本文已参与「新人创作礼」活动，一起开启掘金创作之路。
前言
java中hashmap是线程不安全的，可以使用hashtable是线程安全的，那并发编程juc包里怎么会少了并发安全的hashmap呢，接下来就从源码看看concurrenthashmap到底怎么线程安全？
ConcurrentHashMap JDK 1.8 源码
ConcurrentHashMap重要****成员变量
static final int MOVED = -1; // hash for forwarding nodes
static final int TREEBIN = -2; // hash for roots of trees
static final int RESERVED = -3; // hash for transient reservations
static final int HASH_BITS = 0x7fffffff; // usable bits of normal node hash
解释
- MOVED: 值为-1, 当Node.hash为MOVED时, 代表着table正在扩容。
- TREEBIN, 置为-2, 代表此元素后接红黑树。
/**
* 容器数组。在第一次插入时延迟初始化。大小总是2的幂
*/
transient volatile Node<K,V>[] table;
/**
* 下个可用table; 扩容不为空前提下
*/
private transient volatile Node<K,V>[] nextTable;
/**
* 基本计数器值，主要在没有争用时使用。被CAS更新
*/
private transient volatile long baseCount;
/**
* 表初始化和调整大小控件。
* 当为负数时，表示表正在初始化或调整大小
*/
private transient volatile int sizeCtl;
/**
* table容量从n扩到2n时, 是从索引n->1的元素开始迁移, transferIndex代表当前已经迁移的元 素下标
*/
private transient volatile int transferIndex;
解释
sizeCtl
- 0: table还没有被初始化
- -1: table正在初始化
- 小于-1: table正在扩容
- 大于0: 初始化完成后, 代表table最大存放元素的个数, 默认为0.75*n
ConcurrentHashMap put方法源码
说起一个数据结构就少不了它的增删改查，并发安全往往也就是容易在这里发生问题。
final V putVal(K key, V value, boolean onlyIfAbsent) {
if (key == null || value == null) throw new NullPointerException();
int hash = spread(key.hashCode());
int binCount = 0;
//循环cas获取执行权***************************************************//
for (Node<K,V>[] tab = table;;) {
Node<K,V> f; int n, i, fh;
if (tab == null || (n = tab.length) == 0)
//initTable()中****************************************************//
//cas获取执行权初始化************************************************//
tab = initTable();
else if ((f = tabAt(tab, i = (n - 1) & hash)) == null) {
//cas无锁put元素进空位***********************************************//
if (casTabAt(tab, i, null,
new Node<K,V>(hash, key, value, null)))
break; // no lock when adding to empty bin
}
//MOVED为-1标识正在扩容**********************************************//
//意味着正在向新表移动************************************************//
else if ((fh = f.hash) == MOVED)
tab = helpTransfer(tab, f);
else {
V oldVal = null;
//synchronized锁非空位**********************************************//
//带锁去构建/追加 链表 或者 红黑树*************************************//
//非空位上的元素作为锁************************************************//
synchronized (f) {
//构建/追加 链表 ****************************************************//
if (tabAt(tab, i) == f) {
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;
}
}
}
//构建/追加 红黑树 ****************************************************//
else if (f instanceof TreeBin) {
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) {
if (binCount >= TREEIFY_THRESHOLD)
treeifyBin(tab, i);
if (oldVal != null)
return oldVal;
break;
}
}
}
解释
- 初始化hahsmap用CAS保证线程安全。
- 数组中为空时候，cas添加元素。
- 数组中不为空时候synchronized添加元素到链表/红黑树。
- 一个线程插入数据时, 发现table对应元素的hash=MOVED, 那么调用helpTransfer()协助扩容。如下：
ConcurrentHashMap 扩容方法源码——协助扩容逻辑
final Node<K,V>[] helpTransfer(Node<K,V>[] tab, Node<K,V> f) {
Node<K,V>[] nextTab; int sc;
if (tab != null && (f instanceof ForwardingNode) &&
(nextTab = ((ForwardingNode<K,V>)f).nextTable) != null) {
int rs = resizeStamp(tab.length);
while (nextTab == nextTable && table == tab &&
(sc = sizeCtl) < 0) {
if ((sc >>> RESIZE_STAMP_SHIFT) != rs || sc == rs + 1 ||
sc == rs + MAX_RESIZERS || transferIndex <= 0)
break;
if (U.compareAndSwapInt(this, SIZECTL, sc, sc + 1)) {
//转移************************************************************************//
transfer(tab, nextTab);
break;
}
}
return nextTab;
}
return table;
}
ConcurrentHashMap 扩容方法源码——实际扩容逻辑
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)
//每个线程最少迁移16个数组位**********************************************//
stride = MIN_TRANSFER_STRIDE;
if (nextTab == null) {
try {
@SuppressWarnings("unchecked")
//扩容2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 = n;
}
int nextn = nextTab.length;
ForwardingNode<K,V> fwd = new ForwardingNode<K,V>(nextTab);
boolean advance = true;
boolean finishing = false;
for (int i = 0, bound = 0;;) {
Node<K,V> f; int fh;
while (advance) {
int nextIndex, nextBound;
//本线程没有处理完成，那么就继续处理************************************//
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;
//如果迁移完成*******************************************************//
if (finishing) {
nextTable = null;
table = nextTab;
sizeCtl = (n << 1) - (n >>> 1);
return;
}
//表示当前线程迁移完成了***********************************************//
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)
advance = casTabAt(tab, i, null, fwd);
//别的线程已经在处理***********************************************//
else if ((fh = f.hash) == MOVED)
advance = true;
//如果数组位置上有元素***********************************************//
else {
//需要加锁，防止再向里面放值。
//另外一个线程向这个节点插入数据，此时迁移到这里了，会被阻塞住
synchronized (f) {
if (tabAt(tab, i) == f) {
Node<K,V> ln, hn;
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);
setTabAt(nextTab, i + n, hn);
setTabAt(tab, i, fwd);
advance = true;
}
// 如果是红黑树***************************************//
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) {
//中序遍历红黑树***************************************//
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) :
//判断是否需要转化为树**********************************************//
(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;
}
}
}
}
}
}
