简介
java（版本：1.8.0_161） 中ConcurrentHashMap主要在多线程中使用，dev不需要自己保证数据同步。用法基本上和HashMap相似。本文主要讲解源码中的put方法是怎么工作的。让我们带着问题去看源码。
问题
1. ConcurrentHashMap是如何保证多个线程同时调用put方法时而不出现错误的
2.ConcurrentHashMap是如何进行扩容的。当ConcurrentHashMap正在扩容的时候，当多个线程同时调用put怎么办
3.ConcurrentHashMap是怎么维护元素数量的， 当多个线程同时调用put的时候，ConcurrentHashMap是怎么维护元素的数量的
4.ConcurrentHashMap如何初始化table,当多个线程同时初始化时怎么办
源码
先解释一个很重要的属性
sizeCtl
正数代表threshold(当前ConcurrentHashMap容量的0.75倍)
负数代表正在进行初始化或扩容操作
-1代表正在初始化
-N 表示有N-1个线程正在进行扩容操
下面讲解put方法。
当调用put的时候，直接调用了putVal方法
public V put(K key, V value) {
return putVal(key, value, false);
}putVal主要用于把key-value插入table中
// putVal用于将key-value放入table中
final V putVal(K key, V value, boolean onlyIfAbsent) {
if (key == null || value == null) throw new NullPointerException();
// spread将hash值的高16位和低16位进行异或操作，这样高16位和低16位就都可以参与进hash的计算了
int hash = spread(key.hashCode());
int binCount = 0;
// 当第一次调用put方法的时候，table是null
for (Node<K,V>[] tab = table;;) {
Node<K,V> f; int n, i, fh;
if (tab == null || (n = tab.length) == 0)
tab = initTable(); // 第一次调用的时候，初始化table（initTable方法下面讲）
else if ((f = tabAt(tab, i = (n - 1) & hash)) == null) { // 当hash所在bin是null的时候，尝试将key-value放入bin
if (casTabAt(tab, i, null,
new Node<K,V>(hash, key, value, null)))
break; // no lock when adding to empty bin
}
// 当hash == MOVED，说明table正在进行扩容操作
else if ((fh = f.hash) == MOVED)
tab = helpTransfer(tab, f); // 帮助table进行扩容
else {
V oldVal = null;
synchronized (f) { // 锁住bin的第一个node,就相当于锁住了整个bin
if (tabAt(tab, i) == f) {
if (fh >= 0) { // 如果是bin,就将node放入bin里面（bin的末尾）
binCount = 1;
for (Node<K,V> e = f;; ++binCount) {
K ek;
if (e.hash == hash &&
((ek = e.key) == key ||
(ek != null && key.equals(ek)))) {
oldVal = e.val;
if (!onlyIfAbsent)
e.val = value;
break;
}
Node<K,V> pred = e;
if ((e = e.next) == null) {
pred.next = new Node<K,V>(hash, key,
value, null);
break;
}
}
}
else if (f instanceof TreeBin) { // 如果是红黑树
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) { // 是bin
if (binCount >= TREEIFY_THRESHOLD) // bin里元素的个数超过TREEIFY_THRESHOLD，就将bin弄成红黑树
treeifyBin(tab, i);
if (oldVal != null)
return oldVal;
break;
}
}
}
addCount(1L, binCount);//给table中的元素的数量加一
return null;
}initTable用于初始化一个table
private final Node<K,V>[] initTable() {
Node<K,V>[] tab; int sc;
while ((tab = table) == null || tab.length == 0) {
if ((sc = sizeCtl) < 0) // 当sizeCtl<0时，代表有其他线程正在初始化table,让出CPU让其他线程进行初始化操作
Thread.yield();
else if (U.compareAndSwapInt(this, SIZECTL, sc, -1)) { // 将sizeCtl置为-1,代表正在初始化table,从而告诉其他线程正在初始化
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;
sc = n - (n >>> 2);
}
} finally {
sizeCtl = sc; // 当初始化完成，将sizeCtl置为threshold（capacity*0.75）
}
break;
}
}
return tab;
}addCount主要有两个作用.
