java HashMap源码分析(JDK8)

  这两天在复习JAVA的知识点,想更深层次的了解一下JAVA,所以就看了看JAVA的源码,把自己的分析写在这里,也当做是笔记吧,方便记忆。写的不对的地方也请大家多多指教。

  JDK1.6中HashMap采用的是位桶+链表的方式,即我们常说的散列链表的方式,而JDK1.8中采用的是位桶+链表/红黑树的方式,也是非线程安全的。当某个位桶的链表的长度达到某个阀值的时候,这个链表就将转换成红黑树。

基本的数据结构:

 1 //链表节点
 2 static class Node<K,V> implements Map.Entry<K,V> {
 3         final int hash;
 4         final K key;
 5         V value;
 6         Node<K,V> next;
 7      //省略 
 8 }
 9 //红黑树节点
10 static final class TreeNode<K,V> extends LinkedHashMap.Entry<K,V> {
11         TreeNode<K,V> parent;  // red-black tree links
12         TreeNode<K,V> left;
13         TreeNode<K,V> right;
14         TreeNode<K,V> prev;    // needed to unlink next upon deletion
15         boolean red;
16         TreeNode(int hash, K key, V val, Node<K,V> next) {
17             super(hash, key, val, next);
18         }
19         //省略  
20 }
21 // HashMap的主要属性
22 public class HashMap<K,V> extends AbstractMap<K,V>
23     implements Map<K,V>, Cloneable, Serializable {
24     // 槽数组,Node<K,V>类型,TreeNode extends LinkedHashMap.Entry<K,V>,所以可以存放TreeNode来实现Tree bins
25     transient Node<K,V>[] table;
26     
27     transient Set<Map.Entry<K,V>> entrySet;
28 
29     transient int size;
30     // 去掉了volatile的修饰符
31     transient int modCount;
32 
33     int threshold;
34 
35     final float loadFactor;
36 
37     ...
38 
39 }
   

 

//计算key的hash
static final int hash(Object key) {
        int h;
        return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
 }

 get(key) 函数

 1 public V get(Object key) {
 2         Node<K,V> e;
 3         return (e = getNode(hash(key), key)) == null ? null : e.value;
 4     }
 5      final Node<K,V> getNode(int hash, Object key) {
 6             Node<K,V>[] tab; 
 7             Node<K,V> first, e; 
 8             int n; K k;
 9             //hash & length-1 定位数组下标
10             if ((tab = table) != null && (n = tab.length) > 0 &&
11                 (first = tab[(n - 1) & hash]) != null) 
12             {
13                 if (first.hash == hash && // always check first node
14                     ((k = first.key) == key || (key != null && key.equals(k))))
15                     return first;
16                 if ((e = first.next) != null) {
17                     /*第一个节点是TreeNode,则采用位桶+红黑树结构,
18                      * 调用TreeNode.getTreeNode(hash,key),
19                      *遍历红黑树,得到节点的value
20                      */
21                     if (first instanceof TreeNode)
22                         return ((TreeNode<K,V>)first).getTreeNode(hash, key);
23                     do {
24                         if (e.hash == hash &&
25                             ((k = e.key) == key || (key != null && key.equals(k))))
26                             return e;
27                        } while ((e = e.next) != null);
28                 }
29             }
30             return null;
31         }
32      final TreeNode<K,V> getTreeNode(int h, Object k) {
33              //找到红黑树的根节点并遍历红黑树
34          return ((parent != null) ? root() : this).find(h, k, null);
35      }
36      /*
37       *通过hash值的比较,递归的去遍历红黑树,这里要提的是compareableClassFor(Class k)这个函数的作用,在某些时候
38       *如果红黑树节点的元素are of the same "class C implements Comparable<C>" type 
39       *利用他们的compareTo()方法来比较大小,这里需要通过反射机制来check他们到底是不是属于同一个类,是不是具有可比较性.
40       */
41      final TreeNode<K,V> find(int h, Object k, Class<?> kc) {
42          TreeNode<K,V> p = this;
43          do {
44              int ph, dir; K pk;
45              TreeNode<K,V> pl = p.left, pr = p.right, q;
46              if ((ph = p.hash) > h)
47                  p = pl;
48              else if (ph < h)
49                  p = pr;
50              else if ((pk = p.key) == k || (k != null && k.equals(pk)))
51                  return p;
52              else if (pl == null)
53                  p = pr;
54              else if (pr == null)
55                  p = pl;
56              else if ((kc != null ||
57                        (kc = comparableClassFor(k)) != null) &&
58                       (dir = compareComparables(kc, k, pk)) != 0)
59                  p = (dir < 0) ? pl : pr;
60              else if ((q = pr.find(h, k, kc)) != null)
61                  return q;
62              else
63                  p = pl;
64          } while (p != null);
65          return null;
66      }      

put(K key,V value)函数 

 1 //put(K key,V value)函数 
 2     public V put(K key, V value) {
 3             return putVal(hash(key), key, value, false, true);
 4         }
 5     
 6     final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
 7             boolean evict) {
 8          Node<K,V>[] tab; 
 9          Node<K,V> p; 
10          int n, i;
11          //如果table为空或者长度为0,则resize()
12          if ((tab = table) == null || (n = tab.length) == 0)
13              n = (tab = resize()).length;
14          //找到key值对应的槽并且是第一个,直接加入
15          if ((p = tab[i = (n - 1) & hash]) == null)
16              tab[i] = newNode(hash, key, value, null);
17          else {
18                  Node<K,V> e;
19                  K k;
20                  //第一个node的hash值即为要加入元素的hash
21                  if (p.hash == hash &&
22                      ((k = p.key) == key || (key != null && key.equals(k)))){
23                       e = p;
24                  }else if (p instanceof TreeNode)//第一个节点是TreeNode,即tree-bin
25                     /*Tree version of putVal.
26                      *final TreeNode<K,V> putTreeVal(HashMap<K,V> map, Node<K,V>[] tab,int h, K k, V v)
27                      */
28                      e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
29                      else {
30                          //不是TreeNode,即为链表,遍历链表
31                          for (int binCount = 0; ; ++binCount) {
32                              /*到达链表的尾端也没有找到key值相同的节点,
33                               *则生成一个新的Node,并且判断链表的节点个数是不是到达转换成红黑树的上界
34                               *达到,则转换成红黑树
35                               */
36                              if ((e = p.next) == null) {
37                                  p.next = newNode(hash, key, value, null);
38                                  if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
39                                      treeifyBin(tab, hash);
40                                  break;
41                              }
42                              if (e.hash == hash &&
43                                  ((k = e.key) == key || (key != null && key.equals(k))))
44                                  break;
45                              p = e;
46                          }
47                      }
48                  if (e != null) { // existing mapping for key
49                      V oldValue = e.value;
50                      if (!onlyIfAbsent || oldValue == null)
51                          e.value = value;
52                      afterNodeAccess(e);
53                      //返回旧的value值
54                      return oldValue;
55                  }
56          }
57          ++modCount;
58          if (++size > threshold)
59              resize();
60          afterNodeInsertion(evict);
61          return null;
62 }

 

    原文作者:hfczgo
    原文地址: http://www.cnblogs.com/hfczgo/p/4033283.html
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