Java HashMap，LinkedHashMap 实现原理分
2018-04-01 本文已影响84人
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相关类继承结构
image.png
@startuml
interface Map {
interface Entry
int size()
boolean containersKey()
boolean containersValue()
V get(Object)
V put(K,V)
V remove(Object)
boolean remove(K, V)
boolean replace(K,V,V)
V replace<K,V)
void putAll(Map<? extends K, ? extends V>
void clear()
Set<K> keySet()
Collection<V> values()
Set<Entry<K,V>> entrySet()
}
class HashMap {
static class Node
static final class TreeNode
Node[] table
int size
int modCound
int threshold
loadFactor
void afterNodeAccess()
void afterNodeInsertion()
void afterNodeRemoval()
}
class LinkedHashMap {
static class LinkedHashMapEntry
LinkedHashMapEntry head
LinkedHashMapEntry tail
boolean accessOrder
void afterNodeAccess()
void afterNodeInsertion()
void afterNodeRemoval()
}
interface Map.Entry {
K getKey()
V getValue()
V setValue()
boolean equals
int hashCode()
Comparator comparintByKey()
Comparator comparintByValue()
}
class HashMap.Node{
final int hash;
final K key;
V value;
Node<K,V> next;
}
class HashMap.TreeNode {
TreeNode<K,V> parent; // red-black tree links
TreeNode<K,V> left;
TreeNode<K,V> right;
TreeNode<K,V> prev; // needed to unlink next upon deletion
boolean red;
}
class LinkedHashMap.LinkedHashMapEntry{
LinkedHashMapEntry<K,V> before;
LinkedHashMapEntry<K,V> after;
}
Map <|.. HashMap
HashMap <|-- LinkedHashMap
Map.Entry <|.. HashMap.Node
HashMap.Node <|-- LinkedHashMap.LinkedHashMapEntry
LinkedHashMap.LinkedHashMapEntry <|-- HashMap.TreeNode
@enduml
HashMap原理概述
- JDK1.8之前：HashMap内部是由数组+单向链表实现的，如图HashMap Internal Structure before JDK1.8。
- JDK1.8以后： HashMap内部是由数组+单向链表+红黑树实现的，如图HashMap Internal Structure after JDK1.8。
- 数组和链表存放的都是HashMap.Node对象，Node是一个单链结构，具有hash, key, value, next四个field。
- 具有相同hash值的Node放在同一个链表中，该链表的头放在在table中。
HashMap Internal Structure before JDK1.8
图片出处：How HashMap Works Internally In Java?
HashMap Internal Structure after JDK1.8
图片出处：深入分析hashmap
HashMap源码分析
put(K key, V value)
put方法的大体流程可以概括为：
- 根据key计算hash值；
- 根据hash值在table中找对应的Node：
没有，直接new一个Node，放到table对应的位置上；
有，看key是否equal，equal就给Node赋值新的value，不equal就遍历Node所在链表，找到equal的Node的话就赋值新的value，找不到就new一个Node放到链表末尾。如果链表长度超过阈值，将其转化为红黑树。
public V put(K key, V value) {
return putVal(hash(key), key, value, false, true);
}
/**
* Implements Map.put and related methods
*
* @param hash hash for key
* @param key the key
* @param value the value to put
* @param onlyIfAbsent if true, don't change existing value
* @param evict if false, the table is in creation mode.
* @return previous value, or null if none
*/
final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
boolean evict) {
// Node<K,V>[] tab即是Map中的table
// Node p 为table中与hash值对应的Node
Node<K,V>[] tab; Node<K,V> p; int n, i;
// 如果table为空，初始化table
if ((tab = table) == null || (n = tab.length) == 0)
n = (tab = resize()).length;
// 以i = (n - 1) & hash值为index从table中取出Node，赋值给p
// 如果p为空，则以hash、key、value等入参new出Node对象，赋值给table[i]
if ((p = tab[i = (n - 1) & hash]) == null)
tab[i] = newNode(hash, key, value, null);
else {
// Node e可以理解为Map中key与入参中的key相等的Node
Node<K,V> e; K k;
// p的key与入参的key相等
if (p.hash == hash &&
((k = p.key) == key || (key != null && key.equals(k))))
e = p;
// p的key与入参的key不相等，LinkedHashMap会走下面这个分支，因为LinkedHashMap覆写了newNode()，它的Node是TreeNode
else if (p instanceof TreeNode)
e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
// p的key与入参的key不相等，HashMap会走这个分支
else {
// 遍历以p开头的链表
// 如果找到key与入参相等的节点，结束循环；
// 如果循环结束还没找到key相同的，则new一个Node放到链表的尾部。
for (int binCount = 0; ; ++binCount) {
if ((e = p.next) == null) {
p.next = newNode(hash, key, value, null);
// 当链表深度达到阈值时，将链表转化为红黑树
if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
treeifyBin(tab, hash);
break;
}
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k))))
break;
p = e;
}
}
// 如果e != null（Map中存在key与传入参数相等的Node），直接将入参的value赋值到该Node上，return旧的value
if (e != null) { // existing mapping for key
V oldValue = e.value;
