private final void transfer(Node<K,V>[] tab, Node<K,V>[] nextTab) {
// stride 可以理解为步长 单核模式下为 n，多核模式下为 (n >>> 3) / NCPU，步长最小为16
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")
// 容量翻倍
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 是 ConcurrentHashMap 的属性，用来控制迁移的位置
transferIndex = n;
}
int nextn = nextTab.length;
ForwardingNode<K,V> fwd = new ForwardingNode<K,V>(nextTab);
// 做完一个位置的迁移工作，可以准备做下一个位置的了
boolean advance = true;
boolean finishing = false; // to ensure sweep before committing nextTab
for (int i = 0, bound = 0;;) {
Node<K,V> f; int fh;
// advance 为 true 表示可以进行下一个位置的迁移了
while (advance) {
int nextIndex, nextBound;
if (--i >= bound || finishing)
advance = false;
// transferIndex <= 0 说明原数组的所有位置都有相应的线程去处理了
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) {
// 所有操作已经迁移完成
nextTable = null;
table = nextTab;
// 重新计算 sizeCtl :
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)
// 位置 i 是空的，放入 fwd 节点
advance = casTabAt(tab, i, null, fwd);
else if ((fh = f.hash) == MOVED)
// 该位置是一个 ForwardingNode，表示该位置已经迁移过了
advance = true; // already processed
else {
// 对槽位进行加锁，开始处理该位置处的迁移工作
synchronized (f) {
if (tabAt(tab, i) == f) {
Node<K,V> ln, hn;
// 是链表的节点
if (fh >= 0) {
int runBit = fh & n;
Node<K,V> lastRun = f;
// 这个主要就是为了重用对象，和1.7类似，看我1.7的解析吧
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);
}
// 放一个链表在新数组的位置 i
setTabAt(nextTab, i, ln);
// 放另一个链表在新数组的位置 i + n
setTabAt(nextTab, i + n, hn);
// 在原数组放 fwd 节点，表示该位置已经处理完毕了
setTabAt(tab, i, fwd);
// 该位置已经迁移完毕
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;
}
}
}
}
}
}
