jdk 基于 8 版本
在平时的开发中，我们经常会用到 ConcurrentHashMap, 是并发安全的。
使用方式
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| public class ConcurrentHashMapTest {
@Test
void test() {
Map<String, String> map = new ConcurrentHashMap<>();
map.put("1", "1");
assertThat(map.get("1")).isEqualTo("1");
map.remove("1");
assertThat(map.size()).isEqualTo(0);
}
}
|
put
添加元素。
源码位置: java.util.concurrent.ConcurrentHashMap#putVal
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| final V putVal(K key, V value, boolean onlyIfAbsent) {
if (key == null || value == null) throw new NullPointerException();
// 计算 hash 值
int hash = spread(key.hashCode());
int binCount = 0;
for (Node<K,V>[] tab = table;;) {
Node<K,V> f; int n, i, fh;
if (tab == null || (n = tab.length) == 0)
// 使用 CAS 来初始化 table
tab = initTable();
else if ((f = tabAt(tab, i = (n - 1) & hash)) == null) {
// f: 桶里的第一个元素，如果为空，使用 CAS 来初始化
if (casTabAt(tab, i, null,
new Node<K,V>(hash, key, value, null)))
break;
}
else if ((fh = f.hash) == MOVED)
// 扩容操作，太复杂了，不用关心
tab = helpTransfer(tab, f);
else {
V oldVal = null;
// 对首元素加锁
synchronized (f) {
if (tabAt(tab, i) == f) {
// fh >= 0 表示链表节点
if (fh >= 0) {
binCount = 1;
// 添加新节点，替换旧节点
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;
}
}
}
// 红黑树节点
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) {
// 链表节点大于8，将链表转为红黑树
if (binCount >= TREEIFY_THRESHOLD)
treeifyBin(tab, i);
if (oldVal != null)
return oldVal;
break;
}
}
}
// 添加个数
addCount(1L, binCount);
return null;
}
|
remove
删除元素。
源码位置: java.util.concurrent.ConcurrentHashMap#replaceNode
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| // 参数 value = null, vc = null
final V replaceNode(Object key, V value, Object cv) {
// 计算 hash 值
int hash = spread(key.hashCode());
for (Node<K,V>[] tab = table;;) {
Node<K,V> f; int n, i, fh;
// 桶里的首元素不存在
if (tab == null || (n = tab.length) == 0 ||
(f = tabAt(tab, i = (n - 1) & hash)) == null)
break;
else if ((fh = f.hash) == MOVED)
// 扩容，太复杂，不用关心
tab = helpTransfer(tab, f);
else {
V oldVal = null;
boolean validated = false;
// 对首元素加锁
synchronized (f) {
if (tabAt(tab, i) == f) {
// fh >= 0 表示链表节点
if (fh >= 0) {
validated = true;
// 删除节点
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;
if (cv == null || cv == ev ||
(ev != null && cv.equals(ev))) {
oldVal = ev;
if (value != null)
e.val = value;
else if (pred != null)
pred.next = e.next;
else
setTabAt(tab, i, e.next);
}
break;
}
pred = e;
if ((e = e.next) == null)
break;
}
}
// 红黑树节点
else if (f instanceof TreeBin) {
validated = true;
TreeBin<K,V> t = (TreeBin<K,V>)f;
TreeNode<K,V> r, p;
if ((r = t.root) != null &&
(p = r.findTreeNode(hash, key, null)) != null) {
V pv = p.val;
// 注意删除操作时，cv 和 value 都为 null
if (cv == null || cv == pv ||
(pv != null && cv.equals(pv))) {
oldVal = pv;
if (value != null)
p.val = value;
else if (t.removeTreeNode(p))
setTabAt(tab, i, untreeify(t.first));
}
}
}
}
}
if (validated) {
if (oldVal != null) {
if (value == null)
// 减少 count
addCount(-1L, -1);
return oldVal;
}
break;
}
}
}
return null;
}
|
get
获取元素，无锁。
源码位置: java.util.concurrent.ConcurrentHashMap#get
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| public V get(Object key) {
Node<K,V>[] tab; Node<K,V> e, p; int n, eh; K ek;
// 计算 hash 值
int h = spread(key.hashCode());
// e: 桶里第一个元素
if ((tab = table) != null && (n = tab.length) > 0 &&
(e = tabAt(tab, (n - 1) & h)) != null) {
// 是首元素，返回 val
if ((eh = e.hash) == h) {
if ((ek = e.key) == key || (ek != null && key.equals(ek)))
return e.val;
}
// 表示为 TREEBIN (红黑树节点)
else if (eh < 0)
return (p = e.find(h, key)) != null ? p.val : null;
// 遍历查找 (链表节点)
while ((e = e.next) != null) {
if (e.hash == h &&
((ek = e.key) == key || (ek != null && key.equals(ek))))
return e.val;
}
}
return null;
}
|
size
获取大小，无锁。
源码位置: java.util.concurrent.ConcurrentHashMap#size
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| public int size() {
long n = sumCount();
return ((n < 0L) ? 0 :
(n > (long)Integer.MAX_VALUE) ? Integer.MAX_VALUE :
(int)n);
}
// 遍历每个桶的大小
final long sumCount() {
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;
}
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