ConcurrentHashMap(基于jdk1.8)
- HashMap 无锁 ConcurrentHashMap是有锁的
ConcurrentHashMap结构介绍
/*Key-value entry. This class is never exported out as a
user-mutable Map.Entry (i.e., one supporting setValue; see
MapEntry below), but can be used for read-only traversals used
in bulk tasks. Subclasses of Node with a negative hash field
are special, and contain null keys and values (but are never
exported). Otherwise, keys and vals are never null.*/
/*译:键值结构。这个类不可以导出成为一个用户可变的Map.Entry。但是可以在批量任务中做只读遍历。是一个具有负的hash值的特殊子类。并且包含null的值和键(但是不能导出),否则,键和值不可为空。*/
static class Node<K,V> implements Map.Entry<K,V> {
final int hash;
final K key;
volatile V val;
volatile Node<K,V> next;
Node(int hash, K key, V val, Node<K,V> next) {
this.hash = hash;
this.key = key;
this.val = val;
this.next = next;
}
public final K getKey() { return key; }
public final V getValue() { return val; }
public final int hashCode() { return key.hashCode() ^ val.hashCode(); }
public final String toString(){ return key + "=" + val; }
public final V setValue(V value) {
throw new UnsupportedOperationException();
}
public final boolean equals(Object o) {
Object k, v, u; Map.Entry<?,?> e;
return ((o instanceof Map.Entry) &&
(k = (e = (Map.Entry<?,?>)o).getKey()) != null &&
(v = e.getValue()) != null &&
(k == key || k.equals(key)) &&
(v == (u = val) || v.equals(u)));
}
/**
* Virtualized support for map.get(); overridden in subclasses.
*/
Node<K,V> find(int h, Object k) {
Node<K,V> e = this;
if (k != null) {
do {
K ek;
if (e.hash == h &&
((ek = e.key) == k || (ek != null && k.equals(ek))))
return e;
} while ((e = e.next) != null);
}
return null;
}
}
底层实现采用的是Node和红黑树(红黑树)以及cas实现的
- 首先来看一下get方法
/**
* Returns the value to which the specified key is mapped,
* or {@code null} if this map contains no mapping for the key.
*
* <p>More formally, if this map contains a mapping from a key
* {@code k} to a value {@code v} such that {@code key.equals(k)},
* then this method returns {@code v}; otherwise it returns
* {@code null}. (There can be at most one such mapping.)
*
* @throws NullPointerException if the specified key is null
*/
public V get(Object key) {
Node<K,V>[] tab; Node<K,V> e, p; int n, eh; K ek;
int h = spread(key.hashCode());
if ((tab = table) != null && (n = tab.length) > 0 &&
(e = tabAt(tab, (n - 1) & h)) != null) {
if ((eh = e.hash) == h) {
if ((ek = e.key) == key || (ek != null && key.equals(ek)))
return e.val;
}
else if (eh < 0)
return (p = e.find(h, key)) != null ? p.val : null;
while ((e = e.next) != null) {
if (e.hash == h &&
((ek = e.key) == key || (ek != null && key.equals(ek))))
return e.val;
}
}
return null;
}
看一下这个spread方法是干啥的?
在Java 8 中求hash的方法从hash改为了spread。Key的hashCode值与其高16位作异或并保证最高位为0(从而保证最终结果为正整数)。
(tab = table) != null && (n = tab.length) > 0 &&
(e = tabAt(tab, (n - 1) & h)) != null
/**
* The array of bins. Lazily initialized upon first insertion.
* Size is always a power of two. Accessed directly by iterators.
*/
transient volatile Node<K,V>[] table;
看一下这个比较条件
tab 局部变量的Node数组 table是一个全局的Node数组 被transient 修饰的属性如果实现的是Serializable接口则不被序列化 volatile 保证了 table的可见性,也就是任意一个线程对table做了修改他会马上同步到主存中
e=tabAt(tab, (n - 1) & h)) 作用是获取首节点的的Node元素
也就是说如果table不为null且 长度大于0 且首节点不为空,那么就继续判断
if ((eh = e.hash) == h) { if ((ek = e.key) == key || (ek != null && key.equals(ek))) return e.val; }
这个的意思的如果获取到的e节点的hash值等于要获取值的hash值,那么就判断key值是否相同(hash冲突)如果满足则返回valu值
else if (eh < 0) return (p = e.find(h, key)) != null ? p.val : null;
hash值为负值表示正在扩容,这个时候查的是ForwardingNode的find方法来定位到nextTable来
eh=-1,说明该节点是一个ForwardingNode,正在迁移,此时调用ForwardingNode的find方法去nextTable里找。
eh=-2,说明该节点是一个TreeBin,此时调用TreeBin的find方法遍历红黑树,由于红黑树有可能正在旋转变色,所以find里会有读写锁。
eh>=0,说明该节点下挂的是一个链表,直接遍历该链表即可。
- put方法
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) {
if (key == null || value == null) throw new NullPointerException();//值和value不可以为null
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) //如果为空初始化table
tab = initTable();
else if ((f = tabAt(tab, i = (n - 1) & hash)) == null) { //首节点为空
if (casTabAt(tab, i, null,
new Node<K,V>(hash, key, value, null))) //是否存在该节点不存在就添加
break; // no lock when adding to empty bin
}
else if ((fh = f.hash) == MOVED) // 如果当前结点为forwarding则帮助扩容
tab = helpTransfer(tab, f);
else {
V oldVal = null;
synchronized (f) { // 如果非forwarding且非null则加锁
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;
}
}
}
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;
}
}
}
addCount(1L, binCount);
return null;
}