本文基于jdk1.8
1.简介
ConcurrentHashMap 是HashMap 的线程安全版本。底层跟HashMap类似，采用了数组+链表+红黑树的结构。其继承关系如下
二.源码解析
1.基本属性
主要列出与HashMap不同的属性
// 最小转移分组大小
private static final int MIN_TRANSFER_STRIDE = 16;
private static int RESIZE_STAMP_BITS = 16;
// 最大扩容量
private static final int MAX_RESIZERS = (1 << (32 - RESIZE_STAMP_BITS)) - 1;
// 扩容时邮戳
private static final int RESIZE_STAMP_SHIFT = 32 - RESIZE_STAMP_BITS;
transient volatile Node<K,V>[] table;
// 扩容时使用，正常时为nul
private transient volatile Node<K,V>[] nextTable;
// 基础数量，当前数量的一部分
private transient volatile long baseCount;
// 控制是否扩容,
private transient volatile int sizeCtl;
private transient volatile int transferIndex;
// 表示counterCell是否在扩容或初始化，0表示默认未进行这些操作
private transient volatile int cellsBusy;
private transient volatile CounterCell[] counterCells;
2.构造函数
构造函数与HashMap类似，区别在于不在使用loadFactor，而使用sizeCtl 。
public ConcurrentHashMap() {
}
public ConcurrentHashMap(int initialCapacity) {
if (initialCapacity < 0)
throw new IllegalArgumentException();
int cap = ((initialCapacity >= (MAXIMUM_CAPACITY >>> 1)) ?
MAXIMUM_CAPACITY :
tableSizeFor(initialCapacity + (initialCapacity >>> 1) + 1));
this.sizeCtl = cap;
}
public ConcurrentHashMap(Map<? extends K, ? extends V> m) {
this.sizeCtl = DEFAULT_CAPACITY;
putAll(m);
}
public ConcurrentHashMap(int initialCapacity, float loadFactor) {
this(initialCapacity, loadFactor, 1);
}
public ConcurrentHashMap(int initialCapacity,
float loadFactor, int concurrencyLevel) {
if (!(loadFactor > 0.0f) || initialCapacity < 0 || concurrencyLevel <= 0)
throw new IllegalArgumentException();
if (initialCapacity < concurrencyLevel) // Use at least as many bins
initialCapacity = concurrencyLevel; // as estimated threads
long size = (long)(1.0 + (long)initialCapacity / loadFactor);
int cap = (size >= (long)MAXIMUM_CAPACITY) ?
MAXIMUM_CAPACITY : tableSizeFor((int)size);
this.sizeCtl = cap;
}
3. put添加
public V put(K key, V value) {
return putVal(key, value, false);
}
final V putVal(K key, V value, boolean onlyIfAbsent) {
if (key == null || value == null) throw new NullPointerException();
// 计算hash，用于定位桶位置
int hash = spread(key.hashCode());
int binCount = 0;
for (Node<K,V>[] tab = table;;) {
Node<K,V> f; int n, i, fh;
// 首次添加初始化数组
if (tab == null || (n = tab.length) == 0)
tab = initTable();
// 桶不存在，直接插入
else if ((f = tabAt(tab, i = (n - 1) & hash)) == null) {
if (casTabAt(tab, i, null,
new Node<K,V>(hash, key, value, null)))
break;
}
// 头节点move状态，则帮助扩容
else if ((fh = f.hash) == MOVED)
tab = helpTransfer(tab, f);
else {
V oldVal = null;
synchronized (f) {
// 找到桶
if (tabAt(tab, i) == f) {
// 树的根节点为 -2，这里大于0表示链表
if (fh >= 0) {
// 链表长度
binCount = 1;
for (Node<K,V> e = f;; ++binCount) {
K ek;
// 查到到元素替换
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;
// 链表中不存在，则插入末尾
if ((e = e.next) == null) {
pred.next = new Node<K,V>(hash, key,
value, null);
break;
}
}
}
// 如果是树节点，以树的方式插入
else if (f instanceof TreeBin) {
Node<K,V> p;
// 这里设置2，避免影响后面binCount >= TREEIFY_THRESHOLD的判断
binCount = 2;
if ((p = ((TreeBin<K,V>)f).putTreeVal(hash, key,
value)) != null) {
oldVal = p.val;
