HashMap源码学习——容量与扩容resize

1.相关常量

    static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; //默认初始容量：初始桶数。
    static final int MAXIMUM_CAPACITY = 1 << 30;//最大容量：最大桶数。
    static final float DEFAULT_LOAD_FACTOR = 0.75f;//负载因子：当箱子数达到这个值时，开始扩容。

2.HashMap提供了四个构造函数，不同的构造函数分析初始化容量

（一）HashMap()

   public HashMap() {
        this.loadFactor = DEFAULT_LOAD_FACTOR; //设置负载因子为默认值 0.75
    }

初始化容量为0，负载因子为0.75

final V putVal(int hash, K key, V value, boolean onlyIfAbsent, boolean evict) {   
    Node[] tab; Node p; int n, i;
    if ((tab = table) == null || (n = tab.length) == 0)
        n = (tab = resize()).length;
    ...
}

当第一次put的时候，桶数组table为null,开始resize扩容。

final Node[] resize() {
        Node[] oldTab = table;
        int oldCap = (oldTab == null) ? 0 : oldTab.length;
        int oldThr = threshold;
        int newCap, newThr = 0;
        ...
        else {               // zero initial threshold signifies using defaults
            newCap = DEFAULT_INITIAL_CAPACITY;
            newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY);
        }
        ...
        threshold = newThr;
    ...
}

oldCap = 0; oldThr = 0; 进入else分支，执行完之后newCap = 16；newThr = 12；第一次扩容后桶容量为16，阈值为12。

（二）HashMap(int initialCapacity, float loadFactor)

    public HashMap(int initialCapacity, float loadFactor) {
        //桶容量不能小于0
        if (initialCapacity < 0)
            throw new IllegalArgumentException("Illegal initial capacity: " +
                    initialCapacity);
        //桶容量不能大于最大桶容量
        if (initialCapacity > MAXIMUM_CAPACITY)
            initialCapacity = MAXIMUM_CAPACITY;
        //判断加载因子是否符合规范  大于0的浮点数
        if (loadFactor <= 0 || Float.isNaN(loadFactor))
            throw new IllegalArgumentException("Illegal load factor: " +
                    loadFactor);
        this.loadFactor = loadFactor;
        this.threshold = tableSizeFor(initialCapacity);
    }

此构造函数就是给定初始化桶容量和加载因子，通过tableSizeFor方法计算阈值。

    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;
    }

获取第一个比入参cap大的2的幂指数。例如输入3返回4，输入10返回16。当第一次put的时候，桶数组table仍然为null,开始resize扩容。

    final Node[] resize() {
        Node[] oldTab = table;
        int oldCap = (oldTab == null) ? 0 : oldTab.length;//为0
        int oldThr = threshold;//经过tableSizeFor方法threshold最小为1
        int newCap, newThr = 0;
        ...
        else if (oldThr > 0) {
            newCap = oldThr;
        }
        ...
        if (newThr == 0) {//根据加载因子计算最新阈值
            float ft = (float)newCap * loadFactor;
            newThr = (newCap < MAXIMUM_CAPACITY && ft < (float)MAXIMUM_CAPACITY ?
                    (int)ft : Integer.MAX_VALUE);
        }
        threshold = newThr;
    ...
}
            

（三）HashMap(int initialCapacity)

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

就是用默认加载因子调用HashMap(int initialCapacity, float loadFactor)

（四）HashMap(Map m)

 public HashMap(Map m) {
        this.loadFactor = DEFAULT_LOAD_FACTOR;
        putMapEntries(m, false);
    }

给定默认构造因子，将原有map放入到HashMap中，调用putMapEntries方法

    final void putMapEntries(Map m, boolean evict) {
        int s = m.size();
        if (s > 0) {
            //如果桶数组为空，根据m的长度来初始化阈值
            if (table == null) { // pre-size
                float ft = ((float)s / loadFactor) + 1.0F;
                int t = ((ft < (float)MAXIMUM_CAPACITY) ?
                        (int)ft : MAXIMUM_CAPACITY);
                if (t > threshold)
                    threshold = tableSizeFor(t);
            }
            //m长度超过阈值，进行扩容
            else if (s > threshold)
                resize();
            //将m中元素放入HashMap中
            for (Map.Entry e : m.entrySet()) {
                K key = e.getKey();
                V value = e.getValue();
                putVal(hash(key), key, value, false, evict);
            }
        }
    }

3.非初始化容量扩充

(一)桶数量未到极值

    final Node[] resize() {
        Node[] oldTab = table;
        int oldCap = (oldTab == null) ? 0 : oldTab.length;
        int oldThr = threshold;
        int newCap, newThr = 0;
        if (oldCap > 0) {
            ...
            else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
                    oldCap >= DEFAULT_INITIAL_CAPACITY)
                newThr = oldThr << 1; // double threshold
        }
    ...
    }

桶数量未到16或者桶数量翻倍后大于极值，桶数量翻倍，然后还要走下面代码

final Node[] resize() {
    ...
    if (newThr == 0) {
        float ft = (float)newCap * loadFactor;
        newThr = (newCap < MAXIMUM_CAPACITY && ft < (float)MAXIMUM_CAPACITY ?
            (int)ft : Integer.MAX_VALUE);
    }
    ...
}

根据最新桶数量和加载因子计算阈值，如果阈值或最新桶数量大于极值，则阈值为最大Integer值。

（二）桶数量超过极值

    final Node[] resize() {
        Node[] 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;
            }
            ...
        }
    ...
    }

阈值为最大Integer值。