Java中HashMap扩容机制

前言

jdk1.8中源码位于rt.jar/java/util/HashMap.java

机制

初始值设置从源码可以看出，初始容量16，装载因子0.75，碰撞的结果初始以链表形式存储，当链表长度大于8时，调整为红黑树存储，保证最坏情况下查询、插入、删除的效率[wiki]。

    /**
     * The default initial capacity - MUST be a power of two.
     */
    static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16

    /**
     * The maximum capacity, used if a higher value is implicitly specified
     * by either of the constructors with arguments.
     * MUST be a power of two <= 1<<30.
     */
    static final int MAXIMUM_CAPACITY = 1 << 30;

    /**
     * The load factor used when none specified in constructor.
     */
    static final float DEFAULT_LOAD_FACTOR = 0.75f;

    /**
     * The bin count threshold for using a tree rather than list for a
     * bin.  Bins are converted to trees when adding an element to a
     * bin with at least this many nodes. The value must be greater
     * than 2 and should be at least 8 to mesh with assumptions in
     * tree removal about conversion back to plain bins upon
     * shrinkage.
     */
    static final int TREEIFY_THRESHOLD = 8;

扩容问题肯定在新增数据时出现，直接看putVal()方法。(据说1.8之后链表长度大于8不会先转换红黑树，看数组长度是否小于（？）64，优先扩容。这个边界条件要再查查！)

    final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
                   boolean evict) {
        Node<K,V>[] tab; Node<K,V> p; int n, i;
        if ((tab = table) == null || (n = tab.length) == 0)
            n = (tab = resize()).length;  // 初始容量设置过程
        if ((p = tab[i = (n - 1) & hash]) == null)
            tab[i] = newNode(hash, key, value, null); // 没有碰撞，链表节点形式存储
        else { // 已存在，更新
            Node<K,V> e; K k;
            if (p.hash == hash &&
                ((k = p.key) == key || (key != null && key.equals(k))))
                e = p;
            else if (p instanceof TreeNode)
                e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
            else { // 已经出现过碰撞情况。
                for (int binCount = 0; ; ++binCount) {
                    if ((e = p.next) == null) {
                        p.next = newNode(hash, key, value, null);
                        if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st							// 碰撞过多，链表长度超过8，转换成红黑树
                            treeifyBin(tab, hash);
                        break;
                    }
                    if (e.hash == hash &&
                        ((k = e.key) == key || (key != null && key.equals(k))))
                        break;
                    p = e;
                }
            }
            if (e != null) { // existing mapping for key
                V oldValue = e.value;
                if (!onlyIfAbsent || oldValue == null)
                    e.value = value;
                afterNodeAccess(e);
                return oldValue;
            }
        }
        ++modCount;
        if (++size > threshold)
            resize();
        afterNodeInsertion(evict);
        return null;
    }

resize()函数也怪长的，摘取容量扩张的部分

	if (oldCap > 0) {
          if (oldCap >= MAXIMUM_CAPACITY) {
              threshold = Integer.MAX_VALUE;
              return oldTab;
          }
          else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
                   oldCap >= DEFAULT_INITIAL_CAPACITY)
              newThr = oldThr << 1; // double threshold 
      }
      else if (oldThr > 0) // initial capacity was placed in threshold
          newCap = oldThr;
      else {               // zero initial threshold signifies using defaults	
          newCap = DEFAULT_INITIAL_CAPACITY;
          newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY);
      }

可以看出除了初始容量配置其余容量扩展都是double加倍。初始值除了可以是默认的16以外，还可以自己通过构造函数设置。

     public HashMap(int initialCapacity, float loadFactor) {
        if (initialCapacity < 0)
            throw new IllegalArgumentException("Illegal initial capacity: " +
                                               initialCapacity);
        if (initialCapacity > MAXIMUM_CAPACITY)
            initialCapacity = MAXIMUM_CAPACITY;
        if (loadFactor <= 0 || Float.isNaN(loadFactor))
            throw new IllegalArgumentException("Illegal load factor: " +
                                               loadFactor);
        this.loadFactor = loadFactor;
        this.threshold = tableSizeFor(initialCapacity);
    }

但是注意到自己设置的容量会被tableSizeFor()方法调整成为2的幂。

    /**
     * Returns a power of two size for the given target capacity.
     */
    static final int tableSizeFor(int cap) {
        int n = cap - 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;
    }

所以HashMap容量始终是2的幂。

2的幂?

散列值是不能直接拿来用的。用之前还要先做对数组的长度取模运算，得到的余数才能用来要存放的位置也就是对应的数组下标。这个数组下标的计算方法是“ (n - 1) & hash”。“取余(%)操作中如果除数是2的幂次则等价于与其除数减一的与(&)操作（也就是说 hash%length==hash&(length-1)的前提是 length 是2的 n 次方；）。” 并且 采用二进制位操作 &，相对于%能够提高运算效率，这就解释了 HashMap 的长度为什么是2的幂次方。[JavaGuide]

总结

HashMap容量必然是2的幂，初始默认容量16，容量扩展时直接加倍。