JDK9 ConcurrentHashMap实现原理(一)
数据结构
JDK1.7中采用Segment + HashEntry的方式进行实现.使用ReentrantLock实现加锁操作。
JDK1.8中放弃了Segment臃肿的设计,取而代之的是采用Node + CAS + Synchronized来保证并发安全进行实现.结构类似于HashMap,数组+链表+红黑树。
私有属性
静态属性
-
private static final int MAXIMUM_CAPACITY = 1 << 30;
最大的容量,必须为2的平方。 -
private static final int DEFAULT_CAPACITY = 16;
默认的初始容量,也是2的平方 -
static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
最大的数组大小。 -
private static final int DEFAULT_CONCURRENCY_LEVEL = 16;
默认的并发等级,只在writeObject中用了。 -
private static final float LOAD_FACTOR = 0.75f;
加载因子,只在writeObject中用了。不像HashMap中的用法。 -
static final int TREEIFY_THRESHOLD = 8;
当某个数组位置上的节点数量超过8时,则将单链表结构转换为红黑树结构。 -
static final int UNTREEIFY_THRESHOLD = 6;
当某个数组位置上的节点数量小于6时,则将红黑树结构转换为单链表结构。 -
static final int MIN_TREEIFY_CAPACITY = 64;
在转换成红黑树之前,还需要检测当前table的大小是否大于等于MIN_TREEIFY_CAPACITY,小于不会转换成红黑树,而是重新扩展table的大小。 -
private static final int MIN_TRANSFER_STRIDE = 16;
-
private static final int RESIZE_STAMP_BITS = 16;
-
private static final int MAX_RESIZERS = (1 << (32 - RESIZE_STAMP_BITS)) - 1;
扩容时可利用的最大线程 -
private static final int RESIZE_STAMP_SHIFT = 32 - RESIZE_STAMP_BITS;
-
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 -
static final int NCPU = Runtime.getRuntime().availableProcessors();
当前机器的CPU的核心处理器数量,transfer扩容时会用到。
相关节点
- Node:该类用于构造table[],只读节点(不提供修改方法)。是一个单链表结构。
static class Node<K,V> implements Map.Entry<K,V> {
//节点的哈希值
final int hash;
//建
final K key;
//值
volatile V val;
//指向下一个节点,说明是单链表结构
volatile Node<K,V> next;
Node(int hash, K key, V val) {
this.hash = hash;
this.key = key;
this.val = val;
}
Node(int hash, K key, V val, Node<K,V> next) {
this(hash, key, 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 Helpers.mapEntryToString(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;
}
}
- TreeBin:红黑树结构。
- TreeNode:红黑树节点。
static final class TreeNode<K,V> extends Node<K,V> {
//父节点
TreeNode<K,V> parent; // red-black tree links
//左节点
TreeNode<K,V> left;
//右节点
TreeNode<K,V> right;
//
TreeNode<K,V> prev; // needed to unlink next upon deletion
//节点的颜色
boolean red;
TreeNode(int hash, K key, V val, Node<K,V> next,
TreeNode<K,V> parent) {
super(hash, key, val, next);
this.parent = parent;
}
Node<K,V> find(int h, Object k) {
return findTreeNode(h, k, null);
}
//使用this这个树形节点作为根节点,寻找目标节点。
final TreeNode<K,V> findTreeNode(int h, Object k, Class<?> kc) {}
}
- ForwardingNode:临时节点(扩容时使用)。
构造器
- 无参构造器
public ConcurrentHashMap() {
}
指定初始容量
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;
}
这里使用tableSizeFor将输入的容量转换为2的平方。
private static final int tableSizeFor(int c) {
int n = c - 1;
n |= n >>> 1;
n |= n >>> 2;
n |= n >>> 4;
n |= n >>> 8;
n |= n >>> 16;
return (n < 0) ? 1 : (n >= MAXIMUM_CAPACITY) ? MAXIMUM_CAPACITY : n + 1;
}
输入容量:3 , 输出:4;
输入容量:11 , 输出:16;
输入容量:17 , 输出:32;
不确定这里为什么要这么处理(initialCapacity + (initialCapacity >>> 1) + 1));虽然直接initialCapacity的结果也是一样的。
- 还可以指定加载因子和concurrencyLevel。
可以看到这两个参数在新版本中只是在初始化才会用到,其他地方不会用到,注意这里和HashMap的区别。
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;
}
- 和大多数集合一样,都可以由其他集合元素作为初始元素。
public ConcurrentHashMap(Map<? extends K, ? extends V> m) {
this.sizeCtl = DEFAULT_CAPACITY;
putAll(m);
}
Hash值计算
可以看出 计算出的hash都是正值
static final int HASH_BITS = 0x7fffffff; // usable bits of normal node hash
static final int spread(int h) {
return (h ^ (h >>> 16)) & HASH_BITS;
}
添加元素
1.key值和value都不能null
2.onlyIfAbsent=true : 如果当前key值已经存在,则不 更新为新值, false: 不管什么情况都会更新为新值。
final V putVal(K key, V value, boolean onlyIfAbsent) {
if (key == null || value == null) throw new NullPointerException();
//获取hash值
int hash = spread(key.hashCode());
int binCount = 0;
for (Node<K,V>[] tab = table;;) {
Node<K,V> f; int n, i, fh; K fk; V fv;
//如果table为空,说明之前没有放入元素
if (tab == null || (n = tab.length) == 0)
tab = initTable();
//如果要插入的位置没有节点数据
else if ((f = tabAt(tab, i = (n - 1) & hash)) == null) {
//如果数组的桶是空的,则尝试插入数据,直到成功才中断当前循环,使用CAS算法
if (casTabAt(tab, i, null, new Node<K,V>(hash, key, value)))
break; // no lock when adding to empty bin
}
//到此插入数据成功
//如果在插入的时候,节点是一个forwordingNode状态,表示正在扩容,那么当前线程进行帮助扩容
else if ((fh = f.hash) == MOVED)
tab = helpTransfer(tab, f);
else if (
//如果onlyIfAbsent为true,也就是只要key已经存在,就不写入新值
onlyIfAbsent
//扩容已经结束,fh 和传入key的值一样
&& fh == hash // check first node
//
&& ((fk = f.key) == key || fk != null && key.equals(fk))
&& (fv = f.val) != null)
return fv;
else {
V oldVal = null;
synchronized (f) {
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);
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;
}
}
else if (f instanceof ReservationNode)
throw new IllegalStateException("Recursive update");
}
}
if (binCount != 0) {
if (binCount >= TREEIFY_THRESHOLD)
treeifyBin(tab, i);
if (oldVal != null)
return oldVal;
break;
}
}
}
addCount(1L, binCount);
return null;
}
初始化数组
private final Node<K,V>[] initTable() {
Node<K,V>[] tab; int sc;
while ((tab = table) == null || tab.length == 0) {
if ((sc = sizeCtl) < 0)
Thread.yield(); // lost initialization race; just spin
else if (U.compareAndSetInt(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;
sc = n - (n >>> 2);
}
} finally {
sizeCtl = sc;
}
break;
}
}
return tab;
}
未完待续。。。。。。。。。。