小编给大家分享一下ConcurrentHashMap怎么用，希望大家阅读完这篇文章之后都有所收获，下面让我们一起去探讨吧！
首先看一下putVal方法，
if (tab == null || (n = tab.length) == 0) tab = initTable();
如果还没有table的话，就要先初始化table
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(); // 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;
@SuppressWarnings("unchecked")
Node<K,V>[] nt = (Node<K,V>[])new Node<?,?>[n];
table = tab = nt;
// size 控制在 n的0.75
sc = n - (n >>> 2);
}
} finally {
sizeCtl = sc;
}
break;
}
}
return tab;
}这一段代码相对简单，这里的sizeCtl是整个过程中的一个非常重要的属性，在扩容，初始化等过程过程中会多次遇到。在这里也是充当了一个排他锁的作用，当它为-1的时候，其它线程等待。
else if ((f = tabAt(tab, i = (n - 1) & hash)) == null) {
if (casTabAt(tab, i, null,
new Node<K,V>(hash, key, value, null)))
break; // no lock when adding to empty bin
}如果要插入的槽是空的，那么直接插入就可以了。
else if ((fh = f.hash) == MOVED) tab = helpTransfer(tab, f);
那么如果要插入的hash值为moved状态即-1的时候，那么就要执行helpTransfer方法了，对，就是先让帮助扩容。这里就要扯出来比较多的东西了，我们一点点来进行分析。
首先看看什么时候一个node的hash值变成了-1，一路看下去，只有
static final class ForwardingNode<K,V> extends Node<K,V>
这个类使用到了，它里面有一个属性
final Node<K,V>[] nextTable;
从这里也大概就能看出来，这个时候ConcurrentHashmap处于扩容状态，通过ForwardingNode就可以找到扩容后的table。
接着来看helpTransfer
final Node<K,V>[] helpTransfer(Node<K,V>[] tab, Node<K,V> f) {
Node<K,V>[] nextTab; int sc;
// 这里还要再次检查 当前节点是不是 ForwardingNode 因为如果不是的话，没有办法找到nextTable，也就没有办法帮助扩容了
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;
// 进来一个线程，则对sizeCtl+1,用以标记参与扩容的线程数
if (U.compareAndSwapInt(this, SIZECTL, sc, sc + 1)) {
// 进行扩容操作
transfer(tab, nextTab);
break;
}
}
return nextTab;
}
return table;
}下来看看transfer扩容操作是如何执行的，这里感觉是ConcurrentHashmap的一个精华点，叹为观止。
private final void transfer(Node<K,V>[] tab, Node<K,V>[] nextTab) {
int n = tab.length, stride;
// 首先进行分段，既每个线程每次处理的node数量，最小16
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;
// 在这里创建了ForwardingNode
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;
// 判断是否要继续
while (advance) {
int nextIndex, nextBound;
// 如果已经结束了或者当前已经到了边界
if (--i >= bound || finishing)
advance = false;
// 扩容时用的指针已经小于0，则结束
else if ((nextIndex = transferIndex) <= 0) {
i = -1;
advance = false;
}
// 扩容的指针，从大向小移动，从大向小移动，每次减小stride
else if (U.compareAndSwapInt
(this, TRANSFERINDEX, nextIndex,
nextBound = (nextIndex > stride ?
nextIndex - stride : 0))) {
bound = nextBound;
i = nextIndex - 1;
advance = false;
}
}
// i 小于 0 ，已经结束了
if (i < 0 || i >= n || i + n >= nextn) {
int sc;
// 如果已经结束了，那么把table设置为nextTable
if (finishing) {
nextTable = null;
table = nextTab;
sizeCtl = (n << 1) - (n >>> 1);
return;
}
// 说明当前的线程已经工作结束，sizeCtl - 1
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
}
}
//如果节点为空，设置该节点为fwd
else if ((f = tabAt(tab, i)) == null)
advance = casTabAt(tab, i, null, fwd);
else if ((fh = f.hash) == MOVED)
advance = true; // already processed
else {
synchronized (f) {
if (tabAt(tab, i) == f) {
Node<K,V> ln, hn;
if (fh >= 0) {
// 这里为什么是 fh & n 做 & 运算 因为 15 的二进制是 1111 16是10000 31是 11111
// 所以，扩容前和扩容后只有第一位 & 运算后会变，其它位都不变，所以与 table.length & 就可以了
int runBit = fh & n;
Node<K,V> lastRun = f;
// 先遍历一遍，确定 ni -> n rehash相等的一段，这样下一次重新分配槽的时候这一段就不再遍历
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);
// 把已完成的节点标记为fwd
setTabAt(tab, i, fwd);
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;
}
}
}
}
}
}再来看一下对于元素总数的统计实现。
private final void addCount(long x, int check)
首先我们遇到了CounterCell这个类，结构很简单，只有一个long value，它是存储数量的最小单元。
先看第一次的判断条件，如果conterCells已经不为空，说明之前已经出现了并发增加baseCount，否则counterCell不会被初始化。
if ((as = counterCells) != null || !U.compareAndSwapLong(this, BASECOUNT, b = baseCount, s = b + x))
或者在改变baseCount的时候出现了冲突，执行下面代码。
CounterCell a; long v; int m;
boolean uncontended = true;
// 如果 counterCell未初始化，或者长度为0 亦或者没有这个对应的槽 再或者更新对应槽的时候出现冲突
// 这个时候说明要么 counterCell未初始化，要么说明又出现了对于同一个槽冲突，所以需要 fullAddCount来解决冲突
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();再来看看fullAddCount做了什么
private final void fullAddCount(long x, boolean wasUncontended) {
int h;
// 如果ThreadLocalRandom还没有被初始化过，说明还没有发生过碰撞
if ((h = ThreadLocalRandom.getProbe()) == 0) {
ThreadLocalRandom.localInit(); // force initialization
h = ThreadLocalRandom.getProbe();
wasUncontended = true;
}
boolean collide = false; // True if last slot nonempty
for (;;) {
CounterCell[] as; CounterCell a; int n; long v;
// 如果数组已经被初始化
if ((as = counterCells) != null && (n = as.length) > 0) {
// 随机选取的槽还未被初始化
if ((a = as[(n - 1) & h]) == null) {
// 获取锁
if (cellsBusy == 0) { // Try to attach new Cell
CounterCell r = new CounterCell(x); // Optimistic create
// U.compareAndSwapInt(this, CELLSBUSY, 0, 1)通过cas操作来获取锁
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;
}
// 有竞争的
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;
// 如果槽的数量已经超过了cpu个数，就不会碰撞了
else if (counterCells != as || n >= NCPU)
collide = false; // At max size or stale
else if (!collide)
collide = true;
// 获取锁，并对CounterCell进行扩容操作
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);
}
// counter cell 没有初始化的情况
else if (cellsBusy == 0 && counterCells == as &&
U.compareAndSwapInt(this, CELLSBUSY, 0, 1)) {
boolean init = false;
try { // Initialize table
if (counterCells == as) {
// 进行初始化
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
}
}我们前面讲了扩容的机制，那么扩容的发起者肯定就是在addCount中了
while (s >= (long)(sc = sizeCtl) && (tab = table) != null && (n = tab.length) < MAXIMUM_CAPACITY)
这里有判断 s 为 sumCount 即 baseCount加上各个节点的和为总数。如果s大于sizeCtl或者table不为空而且没有到达最大值，则进行扩容操作。
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