原文日期：2017-10-22
接着第一篇Java源码心得jdk8-ConcurrentHashMap一继续看transfer扩容方法源码，如下
transfer方法
该方法为集合扩容方法，相当于HashMap的resize方法。相比之下它的处理原理和HashMap基本差不多，但是由于需要进行多线程处理，故而代码实现上更加复杂。
ConcurrentHashMap每次扩容操作和HashMap一样，扩容大小都为原先的两倍。ConcurrentHashMap扩容没有对整个桶元素进行加锁处理，而是只对当前桶元素进行加锁处理，并且每一次扩容，只对当前桶元素的子节点进行扩容，对其他桶元素的写操作毫无影响。每次扩容完会将当前桶元素替换为ForwardingNode对象节点，该节点的特征为hash值为-1，key和value值为null，但它有一个nextTable属性，指向新扩容数组结构。
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
//nextTab为空，表示第一个线程进来扩容
try {
//每次扩容大小都为原来两倍
@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;
//扩容从内部数组最后一个桶元素的子节点开始。
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))) {
//计算扩容元素的索引值
bound = nextBound;
i = nextIndex - 1;
advance = false;
}
}
if (i < 0 || i >= n || i + n >= nextn) {
int sc;
if (finishing) {
//集合已完成扩容操作，结束扩容
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;
//其他线程发现扩容已完成，结束扩容
finishing = advance = true;
i = n; // recheck before commit
}
}
else if ((f = tabAt(tab, i)) == null)
//桶元素为空时，扩容
advance = casTabAt(tab, i, null, fwd);
else if ((fh = f.hash) == MOVED)
//桶元素正在扩容，跳过，对下一个桶元素进行扩容
advance = true; // already processed
else {
//实际扩容操作,对当前桶元素进行加锁。这样集合元素扩容分n个锁，可以多个线程并发扩容，当前桶元素扩容不会影响其他桶元素的操作，极大减少锁冲突概率。
synchronized (f) {
if (tabAt(tab, i) == f) {
Node<K,V> ln, hn;
if (fh >= 0) {
//链表扩容操作
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;
//和HashMap处理方式一样，将索引i的桶元素上的节点，拆分到i和i+n两个桶元素上
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) {
//树形结构扩容操作
......
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;
}
}
......
}
}
}
}
}
}