if (!onlyIfAbsent)
p.val = value;
}
}
}
}
if (binCount != 0) {
// 链表数大于树化阈值，则判断是否进一步树化还是扩容
if (binCount >= TREEIFY_THRESHOLD)
treeifyBin(tab, i);
if (oldVal != null)
return oldVal;
break;
}
}
}
// 判断是否需要扩容
addCount(1L, binCount);
return null;
}
private final Node<K,V>[] initTable() {
Node<K,V>[] tab; int sc;
while ((tab = table) == null || tab.length == 0) {
// 当有其他线程先一步进行初始化，则当前线程等待
if ((sc = sizeCtl) < 0)
Thread.yield();
// CAS比较成功，设置 sizeCtl =-1，进行初始化
else if (U.compareAndSwapInt(this, SIZECTL, sc, -1)) {
try {
if ((tab = table) == null || tab.length == 0) {
int n = (sc > 0) ? sc : DEFAULT_CAPACITY;
@SuppressWarnings("unchecked")
Node<K,V>[] nt = (Node<K,V>[])new Node<?,?>[n];
table = tab = nt;
// 设置新的sizeCtrl 为新容量的0.75
sc = n - (n >>> 2);
}
} finally {
sizeCtl = sc;
}
break;
}
}
private final void addCount(long x, int check) {
CounterCell[] as; long b, s;
// counterCells不为空，或者 baseCount 设置失败，说明线程竞争比较激烈
if ((as = counterCells) != null ||
!U.compareAndSwapLong(this, BASECOUNT, b = baseCount, s = b + x)) {
CounterCell a; long v; int m;
// true表示无竞争
boolean uncontended = true;
// counterCells不存在或者分段长度<0，或分段不存在，或更新分段失败
if (as == null || (m = as.length - 1) < 0 ||
(a = as[ThreadLocalRandom.getProbe() & m]) == null ||
!(uncontended =
U.compareAndSwapLong(a, CELLVALUE, v = a.value, v + x))) {
// 更新数量到分段上
fullAddCount(x, uncontended);
return;
}
if (check <= 1)
return;
// 统计当前元素数量
s = sumCount();
}
if (check >= 0) {
Node<K,V>[] tab, nt; int n, sc;
// 数量大于sizeCtl，小于最大值，则进行扩容
while (s >= (long)(sc = sizeCtl) && (tab = table) != null &&
(n = tab.length) < MAXIMUM_CAPACITY) {
// 获取扩容邮戳
int rs = resizeStamp(n);
// 表示正在扩容
if (sc < 0) {
// 这些判断扩容是否已经完成
if ((sc >>> RESIZE_STAMP_SHIFT) != rs || sc == rs + 1 ||
sc == rs + MAX_RESIZERS || (nt = nextTable) == null ||
transferIndex <= 0)
break;
// 扩容线程+1，执行扩容
if (U.compareAndSwapInt(this, SIZECTL, sc, sc + 1))
transfer(tab, nt);
}
// 其他线程没有扩容，设置当前线程进行扩容
else if (U.compareAndSwapInt(this, SIZECTL, sc,
(rs << RESIZE_STAMP_SHIFT) + 2))
transfer(tab, null);
s = sumCount();
}
}
}
private final void fullAddCount(long x, boolean wasUncontended) {
int h;
// 随机值
if ((h = ThreadLocalRandom.getProbe()) == 0) {
ThreadLocalRandom.localInit();
h = ThreadLocalRandom.getProbe();
wasUncontended = true;
}
// 是否碰撞
boolean collide = false;
for (;;) {
CounterCell[] as; CounterCell a; int n; long v;
// 当前counterCells已经初始化过
if ((as = counterCells) != null && (n = as.length) > 0) {
// 当前分区counterCell不存在
if ((a = as[(n - 1) & h]) == null) {
// 其他线程没有进行扩容操作
if (cellsBusy == 0) {
// 创建新的分区
CounterCell r = new CounterCell(x);
// 再次判断是否有其他分区进行扩容，设置当前线程进行扩容
if (cellsBusy == 0 &&
U.compareAndSwapInt(this, CELLSBUSY, 0, 1)) {
boolean created = false;
try {
CounterCell[] rs; int m, j;
// 再次检查
if ((rs = counterCells) != null &&
(m = rs.length) > 0 &&
rs[j = (m - 1) & h] == null) {
rs[j] = r;
created = true;
}
} finally {
cellsBusy = 0;
}
// 创建分区成功退出循环
if (created)
break;
continue;
}
}
collide = false;
}
else if (!wasUncontended) // CAS already known to fail
wasUncontended = true; // Continue after rehash
// CAS更新该分区的值