1.给table中元素的数量加一
2.检查是否需要扩容，如果需要则进行扩容
private final void addCount(long x, int check) {
CounterCell[] as; long b, s;
/**
* 这里有两种情况
* 1.counterCells是空
* 1）.尝试将给baseCount加一
* 2）.如果失败（由于多个线程再改变baseCount的值）,初始化counterCells并且把一加到counterCells里
* 2.counterCells不是空
* 直接把一加到counterCells里
*/
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))) { // 尝试把一加到counterCells里
// fullAddCount用于将一加到counterCells里
fullAddCount(x, uncontended);
return;
}
if (check <= 1)
return;
s = sumCount();
}
// 对table进行扩容操作
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);
// 其他线程正在进行扩容操作
if (sc < 0) {
if ((sc >>> RESIZE_STAMP_SHIFT) != rs || sc == rs + 1 ||
sc == rs + MAX_RESIZERS || (nt = nextTable) == null ||
transferIndex <= 0)
break;
// 和其他线程一起扩容
if (U.compareAndSwapInt(this, SIZECTL, sc, sc + 1))
transfer(tab, nt);
}
// 没有其他线程进行扩容，首先进行扩容
else if (U.compareAndSwapInt(this, SIZECTL, sc,
(rs << RESIZE_STAMP_SHIFT) + 2))
transfer(tab, null);
s = sumCount();
}
}
}fullAddCount主要用于维护table中node的数量
private final void fullAddCount(long x, boolean wasUncontended) {
int h;
if ((h = ThreadLocalRandom.getProbe()) == 0) {
ThreadLocalRandom.localInit(); // force initialization
h = ThreadLocalRandom.getProbe();
wasUncontended = true;
}
boolean collide = false; // True if last slot nonempty
/**
* 1. if counterCells is not null and length is more than 0
* 1) when the counterCell which we hope to add into is null, will set cellsBusy as 1 and init the null counterCell
* 2) when the counterCell is not null, try to add 1L into to the counterCell
* 3) if we try to add several time but failed, will try add increase the capacity of CounterCells
*
* 2. if counterCells is not init(counterCells is null or length is 0)
* 1) set cellsBusy as 1(tell other threads, we are initing)
* 2) init CounterCells and put CounterCell into it
* 3) change cellsBusy to 0
*
* 3. if other threads are initing the CounterCells, we will try to add 1L to baseCount
*/
/**
* 1.如果 counterCells不是null,且长度大于0且没有其他线程再操作counterCells（意思就是counterCells已经完全初始化好了）
* 1）当counterCell是空的时候，将cellsBusy设置成1,然后初始化
* 2）当counterCell不是空，将尝试将x加到counterCell
* 3）如果我们尝试了几次都失败了，就会对将cellsBusy设置成1,然后counterCells进行扩容
* 2.couterCells没有初始化且没有其他线程再操作counterCells
* 将cellsBusy设置成1,然后进行初始化操作
* 3.其他线程正在初始化counterCells, 就尝试着将x加到baseCount
*/
for (;;) {
CounterCell[] as; CounterCell a; int n; long v;
if ((as = counterCells) != null && (n = as.length) > 0) {
if ((a = as[(n - 1) & h]) == null) {
if (cellsBusy == 0) { // Try to attach new Cell
CounterCell r = new CounterCell(x); // Optimistic create
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))
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 {
if (counterCells == as) {// Expand table unless stale
CounterCell[] rs = new CounterCell[n << 1];
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);
}
else if (cellsBusy == 0 && counterCells == as &&
U.compareAndSwapInt(this, CELLSBUSY, 0, 1)) {
boolean init = false;
try { // Initialize table
if (counterCells == as) {
CounterCell[] rs = new CounterCell[2];
rs[h & 1] = new CounterCell(x);
counterCells = rs;
init = true;
}
} finally {
cellsBusy = 0;
}
if (init)
break;
}
else if (U.compareAndSwapLong(this, BASECOUNT, v = baseCount, v + x))
break; // Fall back on using base
}
}transfer主要用于扩容
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
// 初始化nextTable
if (nextTab == null) {
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; // 旧table的长度
}
int nextn = nextTab.length;
ForwardingNode<K,V> fwd = new ForwardingNode<K,V>(nextTab); //ForwardingNode用于表明这个bin已经完成扩容
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;
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;
advance = false;
}
}
if (i < 0 || i >= n || i + n >= nextn) {
int sc;
if (finishing) {//扩容完成后，将sizeCtl回复位capacity*0.75
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
}
}
else if ((f = tabAt(tab, i)) == null)//如果正在扩容的bin是空的，就把它标记成历经扩容完成
advance = casTabAt(tab, i, null, fwd);
else if ((fh = f.hash) == MOVED)//bin已经被处理过了，进行下一个bin的扩容
advance = true;
else {
synchronized (f) { // 锁住bin的第一个节点进行扩容操作
/**
* 假如旧的table的长度是16,新table的长度是32.
* 当扩容的时候旧table的第0个bin,对应的新table的第0个或者第16个bin. 如果最高为是1,则说明node在高16位（16~31），最高为是0，则说明在低16位（0~15）
* lastRun是用来标识是不是一个bin里的所有元素都是高16位或者都是低16位，
* 如果for循环之后，lastRun == f(bin里的第一个node)则说明一个bin里的所有元素都是高16位或者都是低16位
* 否则说明一个bin里的所有元素不都是高16位或者都是低16位
*
* 给bin扩容的思想是：将旧table的bin里的元素，如果是高位，就放到新table的高位，如果是低位，就放到新table的低位，
*/
if (tabAt(tab, i) == f) {
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); // 扩容完成，将旧table的bin的节点设为ForwardingNode
advance = true;
}
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;
}
}
}
}
}
}