我们先看一下它无参构造函数
public ConcurrentHashMap() {
}可以看到并未对hash表进行初始化。
我们再看一下它的put方法
public V put(K key, V value) {
return putVal(key, value, false);
}进入putVal方法。
final V putVal(K key, V value, boolean onlyIfAbsent) {
if (key == null || value == null) throw new NullPointerException(); // key与value不可为null
int hash = spread(key.hashCode()); //key的hash的计算
int binCount = 0;
for (Node<K,V>[] tab = table;;) {
Node<K,V> f; int n, i, fh;
if (tab == null || (n = tab.length) == 0)
tab = initTable(); 第一次put进入这个方法，初始化table
else if ((f = tabAt(tab, i = (n - 1) & hash)) == null) {//如果该桶位置为空，则直接放入
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) //如果发现正在扩容，帮助扩容
tab = helpTransfer(tab, f);
else {//否则就是发生hash冲突
V oldVal = null;
synchronized (f) {
if (tabAt(tab, i) == f) { //判断是不是链表，顺着链表找，若发现相同的key则更新value，否则找到链表尾就添加这个映射
if (fh >= 0) {
binCount = 1;
for (Node<K,V> e = f;; ++binCount) {
K ek;
if (e.hash == hash &&
((ek = e.key) == key ||
(ek != null && key.equals(ek)))) {
oldVal = e.val;
if (!onlyIfAbsent)
e.val = value;
break;
}
Node<K,V> pred = e;
if ((e = e.next) == null) {
pred.next = new Node<K,V>(hash, key,
value, null);
break;
}
}
}
else if (f instanceof TreeBin) { //判断是不是红黑树
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;
}
}
}
}
if (binCount != 0) { //判断需不需要转红黑树
if (binCount >= TREEIFY_THRESHOLD)
treeifyBin(tab, i);
if (oldVal != null)
return oldVal;
break;
}
}
}
addCount(1L, binCount); //hash元素数+1
return null;
}我们看一下key 的计算方法。
static final int spread(int h) {
return (h ^ (h >>> 16)) & HASH_BITS; 用key的hashCode与他自己右移16位的与再与HASH_BITS进行与运算，HASH_BITS = 0x7fffffff
}我们看一下初始化方法initTable。
private final Node<K,V>[] initTable() {
Node<K,V>[] tab; int sc;
while ((tab = table) == null || tab.length == 0) {
if ((sc = sizeCtl) < 0) //表示有线程正在执行初始化操作，将其他线程挂起，hash只能有一个线程执行初始化操作。
Thread.yield(); // lost initialization race; just spin
else if (U.compareAndSwapInt(this, SIZECTL, sc, -1)) {利用CAS操作将sizeCtl值置为-1，表示本线程正在执行初始化
try {
if ((tab = table) == null || tab.length == 0) {
int n = (sc > 0) ? sc : DEFAULT_CAPACITY; //当sc大于0，hash表初始化为sc，否则初始化为DEFAULT_CAPACITY=16
@SuppressWarnings("unchecked")
Node<K,V>[] nt = (Node<K,V>[])new Node<?,?>[n];
table = tab = nt;
sc = n - (n >>> 2); //sc设置为0.75n.
}
} finally {
sizeCtl = sc; //0.75*n，相当于扩容的阈值
}
break;
}
}
return tab;
}我们看一下addCount方法。
private final void addCount(long x, int check) {
CounterCell[] as; long b, s;
if ((as = counterCells) != null || //完成hash元素数+1操作
!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 ||
(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();
}
if (check >= 0) {
Node<K,V>[] tab, nt; int n, sc;
while (s >= (long)(sc = sizeCtl) && (tab = table) != null &&
(n = tab.length) < MAXIMUM_CAPACITY) {
int rs = resizeStamp(n);
if (sc < 0) {
if ((sc >>> RESIZE_STAMP_SHIFT) != rs || sc == rs + 1 ||
sc == rs + MAX_RESIZERS || (nt = nextTable) == null ||
transferIndex <= 0)
break;
if (U.compareAndSwapInt(this, SIZECTL, sc, sc + 1))
transfer(tab, nt);
}
else if (U.compareAndSwapInt(this, SIZECTL, sc,
(rs << RESIZE_STAMP_SHIFT) + 2))
transfer(tab, null); //扩容
s = sumCount();
}
}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)
stride = MIN_TRANSFER_STRIDE; // subdivide range
if (nextTab == null) { // initiating
try {
@SuppressWarnings("unchecked")
Node<K,V>[] nt = (Node<K,V>[])new Node<?,?>[n << 1]; //扩容为原来的2倍，即2*n
nextTab = nt;
} catch (Throwable ex) { // try to cope with OOME
sizeCtl = Integer.MAX_VALUE;
return;
}
nextTable = nextTab;
transferIndex = n; //transferIndex为如果有下一个线程参与扩容，开始迁移的下标位置，从原hash表尾部开始
}
int nextn = nextTab.length;
ForwardingNode<K,V> fwd = new ForwardingNode<K,V>(nextTab); //如果一个桶位置上位fwd，则表示该桶已被迁移
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) { //不停的从原hash表尾部往前找
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)//若原hash表该桶为null，置为fwd，
advance = casTabAt(tab, i, null, fwd);
else if ((fh = f.hash) == MOVED) //若为fwd，表示已迁移过
advance = true; // already processed
else {
synchronized (f) {
if (tabAt(tab, i) == f) { //链表
Node<K,V> ln, hn; //形成两个链表，
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); //一个链表放在i位置
setTabAt(nextTab, i + n, hn);//一个放在i+n位置
setTabAt(tab, i, fwd); //该桶位置置为fwd，表示已经迁移过，下一个线程到这个桶直接跳过
advance = true;
}
else if (f instanceof TreeBin) { //红黑树
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;
}
}
}
}
}
}这里画一张图，帮助理解上面的两个链表的特点和原链表的区别。
这里可以看出，lastrun后面的结点是和原链表一样的，前面的都被逆序了。可以看一下Node的构造方法，有助于理解。
到最后原hash表的所有桶都被置为fwd，迁移就结束了。
其中stride为步长，意思就是一个线程管stride范围内的桶的迁移工作。而transferIndex为下一个线程开始迁移的位置。
