final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
               boolean evict) {
    Node<K,V>[] tab; Node<K,V> p; int n, i;
    //table未初始化或者長度為0，進(jìn)行擴(kuò)容
    if ((tab = table) == null || (n = tab.length) == 0)
        n = (tab = resize()).length;
    //(n - 1) & hash 確定元素存放在哪個桶中，桶為空，
    //新生成結(jié)點(diǎn)放入桶中(此時(shí)，這個結(jié)點(diǎn)是放在數(shù)組中)
    if ((p = tab[i = (n - 1) & hash]) == null)
        tab[i] = newNode(hash, key, value, null);
    //桶中已經(jīng)存在元素
    else {
        Node<K,V> e; K k;
        //key相等，hash相等
        if (p.hash == hash &&
            ((k = p.key) == key || (key != null && key.equals(k))))
            //將元素賦值給e
            e = p;
        //hash值不相等，即key不相等，并且該節(jié)點(diǎn)是紅黑樹節(jié)點(diǎn)
        else if (p instanceof TreeNode)
            //放入樹中
            e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
        //鏈表節(jié)點(diǎn)
        else {
            //在鏈表末尾插入節(jié)點(diǎn)
            for (int binCount = 0; ; ++binCount) {
                if ((e = p.next) == null) {
                    p.next = newNode(hash, key, value, null);
                    //節(jié)點(diǎn)數(shù)量達(dá)到閾值8，執(zhí)行treeifyBin方法
                    //此方法會根據(jù)HashMap的數(shù)組來決定是否要轉(zhuǎn)換為紅黑樹
                    //數(shù)組長度大于等于64才會轉(zhuǎn)換為紅黑樹，否則只會擴(kuò)容數(shù)組
                    if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
                        treeifyBin(tab, hash);
                    //跳出循環(huán)
                    break;
                }
                if (e.hash == hash &&
                    ((k = e.key) == key || (key != null && key.equals(k))))
                    break;
                p = e;
            }
        }
        //桶中有key值和hash值于插入節(jié)點(diǎn)相等的節(jié)點(diǎn)
        if (e != null) { // existing mapping for key
            V oldValue = e.value;
            if (!onlyIfAbsent || oldValue == null)
                e.value = value;
            afterNodeAccess(e);
            //覆蓋以后返回舊值
            return oldValue;
        }
    }
    ++modCount;
    //插入完成后實(shí)際大小大于閾值，需要擴(kuò)容
    if (++size > threshold)
        resize();
    afterNodeInsertion(evict);
    return null;
}

final Node<K,V>[] resize() {
    Node<K,V>[] oldTab = table;
    int oldCap = (oldTab == null) ? 0 : oldTab.length;
    int oldThr = threshold;
    int newCap, newThr = 0;
    //容量超過最大值就不再擴(kuò)容
    if (oldCap > 0) {
        if (oldCap >= MAXIMUM_CAPACITY) {
            threshold = Integer.MAX_VALUE;
            return oldTab;
        }
        //沒超過最大值擴(kuò)容到原來的2倍
        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) {
        //擴(kuò)容完成后，重新進(jìn)行hash分配，寫入數(shù)據(jù)
        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;
}