HashMap自动扩容机制源码详解

一、简介

• HashMap的源码我们之前解读过，数组加链表，链表过长时裂变为红黑树。自动扩容机制没细说，今天详细看一下

往期回顾：

• Java1.7的HashMap源码分析-面试必备技能

• Java1.8的HashMap源码分析-面试必备技能

二、扩容机制

先说结论：

• hashmap的容量都是2的倍数，比如2，4，8，16，32，64 ...
• 每次扩容都是扩一倍，2到4 ，4到8，8到16， 16到32 等等
• 扩容因子：默认是0.75，也可以指定一个小数
• 扩容时间点：当容器内的元素数量到达：容量*扩容因子 开始扩容

三、源码分析

（1）先看构造函数

static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16
static final float DEFAULT_LOAD_FACTOR = 0.75f;

public HashMap() {
    this.loadFactor = DEFAULT_LOAD_FACTOR; // all other fields defaulted
}

默认的构造函数指定了扩容因子：0.75, 默认容量是16

public HashMap(int initialCapacity) {
    this(initialCapacity, DEFAULT_LOAD_FACTOR);
}

指定初始容量，默认扩容因子：0.75

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);
}

同时指定初始容量和扩容因子

/**
    * The next size value at which to resize (capacity * load factor).
    *
    * @serial
    */
int threshold;

• 注意这个变量：下一个要扩容的值，扩容容量，容量*扩容因子
• 看这一句：this.threshold = tableSizeFor(initialCapacity);

/**
 * 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;
}

• 这个方法是取给定值四舍五入之后的2的倍数，比如3—->4 ,15->16, 27->32
• 至此准备工作就做好了，下面看put方法

（2）put方法

public V put(K key, V value) {
    return putVal(hash(key), key, value, false, true);
}

final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
                boolean evict) {
    Node<K,V>[] tab; Node<K,V> p; int n, i;
    // ① 最开始table为null, 调用resize（）方法
    if ((tab = table) == null || (n = tab.length) == 0)
        n = (tab = resize()).length;
    if ((p = tab[i = (n - 1) & hash]) == null)
        tab[i] = newNode(hash, key, value, null);
    else {
        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 {
            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
                        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;
    // ② 结束的时候判断容量是不是大于扩容容量，大于则调用resize方法
    if (++size > threshold)
        resize();
    afterNodeInsertion(evict);
    return null;
}

• ① 最开始table为null, 调用resize（）方法
• ② 结束的时候判断容量是不是大于扩容容量，大于则调用resize（）方法
• 看resize（）方法

final Node<K,V>[] resize() {
    Node<K,V>[] oldTab = table;
    int oldCap = (oldTab == null) ? 0 : oldTab.length;
    int oldThr = threshold;
    int newCap, newThr = 0;
    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);
    }
    if (newThr == 0) {
        float ft = (float)newCap * loadFactor;
        newThr = (newCap < MAXIMUM_CAPACITY && ft < (float)MAXIMUM_CAPACITY ?
                    (int)ft : Integer.MAX_VALUE);
    }
    threshold = newThr;
    @SuppressWarnings({"rawtypes","unchecked"})
        Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap];
    table = newTab;
    if (oldTab != null) {
        for (int j = 0; j < oldCap; ++j) {
            Node<K,V> e;
            if ((e = oldTab[j]) != null) {
                oldTab[j] = null;
                if (e.next == null)
                    newTab[e.hash & (newCap - 1)] = e;
                else if (e instanceof TreeNode)
                    ((TreeNode<K,V>)e).split(this, newTab, j, oldCap);
                else { // preserve order
                    Node<K,V> loHead = null, loTail = null;
                    Node<K,V> hiHead = null, hiTail = null;
                    Node<K,V> next;
                    do {
                        next = e.next;
                        if ((e.hash & oldCap) == 0) {
                            if (loTail == null)
                                loHead = e;
                            else
                                loTail.next = e;
                            loTail = e;
                        }
                        else {
                            if (hiTail == null)
                                hiHead = e;
                            else
                                hiTail.next = e;
                            hiTail = e;
                        }
                    } while ((e = next) != null);
                    if (loTail != null) {
                        loTail.next = null;
                        newTab[j] = loHead;
                    }
                    if (hiTail != null) {
                        hiTail.next = null;
                        newTab[j + oldCap] = hiHead;
                    }
                }
            }
        }
    }
    return newTab;
}

• 先分析第一种情况：Map map = new HashMap();
• 走最后一个分支 , 容量为16，扩容容量为12

else {
    newCap = DEFAULT_INITIAL_CAPACITY;
    newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY);
}

• 分析第二种情况：Map map = new HashMap(20);
• 走第二个分支，前面分析过，threshold = tableSizeFor(20) 为 32
• 新容量newcap = oldThr 为32

// 容量
else if (oldThr > 0) // initial capacity was placed in threshold
        newCap = oldThr;

• 新扩容容量newThr = newCap * loadFactor 为 24

// 扩容容量
if (newThr == 0) {
    float ft = (float)newCap * loadFactor;
    newThr = (newCap < MAXIMUM_CAPACITY && ft < (float)MAXIMUM_CAPACITY ?
                (int)ft : Integer.MAX_VALUE);
}
threshold = newThr;

• 分析第三种情况：上面的map已经插入24个元素，新插入一个要扩容
• 走第一个分支，oldCap=32，oldThr=24
• 扩容：newCap = oldCap << 1 为64
• 扩扩容容量newThr = oldThr << 1 为48

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
}

• 最后是复制元素到新的table
• 单个元素直接复制
• 如果是树，调用树的复制方法
• 如果是链表，循环链表复制

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