HashMap1.8源码学习之：resize()

一、流程

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) {//对应数组扩容
            if (oldCap >= MAXIMUM_CAPACITY) {
                threshold = Integer.MAX_VALUE;
                return oldTab;
            }
            //将数组扩大一倍
            else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
                     oldCap >= DEFAULT_INITIAL_CAPACITY)
            //将阈值扩大一倍
                newThr = oldThr << 1; // double threshold
        }
        //对应使用new HashMap(int initialCapacity)初始化后，第一次put的时候
        else if (oldThr > 0) // initial capacity was placed in threshold
            newCap = oldThr;
          //对应使用new HashMap()初始化后，第一次put的时候
        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"})
        //用新的数组大小初始化新的数组
        Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap];
        table = newTab;//如果是初始化数组，到这里就结束了，返回newTab即可
        if (oldTab != null) {
            //开始遍历原数组，进行数据迁移
            for (int j = 0; j < oldCap; ++j) {
                Node<K,V> e;
                if ((e = oldTab[j]) != null) {
                    oldTab[j] = null;
                    //如果该数组位置上只有单个元素，直接进行迁移
                    if (e.next == null)
                        newTab[e.hash & (newCap - 1)] = e;
                    //如果是红黑树，略 
                    else if (e instanceof TreeNode)
                        ((TreeNode<K,V>)e).split(this, newTab, j, oldCap);
                    //处理链表的情况：
                    //将链表拆分成两个链表，放到新的数组中
                    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;
                            //如果旧的容量和原有容量与运算为0，则原链表不动
                            if ((e.hash & oldCap) == 0) {
                                if (loTail == null)
                                    loHead = e;
                                else
                                    loTail.next = e;
                                loTail = e;
                            }
                            //如果旧的容量和原有容量与运算不为0，则新链表移动到oldCap+j的位置
                            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;

二、常见问题
1.多线程中发生的死循环问题