ConcurrentHashMap源码
JDK8源码
put操作
public V put(K key, V value) {
return putVal(key, value, false); //调用putVal
}
putVal：
final V putVal(K key, V value, boolean onlyIfAbsent) {
//key和value都不能为null
if (key == null || value == null) throw new NullPointerException();
int hash = spread(key.hashCode());
int binCount = 0; //表示链表长度
for (Node<K,V>[] tab = table;;) {
//f：链表头节点 fh：头节点的hash i：链表在table中的下标 n：table长度
Node<K,V> f; int n, i, fh;
if (tab == null || (n = tab.length) == 0) //判断table是否创建
tab = initTable(); //初始化table，使用CAS操作，无需加锁。创建后进入下一轮循环
else if ((f = tabAt(tab, i = (n - 1) & hash)) == null) { //判断头节点是否为空
//如果头节点为空，则创建头节点，添加的元素为头节点。创建使用cas操作
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) //hash为负数MOVED，表示有其他线程正在扩容
//当前put不是阻塞等待，而是会去帮助扩容
tab = helpTransfer(tab, f);
else { //出现hash冲突，链地址法put操作
V oldVal = null;
synchronized (f) { //只锁住当前的链表头节点
if (tabAt(tab, i) == f) { //再次确认头节点没有改变
if (fh >= 0) { //>=0表示是普通节点
binCount = 1;
for (Node<K,V> e = f;; ++binCount) { //遍历链表
K ek;
if (e.hash == hash &&
((ek = e.key) == key ||
(ek != null && key.equals(ek)))) { //如果存在相同的key，则覆盖旧值
oldVal = e.val;
if (!onlyIfAbsent)
e.val = value;
break;
}
Node<K,V> pred = e;
if ((e = e.next) == null) { //不存在相同的key，则追加节点
pred.next = new Node<K,V>(hash, key,
value, null);
break;
}
}
}
else if (f instanceof TreeBin) { //节点是红黑树
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;
}
}
}
} //释放锁
if (binCount != 0) {
if (binCount >= TREEIFY_THRESHOLD) //判断链表长度是否大于阈值，决定是否把链表转为红黑树
treeifyBin(tab, i);
if (oldVal != null)
return oldVal;
break;
}
}
}
addCount(1L, binCount); //元素个数加1
return null;
}
initTable（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)) { //试图将sizeCtl改为-1，表示正在创建table
try {
if ((tab = table) == null || tab.length == 0) {
int n = (sc > 0) ? sc : DEFAULT_CAPACITY; //获取要创建的table的容量大小
@SuppressWarnings("unchecked")
Node<K,V>[] nt = (Node<K,V>[])new Node<?,?>[n];
table = tab = nt;
sc = n - (n >>> 2); //计算下次需要扩容的阈值
}
} finally {
sizeCtl = sc;
}
break;
}
}
return tab;
}
addCount（table中元素个数增加，并检查是否需要扩容，如果需要则进行扩容）
private final void addCount(long x, int check) {
CounterCell[] as; long b, s;
if ((as = counterCells) != null || //如果累加数组as不为空，表示有其他线程也在累加
//没有其他线程累加，则在baseCount上累加。如果baseCount累加失败，则进入
!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 || //累加数组还未创建
(a = as[ThreadLocalRandom.getProbe() & m]) == null || //没有累加单元
!(uncontended =
U.compareAndSwapLong(a, CELLVALUE, v = a.value, v + x))) { //有累加数组和累加单元，则执行cas累加
//如果上述都失败，创建累加数组和累加单元，循环尝试累加
fullAddCount(x, uncontended);
return;
}
if (check <= 1) //检查链表长度，如果大于1，可能需要扩容
return;
s = sumCount(); //获取元素个数
}
if (check >= 0) {
Node<K,V>[] tab, nt; int n, sc;
while (s >= (long)(sc = sizeCtl) && (tab = table) != null && //元素个数超过阈值，需要进行扩容
(n = tab.length) < MAXIMUM_CAPACITY) {
int rs = resizeStamp(n) << RESIZE_STAMP_SHIFT;
if (sc < 0) { //sc为负数，有线程正在扩容
if (sc == rs + MAX_RESIZERS || sc == rs + 1 ||
(nt = nextTable) == null || transferIndex <= 0) //新table还未创建
break;
if (U.compareAndSwapInt(this, SIZECTL, sc, sc + 1)) //新table已经创建，则帮忙扩容
transfer(tab, nt);
}
else if (U.compareAndSwapInt(this, SIZECTL, sc, rs + 2)) //cas操作，sc改为负数
transfer(tab, null); //创建新table，进行扩容
s = sumCount();
}
}
}
transfer（扩容操作，将每个节点复制到新的table数组中）
private final void transfer(Node<K,V>[] tab, Node<K,V>[] nextTab) { //tab：原始table，nextTab：扩容后的新的table
int n = tab.length, stride;
if ((stride = (NCPU > 1) ? (n >>> 3) / NCPU : n) < MIN_TRANSFER_STRIDE)
stride = MIN_TRANSFER_STRIDE; // subdivide range
//新的table还未创建，则创建新table，大小为原table<<1
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;
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) {
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) //如果链表的头节点为null，说明该链表已经移动完毕，将链表头替换为fwd节点（hash为MOVED=-1），表示已经移动
advance = casTabAt(tab, i, null, fwd);
else if ((fh = f.hash) == MOVED) //此链表已经移动完毕，循环处理下一个链表
advance = true; // already processed
else { //链表中有元素
synchronized (f) { //加锁移动元素
if (tabAt(tab, i) == f) {
Node<K,V> ln, hn;
if (fh >= 0) { //普通链表节点
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) { //红黑树节点
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;
}
}
}
}
}
}
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