put方法
直接進入put方法，同HashMap父阻，主要內(nèi)容都在putVal方法中。
putVal方法主要思路如下：
- 計算Hash值
- 判斷當(dāng)前的table是否為空，如果為空則進行初始化操作所森。
- table不為空則根據(jù)Hash值找到對應(yīng)下標(biāo)的節(jié)點
- 下標(biāo)節(jié)點為空則通過cas將新節(jié)點放入，失敗進入循環(huán)
- 如果為ForwardingNode類型夯接，則表示當(dāng)前其他線程正在擴容焕济，則進入helpTransfer()協(xié)助擴容
- 如果不為空且是普通節(jié)點，則對節(jié)點上鎖盔几，往鏈表或者紅黑樹添加晴弃。
- cas更新baseCount，并判斷是否需要擴容
//put方法
public V put(K key, V value) {
return putVal(key, value, false);
}
//第三個參數(shù)若為true, 只有在不存在key的時候才進行put
final V putVal(K key, V value, boolean onlyIfAbsent) {
if (key == null || value == null) throw new NullPointerException();
//計算hash值
int hash = spread(key.hashCode());
int binCount = 0; //記錄鏈表的長度
for (Node<K,V>[] tab = table;;) {
Node<K,V> f; int n, i, fh;
if (tab == null || (n = tab.length) == 0)
tab = initTable(); //數(shù)組為空進行初始化
else if ((f = tabAt(tab, i = (n - 1) & hash)) == null) { //找到hash值對應(yīng)下標(biāo)
if (casTabAt(tab, i, null, //該位置若為空則通過cas將該值放入，失敗則再進入循環(huán)
new Node<K,V>(hash, key, value, null)))
break; // no lock when adding to empty bin
}
else if ((fh = f.hash) == MOVED) //說明是ForwardingNode類型上鞠，需要進行協(xié)助擴容
tab = helpTransfer(tab, f);
else { //f為該位置的頭節(jié)點际邻，并且不為空
V oldVal = null;
synchronized (f) { //獲取頭節(jié)點的鎖
if (tabAt(tab, i) == f) {
if (fh >= 0) { //頭節(jié)點的hash值大于0，說明是鏈表
binCount = 1;
for (Node<K,V> e = f;; ++binCount) { //遍歷鏈表
K ek;
if (e.hash == hash &&
((ek = e.key) == key ||
(ek != null && key.equals(ek)))) { //key相同則覆蓋
oldVal = e.val;
if (!onlyIfAbsent)
e.val = value;
break;
}
Node<K,V> pred = e;
if ((e = e.next) == null) { //到了最末端則直接放到最后面
pred.next = new Node<K,V>(hash, key,
value, null);
break;
}
}
}
else if (f instanceof TreeBin) { //紅黑樹
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) //判斷是否轉(zhuǎn)為紅黑樹 閾值為8
treeifyBin(tab, i); // 不僅僅是紅黑樹轉(zhuǎn)換旗国，如果數(shù)組長度小于64則進行數(shù)組擴容
if (oldVal != null)
return oldVal;
break;
}
}
}
addCount(1L, binCount); // cas更新baseCount 并判斷是否需要擴容
return null;
}
initTable() 表初始化
initTable() 初始化代碼如下枯怖，通過對sizeCtl進行cas操作判斷是否搶到鎖，如果成功將sizeCtl設(shè)置為-1則成功搶到能曾。
private final Node<K,V>[] initTable() {
Node<K,V>[] tab; int sc;
while ((tab = table) == null || tab.length == 0) {
if ((sc = sizeCtl) < 0) //主要是通過cas對 sizeCtl進行賦值 若為-1則表示已經(jīng)被其它線程搶到
Thread.yield(); // 讓出cpu等待系統(tǒng)調(diào)度
else if (U.compareAndSwapInt(this, SIZECTL, sc, -1)) { // cas將線程設(shè)置為-1度硝，成功進入以下代碼，失敗則進入循環(huán)
try {
if ((tab = table) == null || tab.length == 0) {
int n = (sc > 0) ? sc : DEFAULT_CAPACITY; //默認(rèn)為16
@SuppressWarnings("unchecked")
Node<K,V>[] nt = (Node<K,V>[])new Node<?,?>[n]; //創(chuàng)建數(shù)組
table = tab = nt; //賦值給table
sc = n - (n >>> 2); //sc 為0.75*n 也就是12
}
} finally {
sizeCtl = sc; //設(shè)置sizeCtl為sc 12
}
break;
