1，表示正在进行初始化
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;
sc = n - (n >>> 2); //无符号右移2位，此即0.75*n
}
} finally {
sizeCtl = sc; // 更新扩容阀值
}
}
}
// CASE2: c <= sc说明已经被扩容过了；n >= MAXIMUM_CAPACITY说明table数组已达到最大容量
else if (c <= sc || n >= MAXIMUM_CAPACITY)
break;
else if (tab == table) { // CASE3: 进行table扩容
int rs = resizeStamp(n); // 根据容量n生成一个随机数，唯一标识本次扩容操作
if (sc < 0) { // sc < 0 表明此时有别的线程正在进行扩容
Node<K,V>[] nt;
// 如果当前线程无法协助进行数据转移, 则退出
if ((sc >>> RESIZE_STAMP_SHIFT) != rs || sc == rs + 1 ||
sc == rs + MAX_RESIZERS || (nt = nextTable) == null ||
transferIndex <= 0) //RESIZE_STAMP_SHIFT=16,MAX_RESIZERS=2^15-1
break;
// 协助数据转移, 把正在执行transfer任务的线程数加1
if (U.compareAndSwapInt(this, SIZECTL, sc, sc + 1))
transfer(tab, nt);
}
// sc置为负数, 当前线程自身成为第一个执行transfer(数据转移)的线程
// 这个CAS操作可以保证，仅有一个线程会执行扩容
else if (U.compareAndSwapInt(this, SIZECTL, sc,
(rs << RESIZE_STAMP_SHIFT) + 2))
transfer(tab, null);
}
}
}
helpTransfer, 如果发现正在扩容, 就帮助扩容.
//先回顾几个概念
RESIZE_STAMP_BITS = 16;
低5位|低15位
int resizeStamp(){return Integer.numberOfLeadingZeros(n) | (1 << (RESIZE_STAMP_BITS - 1));}
低15位均为1
最多有几个线程协助resize
int MAX_RESIZERS = (1 << (32 - RESIZE_STAMP_BITS)) - 1;
int RESIZE_STAMP_SHIFT = 32 - RESIZE_STAMP_BITS;
final Node<K,V>[] helpTransfer(Node<K,V>[] tab, Node<K,V> f) {
Node<K,V>[] nextTab; int sc;
if (tab != null && (f instanceof ForwardingNode) &&
(nextTab = ((ForwardingNode<K,V>)f).nextTable) != null) {
//rs: 扩容戳
int rs = resizeStamp(tab.length);
while (nextTab == nextTable && table == tab &&
//sizeCtl: -1 初始化, -(1+n) n个线程在resize
(sc = sizeCtl) < 0) {
//sc高位生成戳和rs是否相同,
//sc=rs+1 扩容结束
//sc=rs+MAX_RESIZERS 帮助线程达到最大
//transferIndex<=0 所有的transfer任务都被领取完了
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;
}
transfer
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 range
if (nextTab == null) { // initiating
try {
@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;
}
int nextn = nextTab.length;
ForwardingNode<K,V> fwd = new ForwardingNode<K,V>(nextTab);
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) {//预处理
int nextIndex, nextBound;
if (--i >= bound || finishing)//一次transfer任务还没有执行完毕
advance = false;
else if ((nextIndex = transferIndex) <= 0) {//transfer任务已经没有了
i = -1;
advance = false;
}
else if (U.compareAndSwapInt
(this, TRANSFERINDEX, nextIndex,
nextBound = (nextIndex > stride ?
nextIndex - stride : 0))) {
bound = nextBound;//申请到自己的区间
i = nextIndex - 1;
advance = false;
}
}
//扩容重叠处理, 但实际不会出现扩容重叠
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
}
}
//Node为null, 不需要迁移,直接安放fwd
else if ((f = tabAt(tab, i)) == null)
advance = casTabAt(tab, i, null, fwd);
//该任务被领取了
else if ((fh = f.hash) == MOVED)
advance = true; // already processed
else {
synchronized (f) {
if (tabAt(tab, i) == f) {
//使用高低位
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);
else
hn = new Node<K,V>(ph, pk, pv, hn);
}
setTabAt(nextTab, i, ln);
setTabAt(nextTab, i + n, hn);
setTabAt(tab, i, fwd);
advance = true;
}
//红黑树, 先使用链表遍历, 复制节点, 然后高低位
//组成两个链表, 并判断是否需要红黑树变换, 最后放入Node中
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;
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;
}
}
}
}
}
}