Java8 中的 ConcurrentHashMap 的put()方法简单解析
1.put()方法：大方向上可以分为三块：1.初始化，2扩容，3数据迁移
public V put(K key, V value) {
return putVal(key, value, false);
}
/** Implementation for put and putIfAbsent */
final V putVal(K key, V value, boolean onlyIfAbsent) {
//key和value都不允许null
if (key == null || value == null) throw new NullPointerException();
int hash = spread(key.hashCode());
// 用于记录相应链表的长度
int binCount = 0;
//遍历数组
for (Node[] tab = table;;) {
Node f; int n, i, fh;
//数组不存在或者没数据的情况，初始化
if (tab == null || (n = tab.length) == 0)
tab = initTable();
//如果数组当前位置为null,用一次 CAS 操作将这个新值放入其中
else if ((f = tabAt(tab, i = (n - 1) & hash)) == null) {
if (casTabAt(tab, i, null,
new Node(hash, key, value, null)))
break; // no lock when adding to empty bin
}
//
else if ((fh = f.hash) == MOVED)
tab = helpTransfer(tab, f);
else {
//如果f是头结点
V oldVal = null;
//加锁
synchronized (f) {
//
if (tabAt(tab, i) == f) {
//头结点的 hash 值大于 0，说明是链表
if (fh >= 0) {
//链表长度+1
binCount = 1;
//遍历链表
for (Node e = f;; ++binCount) {
K ek;
if (e.hash == hash &&
((ek = e.key) == key ||
(ek != null && key.equals(ek)))) {
// 如果发现了"相等"的 key，判断是否要进行值覆盖
oldVal = e.val;
if (!onlyIfAbsent)
e.val = value;
break;
}
// 到了链表的最末端，将新值放到链表的最末端
Node pred = e;
if ((e = e.next) == null) {
pred.next = new Node(hash, key,
value, null);
break;
}
}
}
//如果是一个红黑树的结构，将新值放到红黑树中
else if (f instanceof TreeBin) {
Node p;
binCount = 2;
if ((p = ((TreeBin)f).putTreeVal(hash, key,
value)) != null) {
oldVal = p.val;
if (!onlyIfAbsent)
p.val = value;
}
}
}
}
//链表的长度大于8；转换为红黑树
if (binCount != 0) {
if (binCount >= TREEIFY_THRESHOLD)
treeifyBin(tab, i);
if (oldVal != null)
return oldVal;
break;
}
}
}
addCount(1L, binCount);
return null;
} 1.1 private final void treeifyBin(Node
private final void treeifyBin(Node[] tab, int index) {
Node b; int n, sc;
if (tab != null) {
//判断当前表容量
if ((n = tab.length) < MIN_TREEIFY_CAPACITY)
tryPresize(n << 1);
else if ((b = tabAt(tab, index)) != null && b.hash >= 0) {
//加锁
synchronized (b) {
//找到对应的数组
if (tabAt(tab, index) == b) {
TreeNode hd = null, tl = null;
//遍历链表
for (Node e = b; e != null; e = e.next) {
TreeNode p =
new TreeNode(e.hash, e.key, e.val,
null, null);
if ((p.prev = tl) == null)
hd = p;
else
tl.next = p;
tl = p;
}
//建立一个红黑树
setTabAt(tab, index, new TreeBin(hd));
}
}
}
}
} 1.1.1 private final Node
private final Node[] initTable() {
Node[] tab; int sc;
while ((tab = table) == null || tab.length == 0) {
if ((sc = sizeCtl) < 0)
Thread.yield(); // lost initialization race; just spin
// CAS，将 sizeCtl 设置为 -1，代表抢到了锁
else if (U.compareAndSwapInt(this, SIZECTL, sc, -1)) {
try {
//表为空或者没有数据
if ((tab = table) == null || tab.length == 0) {
int n = (sc > 0) ? sc : DEFAULT_CAPACITY;
@SuppressWarnings("unchecked")
//初始化一个表（初始化一个数组）
Node[] nt = (Node[])new Node[n];
table = tab = nt;
//如果 n 为 16 的话，那么这里 sc = 12 也就是 0.75 * n
sc = n - (n >>> 2);
}
} finally {
//赋值到主内存中的sizeCtl
sizeCtl = sc;
}
break;
}
}
return tab;
} 1.2 private final void tryPresize(int size)：按数组合链表结构创建map
private final void tryPresize(int size) {
//size+0.5size+1,再往上取最近的 2 的 n 次方
int c = (size >= (MAXIMUM_CAPACITY >>> 1)) ? MAXIMUM_CAPACITY :
tableSizeFor(size + (size >>> 1) + 1);
int sc;
//sizeCtl 是主内存中的一个值
while ((sc = sizeCtl) >= 0) {
//获取当前表
Node[] tab = table; int n;
if (tab == null || (n = tab.length) == 0) {
//可以理解为当前的表空间
n = (sc > c) ? sc : c;
//CAS 线程安全限制，提高效率，其实是加锁-1
if (U.compareAndSwapInt(this, SIZECTL, sc, -1)) {
