小編給大家分享一下ConcurrentHashMap怎么用，希望大家閱讀完這篇文章之后都有所收獲，下面讓我們一起去探討吧！
首先看一下putVal方法，
if (tab == null || (n = tab.length) == 0) tab = initTable();
如果還沒有table的話，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;
}這一段代碼相對簡單，這里的sizeCtl是整個過程中的一個非常重要的屬性，在擴容，初始化等過程過程中會多次遇到。在這里也是充當了一個排他鎖的作用，當它為-1的時候，其它線程等待。
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值為moved狀態即-1的時候，那么就要執行helpTransfer方法了，對，就是先讓幫助擴容。這里就要扯出來比較多的東西了，我們一點點來進行分析。
首先看看什么時候一個node的hash值變成了-1，一路看下去，只有
static final class ForwardingNode<K,V> extends Node<K,V>
這個類使用到了，它里面有一個屬性
final Node<K,V>[] nextTable;
從這里也大概就能看出來，這個時候ConcurrentHashmap處于擴容狀態，通過ForwardingNode就可以找到擴容后的table。
接著來看helpTransfer
final Node<K,V>[] helpTransfer(Node<K,V>[] tab, Node<K,V> f) {
Node<K,V>[] nextTab; int sc;
// 這里還要再次檢查 當前節點是不是 ForwardingNode 因為如果不是的話，沒有辦法找到nextTable，也就沒有辦法幫助擴容了
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;
// 進來一個線程，則對sizeCtl+1,用以標記參與擴容的線程數
if (U.compareAndSwapInt(this, SIZECTL, sc, sc + 1)) {
// 進行擴容操作
transfer(tab, nextTab);
break;
}
}
return nextTab;
}
return table;
}下來看看transfer擴容操作是如何執行的，這里感覺是ConcurrentHashmap的一個精華點，嘆為觀止。
private final void transfer(Node<K,V>[] tab, Node<K,V>[] nextTab) {
int n = tab.length, stride;
// 首先進行分段，既每個線程每次處理的node數量，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;
// 在這里創建了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;
// 判斷是否要繼續
while (advance) {
int nextIndex, nextBound;
// 如果已經結束了或者當前已經到了邊界
if (--i >= bound || finishing)
advance = false;
// 擴容時用的指針已經小于0，則結束
else if ((nextIndex = transferIndex) <= 0) {
i = -1;
advance = false;
}
// 擴容的指針，從大向小移動，從大向小移動，每次減小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 ，已經結束了
if (i < 0 || i >= n || i + n >= nextn) {
int sc;
// 如果已經結束了，那么把table設置為nextTable
if (finishing) {
nextTable = null;
table = nextTab;
sizeCtl = (n << 1) - (n >>> 1);
return;
}
// 說明當前的線程已經工作結束，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
}
}
//如果節點為空，設置該節點為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) {
// 這里為什么是 fh & n 做 & 運算 因為 15 的二進制是 1111 16是10000 31是 11111
// 所以，擴容前和擴容后只有第一位 & 運算后會變，其它位都不變，所以與 table.length & 就可以了
int runBit = fh & n;
Node<K,V> lastRun = f;
// 先遍歷一遍，確定 ni -> n rehash相等的一段，這樣下一次重新分配槽的時候這一段就不再遍歷
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);
// 把已完成的節點標記為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;
}
}
}
}
}
}再來看一下對于元素總數的統計實現。
private final void addCount(long x, int check)
首先我們遇到了CounterCell這個類，結構很簡單，只有一個long value，它是存儲數量的最小單元。
先看第一次的判斷條件，如果conterCells已經不為空，說明之前已經出現了并發增加baseCount，否則counterCell不會被初始化。
if ((as = counterCells) != null || !U.compareAndSwapLong(this, BASECOUNT, b = baseCount, s = b + x))
或者在改變baseCount的時候出現了沖突，執行下面代碼。
CounterCell a; long v; int m;
boolean uncontended = true;
// 如果 counterCell未初始化，或者長度為0 亦或者沒有這個對應的槽 再或者更新對應槽的時候出現沖突
// 這個時候說明要么 counterCell未初始化，要么說明又出現了對于同一個槽沖突，所以需要 fullAddCount來解決沖突
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();再來看看fullAddCount做了什么
private final void fullAddCount(long x, boolean wasUncontended) {
int h;
// 如果ThreadLocalRandom還沒有被初始化過，說明還沒有發生過碰撞
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;
// 如果數組已經被初始化
if ((as = counterCells) != null && (n = as.length) > 0) {
// 隨機選取的槽還未被初始化
if ((a = as[(n - 1) & h]) == null) {
// 獲取鎖
if (cellsBusy == 0) { // Try to attach new Cell
CounterCell r = new CounterCell(x); // Optimistic create
// U.compareAndSwapInt(this, CELLSBUSY, 0, 1)通過cas操作來獲取鎖
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;
}
// 有競爭的
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;
// 如果槽的數量已經超過了cpu個數，就不會碰撞了
else if (counterCells != as || n >= NCPU)
collide = false; // At max size or stale
else if (!collide)
collide = true;
// 獲取鎖，并對CounterCell進行擴容操作
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 沒有初始化的情況
else if (cellsBusy == 0 && counterCells == as &&
U.compareAndSwapInt(this, CELLSBUSY, 0, 1)) {
boolean init = false;
try { // Initialize table
if (counterCells == as) {
// 進行初始化
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
}
}我們前面講了擴容的機制，那么擴容的發起者肯定就是在addCount中了
while (s >= (long)(sc = sizeCtl) && (tab = table) != null && (n = tab.length) < MAXIMUM_CAPACITY)
這里有判斷 s 為 sumCount 即 baseCount加上各個節點的和為總數。如果s大于sizeCtl或者table不為空而且沒有到達最大值，則進行擴容操作。
看完了這篇文章，相信你對“ConcurrentHashMap怎么用”有了一定的了解，如果想了解更多相關知識，歡迎關注億速云行業資訊頻道，感謝各位的閱讀！
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