目錄
ConcurrentHashMap是jdk1.8中非常重要以及常用的工具類，通過CAS機制高性能的進行線程安全地添加、獲取、擴容等操作，這裏簡單瞭解一下實現機制
1 幾個重要的狀態值
1.1 每個槽點頭結點的hash值
>0：普通Node，該位置是一個鏈表
-1：ForwardingNode，表示正在擴容
-2：TreeNode，該位置是一個紅黑樹
1.2 sizeCtl
-1 代表table正在初始化
-N 表示有N-1個線程正在進行擴容操作
> 0
1 如果table未初始化，表示table需要初始化的大小。
2 如果table初始化完成，表示擴容閾值，默認是table容量 * 0.75
2 源碼鑑賞
2.1 put方法
final V putVal(K key, V value, boolean onlyIfAbsent) {
if (key == null || value == null) throw new NullPointerException();
// 離散哈希，使生成的hash更有隨機性
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)
// 初始化table
tab = initTable();
else if ((f = tabAt(tab, i = (n - 1) & hash)) == null) {
// 如果某個位置鏈表爲空則初始化
if (casTabAt(tab, i, null,
new Node<K,V>(hash, key, value, null)))
break; // no lock when adding to empty bin
}
else if ((fh = f.hash) == MOVED)
// 如果鏈表頭結點hash值=-1，則表示當前table正在擴容(下文會分析擴容代碼)，則當前線程協助擴容。
tab = helpTransfer(tab, f);
else {
// 終於進入正題了，開始put元素
V oldVal = null;
synchronized (f) {
if (tabAt(tab, i) == f) {
if (fh >= 0) {
// 如果fh >0 表示f是鏈表(fh = -2表示紅黑樹)
binCount = 1;
for (Node<K,V> e = f;; ++binCount) {
K ek;
if (e.hash == hash &&
((ek = e.key) == key ||
(ek != null && key.equals(ek)))) {
// 如果待添加的key和某個節點key相同，並且允許覆蓋，則覆蓋
oldVal = e.val;
if (!onlyIfAbsent)
e.val = value;
break;
}
Node<K,V> pred = e;
if ((e = e.next) == null) {
// 如果當前節點下一個元素爲空，則說明找到尾節點，把新節點添加到尾節點後面即可
pred.next = new Node<K,V>(hash, key,
value, null);
break;
}
}
}
else if (f instanceof TreeBin) {
Node<K,V> p;
binCount = 2;
// 如果該該位置是紅黑樹則通過紅黑樹追加元素
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)
// 如果鏈表長度 > 8則嘗試轉紅黑樹
treeifyBin(tab, i);
if (oldVal != null)
return oldVal;
break;
}
}
}
addCount(1L, binCount);
return null;
}
2.2 初始化table
初始化table比較簡單，計算好table容量大小後創建一個Node數組即可
private final Node<K,V>[] initTable() {
Node<K,V>[] tab; int sc;
while ((tab = table) == null || tab.length == 0) {
if ((sc = sizeCtl) < 0)
// 如果sizeCtl < 0表示其他線程在初始化table，這裏線程讓步
Thread.yield();
else if (U.compareAndSwapInt(this, SIZECTL, sc, -1)) {
// 開始初始化table，把sizeCtl的值設置爲-1，表示正在初始化
try {
if ((tab = table) == null || tab.length == 0) {
// 計算需要生成的table大小
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;
}
break;
}
}
return tab;
}
2.3 擴容
2.3.1 協助擴容(helpTransfer)
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) {
// 擴容的時候會給擴容完成的槽點放一個ForwardingNode，表示正在擴容
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;
if (U.compareAndSwapInt(this, SIZECTL, sc, sc + 1)) {
// 擴容操作
transfer(tab, nextTab);
break;
}
}
return nextTab;
}
return table;
}
2.3.2 擴容(transfer)
擴容的時候根據處理器個數分配擴容槽點，創建一個二倍容量的新table
每個槽點擴容到時候加鎖，擴容完成後方放一個ForwardingNode，表示該槽點已經擴容完畢。
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)
// 計算每個線程需要擴容的槽點個數
stride = MIN_TRANSFER_STRIDE; // subdivide range
if (nextTab == null) { // initiating
try {
@SuppressWarnings("unchecked")
// 創建一個nextTable，大小爲table兩倍
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) {
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) {
// 如果已經完成擴容，則把table指向nextTab，sizeCtl設置爲n的1.5倍，即newTable的0.75倍
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)
// 如果當前位置節點爲空，則放置fwd，表示該鏈表已經遷移完成
advance = casTabAt(tab, i, null, fwd);
else if ((fh = f.hash) == MOVED)
// 該槽點已經遷移完成
advance = true;
else {
synchronized (f) {
// 遷移f節點所屬的鏈表
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);
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;
}
}
}
}
}
}
2.4 嘗試轉二叉樹(treeifyBin)
private final void treeifyBin(Node<K,V>[] tab, int index) {
Node<K,V> b; int n, sc;
if (tab != null) {
if ((n = tab.length) < MIN_TREEIFY_CAPACITY)
// 如果table長度小於轉二叉樹閾值則進行擴容
tryPresize(n << 1);
else if ((b = tabAt(tab, index)) != null && b.hash >= 0) {
// 鏈表轉二叉樹
synchronized (b) {
if (tabAt(tab, index) == b) {
TreeNode<K,V> hd = null, tl = null;
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;
}
// 設置二叉樹，頭結點是一個TreeBin,hash=-2
setTabAt(tab, index, new TreeBin<K,V>(hd));
}
}
}
}
}
2.5 get元素
get方法比較簡單，計算出key所在的槽點後取元素即可
public V get(Object key) {
Node<K,V>[] tab; Node<K,V> e, p; int n, eh; K ek;
int h = spread(key.hashCode());
if ((tab = table) != null && (n = tab.length) > 0 &&
(e = tabAt(tab, (n - 1) & h)) != null) {
if ((eh = e.hash) == h) {
// 如果槽點node的哈希值剛好等於key的哈希值則直接返回
if ((ek = e.key) == key || (ek != null && key.equals(ek)))
return e.val;
}
else if (eh < 0)
// 如果槽點處的哈希值小於0則表示是紅黑樹，通過紅黑樹取元素
return (p = e.find(h, key)) != null ? p.val : null;
// 通過鏈表取元素
while ((e = e.next) != null) {
if (e.hash == h &&
((ek = e.key) == key || (ek != null && key.equals(ek))))
return e.val;
}
}
return null;
}
這裏說一下tabAt方法，獲取某個位置node的時候不用table[(n - 1) & h]的原因是雖然table是volatile的，每個線程可以取到最新的table，
但不能表示table數組中每一個元素是volatile的，所以可能取不到最新的元素。
通過(Node<K,V>)U.getObjectVolatile(tab, ((long)i << ASHIFT) + ABASE)方法直接讀取指定內存的數據，可以保證獲取的元素是最新的