JDK 8 HashMap 扩容算法研究

Posted 2021-09-03 子瞻

final Node<K,V>[] resize() {
    Node<K,V>[] oldTab = table;
    int oldCap = (oldTab == null) ? 0 : oldTab.length;
    int oldThr = threshold;
    int newCap, newThr = 0;
    //扩容之前数组不为空
    if (oldCap > 0) {
        //容量超过最大值
        if (oldCap >= MAXIMUM_CAPACITY) {
            threshold = Integer.MAX_VALUE;
            return oldTab;
        }
        //double之后的新容量小于2的30次方且old容量大于等于16
        else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
                 oldCap >= DEFAULT_INITIAL_CAPACITY)
            // 新的threshold double
            newThr = oldThr << 1; 
    }
    //如果old thereshold 大于 0
    else if (oldThr > 0) 
        //旧的threshold 赋给 新的容量
        newCap = oldThr;
    else {//进行初始化容量和阈值赋值               
        newCap = DEFAULT_INITIAL_CAPACITY;
        newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY);
    }
    if (newThr == 0) {//如果old容量为0，
        //新的阈值计算公式
        float ft = (float)newCap * loadFactor;
        //赋值
        newThr = (newCap < MAXIMUM_CAPACITY && ft < (float)MAXIMUM_CAPACITY ?
                  (int)ft : Integer.MAX_VALUE);
    }
    //新的阈值复制到当前hashMap实例中
    threshold = newThr;
    @SuppressWarnings({"rawtypes","unchecked"})
    Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap];
    table = newTab;
    if (oldTab != null) {
        //遍历old数组
        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;
                        /**
                        设计很巧妙，整个扩容之后的数组分为两部分：
                                 低位      高位
                        newCap = oldCap+扩容之后的Cap
                        **/
                        //判断hash命中低位分布或者高位部分
                        if ((e.hash & oldCap) == 0) {//命中低位部分
                            if (loTail == null)
                                //第一次遍历对低位部分头节点进行赋值
                                loHead = e;
                            else
                                //当前节点放在链表最后
                                loTail.next = e;
                            //尾节点指针后移    
                            loTail = e;
                        }
                        //命中高位部分
                        else {
                            if (hiTail == null)
                                //第一次遍历对低位部分头节点进行赋值
                                hiHead = e;
                            else
                                //当前节点放在链表最后
                                hiTail.next = e;
                            hiTail = e;
                        }
                    } while ((e = next) != null);
                    if (loTail != null) {
                        loTail.next = null;
                        //将低位部分链表头节点赋值在newTab[j]
                        newTab[j] = loHead;
                    }
                    if (hiTail != null) {
                        hiTail.next = null;
                        //将高位部分链表头节点赋值在newTab[j+oldCap]
                        newTab[j + oldCap] = hiHead;
                    }
                }
            }
        }
    }
    return newTab;
}