if (!onlyIfAbsent || oldValue == null)
e.value = value;
afterNodeAccess(e);
return oldValue;
}
}
// 往table中新增了new出来的p会走到这里
// 根据现在的table的size判断是否要resize
++modCount;
if (++size > threshold)
resize();
afterNodeInsertion(evict);
return null;
}
为了方便理解，下面是put(key, value)执行的伪代码
put(key,value);
int hash = hash(key);
putVal(hash, key, value, ...);
if (Node[] table is Empty?) {
init table
}
int index = (n - 1) & hash;
Node p = table[index];
if (p == null) {
table[index] = new Node(hash, key, value, null);
}else {
Node e;
if (p.key equals key) {
e = p;
} else if(is LinkedHashMap?) {
e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
} else {
// literate linked list
for (int binCount = 0; true ; ++binCount) {
e = p.next;
if (e == null) {
p.next = newNode(hash, key, value, null);
if (binCount >= TREEIFY_THRESHOLD - 1){
treeifyBin(tab, hash);
}
break;
}
if (e.key equals key)
break;
p = e;
}
}
if (e != null) {
oldValue = e.value;
e.value = value;
return oldValue;
}
}
if (++size > threshold) {
resize();
}
return null;
get(key)
get过程：
- 根据key计算hash值；
- 根据hash值在table中找对应的Node：
没有，return null
有，看key是否equal，equal，返回Node的value，不equal，再遍历链表，看链表中是否有equal的node，没有返回null，有返回Node的value；
public V get(Object key) {
Node<K,V> e;
return (e = getNode(hash(key), key)) == null ? null : e.value;
}
/**
* Implements Map.get and related methods
*
* @param hash hash for key
* @param key the key
* @return the node, or null if none
*/
final Node<K,V> getNode(int hash, Object key) {
Node<K,V>[] tab; Node<K,V> first, e; int n; K k;
if ((tab = table) != null && (n = tab.length) > 0 &&
(first = tab[(n - 1) & hash]) != null) {
if (first.hash == hash && // always check first node
((k = first.key) == key || (key != null && key.equals(k))))
return first;
if ((e = first.next) != null) {
if (first instanceof TreeNode)
return ((TreeNode<K,V>)first).getTreeNode(hash, key);
do {
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k))))
return e;
} while ((e = e.next) != null);
}
}
return null;
}
LinkedHashMap原理概述
LinkedHashMap是在HashMap的基础上演化来的，主要区别在于LinkedHashMap的Node是LinkedHashMapEntry，是双向链表。
image.png
图片出处：https://yikun.github.io/2015/04/01/Java-HashMap%E5%B7%A5%E4%BD%9C%E5%8E%9F%E7%90%86%E5%8F%8A%E5%AE%9E%E7%8E%B0/
LinkedHashMap 特点
- Node是有序的，顺序由insert的顺序决定；
- 可以设置AccessOrder，最近被访问的节点会被移到链表末尾，非常适合做LRUCache；
* This implementation differs from
* <tt>HashMap</tt> in that it maintains a doubly-linked list running through
* all of its entries. This linked list defines the iteration ordering,
* which is normally the order in which keys were inserted into the map
* (<i>insertion-order</i>).
* <p>A special {@link #LinkedHashMap(int,float,boolean) constructor} is
* provided to create a linked hash map whose order of iteration is the order
* in which its entries were last accessed, from least-recently accessed to
* most-recently (<i>access-order</i>). This kind of map is well-suited to
* building LRU caches. Invoking the {@code put}, {@code putIfAbsent},
* {@code get}, {@code getOrDefault}, {@code compute}, {@code computeIfAbsent},
* {@code computeIfPresent}, or {@code merge} methods results
* in an access to the corresponding entry (assuming it exists after the
* invocation completes).
LinkedHashMap 源码分析
image.png
从类图可以看出，LinkedHashMap与HashMap相比，区别主要在于：
- Node是LinkedHashMapEntry（双向链表结构）；
- 多了3个field：head，tail，accessOrder。
- 重写了HashMap的三个空实现方法：afterNodeAccess、afterNodeInsertion、afterNodeRemoval；
当访问过某个节点后，该节点会被放置到链表的末尾。
void afterNodeAccess(Node<K,V> e) { // move node to last
LinkedHashMapEntry<K,V> last;
if (accessOrder && (last = tail) != e) {
LinkedHashMapEntry<K,V> p =
(LinkedHashMapEntry<K,V>)e, b = p.before, a = p.after;
p.after = null;
if (b == null)
head = a;
else
b.after = a;
if (a != null)
a.before = b;
else
last = b;
if (last == null)
head = p;
else {
p.before = last;
last.after = p;
}
tail = p;
++modCount;
}
}
参考资料：
Java HashMap工作原理及实现 by 杨文杰
Java HashMap工作原理及实现 by Yikun
Java LinkedHashMap工作原理及实现
深入分析hashmap