else if (U.compareAndSwapLong(a, CELLVALUE, v = a.value, v + x))
break;
else if (counterCells != as || n >= NCPU)
collide = false; // At max size or stale
else if (!collide)
collide = true;
// counterCell未进行初始化或扩容，更新cellsBusy成功
else if (cellsBusy == 0 &&
U.compareAndSwapInt(this, CELLSBUSY, 0, 1)) {
try {
// 扩容counterCells
if (counterCells == as) {
CounterCell[] rs = new CounterCell[n << 1];
for (int i = 0; i < n; ++i)
rs[i] = as[i];
counterCells = rs;
}
} finally {
cellsBusy = 0;
}
collide = false;
continue; // Retry with expanded table
}
h = ThreadLocalRandom.advanceProbe(h);
}
// counterCell未进行初始化或扩容，更新cellsBusy成功
else if (cellsBusy == 0 && counterCells == as &&
U.compareAndSwapInt(this, CELLSBUSY, 0, 1)) {
boolean init = false;
try {
// 初始化counterCell0s，初始化完成退出循环
if (counterCells == as) {
CounterCell[] rs = new CounterCell[2];
rs[h & 1] = new CounterCell(x);
counterCells = rs;
init = true;
}
} finally {
cellsBusy = 0;
}
if (init)
break;
}
// 竞争不激烈，直接更新BASECOUNT
else if (U.compareAndSwapLong(this, BASECOUNT, v = baseCount, v + x))
break; // Fall back on using base
}
}
4. helpTransfer和transfer扩容方法
final Node<K,V>[] helpTransfer(Node<K,V>[] tab, Node<K,V> f) {
Node<K,V>[] nextTab; int sc;
// 当前桶转移完成，总的扩容未完成
if (tab != null && (f instanceof ForwardingNode) &&
(nextTab = ((ForwardingNode<K,V>)f).nextTable) != null) {
// 获取扩容邮戳
int rs = resizeStamp(tab.length);
while (nextTab == nextTable && table == tab &&
(sc = sizeCtl) < 0) {
// 校验扩容是否完成了
if ((sc >>> RESIZE_STAMP_SHIFT) != rs || sc == rs + 1 ||
sc == rs + MAX_RESIZERS || transferIndex <= 0)
break;
// 扩容线程+1，并进行扩容
if (U.compareAndSwapInt(this, SIZECTL, sc, sc + 1)) {
transfer(tab, nextTab);
break;
}
}
return nextTab;
}
return table;
}
private final void transfer(Node<K,V>[] tab, Node<K,V>[] nextTab) {
int n = tab.length, stride;
// 计算转移分组长度
if ((stride = (NCPU > 1) ? (n >>> 3) / NCPU : n) < MIN_TRANSFER_STRIDE)
stride = MIN_TRANSFER_STRIDE;
// 初始化新数组长度
if (nextTab == null) {
try {
// 新数组容量是原数组的两倍
@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;
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;
// 计算转移的两个边界值
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))) {
bound = nextBound;
i = nextIndex - 1;
advance = false;
}
}
if (i < 0 || i >= n || i + n >= nextn) {
int sc;
// 扩容完成
if (finishing) {
nextTable = null;
table = nextTab;
sizeCtl = (n << 1) - (n >>> 1);
return;
}
// 当前线程扩容完成线程数-1
if (U.compareAndSwapInt(this, SIZECTL, sc = sizeCtl, sc - 1)) {
if ((sc - 2) != resizeStamp(n) << RESIZE_STAMP_SHIFT)
return;
finishing = advance = true;
// 重新检查一遍是否转移完成
i = n; // recheck before commit
}
}
// 当前桶没有数据，设置当前处于完成状态
else if ((f = tabAt(tab, i)) == null)
advance = casTabAt(tab, i, null, fwd);
// 当前已处于完成状态
else if ((fh = f.hash) == MOVED)
advance = true; // already processed
else {
synchronized (f) {
if (tabAt(tab, i) == f) {
Node<K,V> ln, hn;
// 链表元素的转移
if (fh >= 0) {
int runBit = fh & n;
Node<K,V> lastRun = f;
// lastRun 是表示从该节点后面不需要特殊处理的首个节点
for (Node<K,V> p = f.next; p != null; p = p.next) {
int b = p.hash & n;
if (b != runBit) {
runBit = b;
