Java HashMap，LinkedHashMap 实现原理分

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：

1. 根据key计算hash值；
2. 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：

1. 根据key计算hash值；
2. 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

1. Node是有序的，顺序由insert的顺序决定；
2. 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：

1. Node是LinkedHashMapEntry（双向链表结构）；
2. 多了3个field：head，tail，accessOrder。
3. 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