putVal()

final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
                   boolean evict) {
    Node<K,V>[] tab; Node<K,V> p; int n, i;
    Node<K,V>[] tab; Node<K,V> p; int n, i;
    if ((tab = table) == null || (n = tab.length) == 0)
        n = (tab = resize()).length;
    // 下面的判断中巧妙地使用位运算，避免的超过数组的最大值
    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) {
                // 找到链表的最后也没匹配上
                if ((e = p.next) == null) {
                    p.next = newNode(hash, key, value, null);
                    if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
                        // 如果数组对应的链表长度到达TREEIFY_THRESHOLD -1，
                        // 转为红黑树，TREEIFY_THRESHOLD（默认为8）
                        treeifyBin(tab, hash);
                    // 注意这里不会对e赋值，只有针对节点已存在的情况e才有值。
                    break;
                }
                // 发现已存在该节点，退出循环
                if (e.hash == hash &&
                    ((k = e.key) == key || (key != null && key.equals(k))))
                    break;
                p = e;
            }
        }
        // e不为null，说明之前就存在这个Node
        if (e != null) { // existing mapping for key
          V oldValue = e.value;
          if (!onlyIfAbsent || oldValue == null)
            // 已存在，不是空缺才赋值的话，给e的value赋新值
            e.value = value;
          // hashMap的这个方法为空实现，实际上不做处理
          afterNodeAccess(e);
          return oldValue;
        }
    } 
    // 记录节点增减次数
    ++modCount;
    // 如果map内数量超过阈值，需要扩容
    // 如果只是替换value，不会走到这一步，
    // 所以size的变更肯定是因为新节点的插入
    if (++size > threshold)
      resize();
    // hashMap的这个方法为空实现，实际上不做处理
    afterNodeInsertion(evict);
    return null;
}

resize()

/**
 * Initializes or doubles table size.  If null, allocates in
 * accord with initial capacity target held in field threshold.
 * Otherwise, because we are using power-of-two expansion, the
 * elements from each bin must either stay at same index, or move
 * with a power of two offset in the new table.
 *
 * @return the table
 */
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) {
            // 已经达到最大容量，忽略负载因子
                // 放开预置到最大值 size为int 最大值只能到 Integer.MAX_VALUE
            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
        // 初始化
        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)
                    // 拆开红黑树（当节点过少时也会调用）
                    ((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;
                       // cap如果是2的幂次，原有节点会被分成相对平均的两部分
                       // 分别放在table的前半段和后半段
                        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);
                    // 原有节点会被分成相对平均的两部分
                    // 分别放在table的前半段和后半段
                    if (loTail != null) {
                        loTail.next = null;
                        newTab[j] = loHead;
                    }
                    if (hiTail != null) {
                        hiTail.next = null;
                        newTab[j + oldCap] = hiHead;
                    }
                }
            }
        }
    }
    return newTab;
}