lastRun = p;
}
}
// 判断lastRun是低位链表还是高位链表
if (runBit == 0) {
ln = lastRun;
hn = null;
}
else {
hn = lastRun;
ln = null;
}
// 遍历链表，(ph & n) == 0放入低位链表
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);
// 设置当前链表迁移完成
setTabAt(tab, i, fwd);
advance = true;
}
// 树元素的转移，原理跟链表类似
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;
}
}
}
}
}
}
5. remove移除
public V remove(Object key) {
return replaceNode(key, null, null);
}
final V replaceNode(Object key, V value, Object cv) {
// 获取hash值
int hash = spread(key.hashCode());
for (Node<K,V>[] tab = table;;) {
Node<K,V> f; int n, i, fh;
// 数组未进行初始化，或者当前桶不存在，退出循环
if (tab == null || (n = tab.length) == 0 ||
(f = tabAt(tab, i = (n - 1) & hash)) == null)
break;
// 当前在扩容，帮助扩容
else if ((fh = f.hash) == MOVED)
tab = helpTransfer(tab, f);
else {
V oldVal = null;
boolean validated = false;
synchronized (f) {
// 找到对应的桶
if (tabAt(tab, i) == f) {
// 当前是链表
if (fh >= 0) {
validated = true;
// 遍历俩表，查找节点
for (Node<K,V> e = f, pred = null;;) {
K ek;
if (e.hash == hash &&
((ek = e.key) == key ||
(ek != null && key.equals(ek)))) {
V ev = e.val;
// 找到目标节点
if (cv == null || cv == ev ||
(ev != null && cv.equals(ev))) {
oldVal = ev;
// value不为null 替换值
if (value != null)
e.val = value;
// 前置节点不为null，则删除当前节点
else if (pred != null)
pred.next = e.next;
else
// 前置节点为null，则设置下一个节点首节点
setTabAt(tab, i, e.next);
}
break;
}
pred = e;
// 到链表尾未找到节点，中断循环
if ((e = e.next) == null)
break;
}
}
// 当前是红黑树
else if (f instanceof TreeBin) {
validated = true;
TreeBin<K,V> t = (TreeBin<K,V>)f;
TreeNode<K,V> r, p;
// 遍历树，查找节点
if ((r = t.root) != null &&
(p = r.findTreeNode(hash, key, null)) != null) {
V pv = p.val;
if (cv == null || cv == pv ||
(pv != null && cv.equals(pv))) {
oldVal = pv;
// value不为null 替换值
if (value != null)
p.val = value;
// 移除节点
else if (t.removeTreeNode(p))
setTabAt(tab, i, untreeify(t.first));
}
}
}
}
}
// 找到元素，移除，并减少数量
if (validated) {
if (oldVal != null) {
if (value == null)
addCount(-1L, -1);
return oldVal;
}
break;
}
}
}
return null;
}
6. get获取元素
public V get(Object key) {
Node<K,V>[] tab; Node<K,V> e, p; int n, eh; K ek;
int h = spread(key.hashCode());
// 遍历数组查找
if ((tab = table) != null && (n = tab.length) > 0 &&
(e = tabAt(tab, (n - 1) & h)) != null) {
// 首个元素符合，直接返回
if ((eh = e.hash) == h) {
if ((ek = e.key) == key || (ek != null && key.equals(ek)))
return e.val;
}
// 小于0表示红黑树或者正在扩容
else if (eh < 0)
return (p = e.find(h, key)) != null ? p.val : null;
// 链表遍历查找
while ((e = e.next) != null) {
if (e.hash == h &&
((ek = e.key) == key || (ek != null && key.equals(ek))))
return e.val;
}
}
return null;
}
7. size大小
ConcurrentHashMap的容量等于basecount+各个CounterCell之和。
public int size() {
long n = sumCount();
return ((n < 0L) ? 0 :
(n > (long)Integer.MAX_VALUE) ? Integer.MAX_VALUE :
(int)n);
}
final long sumCount() {
// 获取分区数组
CounterCell[] as = counterCells; CounterCell a;
long sum = baseCount;
if (as != null) {
for (int i = 0; i < as.length; ++i) {
if ((a = as[i]) != null)
sum += a.value;
}
}
return sum;
}
三.总结
ConcurrentHashMap 是HashMap 的线程安全版本，根据源码可知采用CAS+自旋来保证线程安全。
为了提高效率采用分段锁的思想，多线程协助扩容。