}
}
return tab;
}
helpTransfer()協(xié)助擴容
其中擴容狀態(tài)的sizeCtl的高16位為標(biāo)識符寿冕，低16位為正在擴容的線程數(shù)蕊程。
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) && // table不為空 且 f為ForwardingNode類型 且f.nextTable不為空，嘗試幫助擴容驼唱。
(nextTab = ((ForwardingNode<K,V>)f).nextTable) != null) {
int rs = resizeStamp(tab.length); // 返回一個16位長度的擴容校驗標(biāo)識
while (nextTab == nextTable && table == tab && // 說明還在擴容
(sc = sizeCtl) < 0) {
//sizeCtl 如果處于擴容狀態(tài)的話
//前 16 位是數(shù)據(jù)校驗標(biāo)識藻茂，后 16 位是當(dāng)前正在擴容的線程總數(shù)
//這里判斷校驗標(biāo)識是否相等，如果校驗符不等或者擴容操作已經(jīng)完成了（sc == rs+1）或者擴容線程數(shù)已經(jīng)滿了（sc == rs + MAX_RESIZERS）或者不需要幫忙（transferIndex <= 0）玫恳，直接退出循環(huán)辨赐，不用協(xié)助它們擴容了
if ((sc >>> RESIZE_STAMP_SHIFT) != rs || sc == rs + 1 ||
sc == rs + MAX_RESIZERS || transferIndex <= 0)
break;
if (U.compareAndSwapInt(this, SIZECTL, sc, sc + 1)) { // 幫助擴容的線程數(shù)加一
transfer(tab, nextTab);
break;
}
}
return nextTab;
}
return table;
}
transfer()擴容方法
該方法主要是對原數(shù)組進行分段，供線程處理京办。其中transferIndex為轉(zhuǎn)移的下標(biāo)掀序，一開始為原數(shù)組的末尾。即每段為[transferIndex-stride, transferIndex]惭婿。
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) // 將 length/8 然后除以 CPU核心數(shù)不恭。如果得到的結(jié)果小于 16，那么就使用 16财饥。
stride = MIN_TRANSFER_STRIDE; // subdivide range
if (nextTab == null) { // initiating 新table初始化
try {
@SuppressWarnings("unchecked")
Node<K,V>[] nt = (Node<K,V>[])new Node<?,?>[n << 1]; // 兩倍擴容
nextTab = nt;
} catch (Throwable ex) { // try to cope with OOME
sizeCtl = Integer.MAX_VALUE;
return;
}
nextTable = nextTab;
transferIndex = n; // 更新轉(zhuǎn)移下標(biāo)换吧，為老的tab的length 指向最后一個桶
}
int nextn = nextTab.length;
ForwardingNode<K,V> fwd = new ForwardingNode<K,V>(nextTab); // 用于標(biāo)記遷移完成的桶
boolean advance = true;
boolean finishing = false; // to ensure sweep before committing nextTab
for (int i = 0, bound = 0;;) { //i 指向當(dāng)前桶，bound 指向當(dāng)前線程需要處理的桶結(jié)點的區(qū)間下限
Node<K,V> f; int fh;
while (advance) { // 這個循環(huán)用來分配任務(wù)區(qū)間 以及--i往下推進
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) { // 判斷是否擴容是否結(jié)束
int sc;
if (finishing) {
nextTable = null;
table = nextTab;
sizeCtl = (n << 1) - (n >>> 1);
return;
}
if (U.compareAndSwapInt(this, SIZECTL, sc = sizeCtl, sc - 1)) { // 如果沒完成則將自己的幫助線程數(shù)減一
if ((sc - 2) != resizeStamp(n) << RESIZE_STAMP_SHIFT) // 相等則說明沒有幫忙的線程了钥星，則擴容結(jié)束
return;
finishing = advance = true;
i = n; // recheck before commit 重新檢查一次
}
}
else if ((f = tabAt(tab, i)) == null) // 節(jié)點為空則放入fwd占位
advance = casTabAt(tab, i, null, fwd); // 進入任務(wù)分配往下推進
else if ((fh = f.hash) == MOVED) // 說明已經(jīng)已經(jīng)處理過了
advance = true; // already processed
else { // 有實際值沾瓦，并且不是占位符
synchronized (f) { 上鎖
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;
}
}
}
}
}
}