try {
if (table == tab) {
@SuppressWarnings("unchecked")
//创建数组空间
Node[] nt = (Node[])new Node[n];
table = nt;
sc = n - (n >>> 2);
}
} finally {
//赋值到主内存中的sizeCtl
sizeCtl = sc;
}
}
}
//
else if (c <= sc || n >= MAXIMUM_CAPACITY)
break;
else if (tab == table) {
int rs = resizeStamp(n);
if (sc < 0) {
Node[] nt;
if ((sc >>> RESIZE_STAMP_SHIFT) != rs || sc == rs + 1 ||
sc == rs + MAX_RESIZERS || (nt = nextTable) == null ||
transferIndex <= 0)
break;
//用 CAS 将 sizeCtl 加 1，然后执行 transfer 方法
if (U.compareAndSwapInt(this, SIZECTL, sc, sc + 1))
transfer(tab, nt);
}
//将 sizeCtl 设置为 (rs << RESIZE_STAMP_SHIFT) + 2)
else if (U.compareAndSwapInt(this, SIZECTL, sc,
(rs << RESIZE_STAMP_SHIFT) + 2))
transfer(tab, null);
}
}
} 1.3 private final void transfer(Node
private final void transfer(Node[] tab, Node[] nextTab) {
int n = tab.length, stride;
// stride 在单核下直接等于 n，多核模式下为 (n>>>3)/NCPU，最小值是 16
if ((stride = (NCPU > 1) ? (n >>> 3) / NCPU : n) < MIN_TRANSFER_STRIDE)
//设置16
stride = MIN_TRANSFER_STRIDE; // subdivide range
//入参为null
if (nextTab == null) { // initiating
try {
@SuppressWarnings("unchecked")
//位移一位，表空间扩大一倍
Node[] nt = (Node[])new Node[n << 1];
nextTab = nt;
} catch (Throwable ex) { // try to cope with OOME
sizeCtl = Integer.MAX_VALUE;
return;
}
//给到当前新表
nextTable = nextTab;
//控制迁移的位置
transferIndex = n;
}
//数组长度
int nextn = nextTab.length;
ForwardingNode fwd = new ForwardingNode(nextTab);
boolean advance = true;
boolean finishing = false; // to ensure sweep before committing nextTab
//索引和边界从零开始
for (int i = 0, bound = 0;;) {
Node f; int fh;
while (advance) {
int nextIndex, nextBound;
//如果索引值比边界值大或者相同或者结束了，迁移就结束了
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;
// 所有的迁移操作已经完成
if (finishing) {
nextTable = null;
// 将新的 nextTab 赋值给 table 属性，完成迁移
table = nextTab;
// 重新计算 sizeCtl：n 是原数组长度，所以 sizeCtl 是新数组长度
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
}
}
// 如果位置 i 处是空的，cas节点出来放入刚刚初始化的 ForwardingNode 空节点
else if ((f = tabAt(tab, i)) == null)
advance = casTabAt(tab, i, null, fwd);
// 该位置处是一个 ForwardingNode，代表该位置已经迁移过了
else if ((fh = f.hash) == MOVED)
advance = true; // already processed
else {
// 对数组该位置处的结点加锁，开始处理数组该位置处的迁移工作
synchronized (f) {
if (tabAt(tab, i) == f) {
Node ln, hn;
// 头结点的 hash 大于 0，说明是链表的 Node 节点
if (fh >= 0) {
//和jdk1.7类似，分两个链表处理
int runBit = fh & n;
Node lastRun = f;
for (Node 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 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(ph, pk, pv, ln);
else
hn = new Node(ph, pk, pv, hn);
}
setTabAt(nextTab, i, ln);
setTabAt(nextTab, i + n, hn);
setTabAt(tab, i, fwd);
advance = true;
}
//判断是不是红黑树，
else if (f instanceof TreeBin) {
//按红黑树规则插入红黑树的节点
TreeBin t = (TreeBin)f;
TreeNode lo = null, loTail = null;
TreeNode hi = null, hiTail = null;
int lc = 0, hc = 0;
for (Node e = t.first; e != null; e = e.next) {
int h = e.hash;
TreeNode p = new TreeNode
(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;
}
}
// 如果一分为二后，节点数少于 8，那么将红黑树转换回链表
ln = (lc <= UNTREEIFY_THRESHOLD) ? untreeify(lo) :
(hc != 0) ? new TreeBin(lo) : t;
// // 如果一分为二后，节点数少于 8，那么将红黑树转换回链表
hn = (hc <= UNTREEIFY_THRESHOLD) ? untreeify(hi) :
(lc != 0) ? new TreeBin(hi) : t;
// 将 ln 放置在新数组的位置 i
setTabAt(nextTab, i, ln);
// 将 hn 放置在新数组的位置 i+n
setTabAt(nextTab, i + n, hn);
//将原数组该位置处设置为 fwd，代表该位置已经处理完毕
setTabAt(tab, i, fwd);
//设置处理完成的标志
advance = true;
}
}
}
}
}
}