put方法

final V putVal(K key, V value, boolean onlyIfAbsent) {
//如果key与value为空，直接抛出异常
    if (key == null || value == null) throw new NullPointerException();
    int hash = spread(key.hashCode());
    int binCount = 0;
    for (Node<K,V>[] tab = table;;) {
        Node<K,V> f; int n, i, fh;
        if (tab == null || (n = tab.length) == 0)
            tab = initTable();
            //如果要插入的元素所在的桶还没有元素，则把这个元素插入到桶中
        else if ((f = tabAt(tab, i = (n - 1) & hash)) == null) {
        //如果cas放入桶中成功则跳出循环，否则进入下一次循环
            if (casTabAt(tab, i, null,
                         new Node<K,V>(hash, key, value, null)))
                break;                   // no lock when adding to empty bin
        }
        else if ((fh = f.hash) == MOVED)
            tab = helpTransfer(tab, f);
        else {
            V oldVal = null;
            //利用分段锁的思想锁住桶
            synchronized (f) {
                if (tabAt(tab, i) == f) {
                //fh>0表示利用链表的方式存储
                    if (fh >= 0) {
                        binCount = 1;
                        for (Node<K,V> e = f;; ++binCount) {
                            K ek;
                            //判断要添加的元素是否存在，存在则替换，不存在则新增
                            if (e.hash == hash &&
                                ((ek = e.key) == key ||
                                 (ek != null && key.equals(ek)))) {
                                oldVal = e.val;
                                if (!onlyIfAbsent)
                                    e.val = value;
                                break;
                            }
                            Node<K,V> pred = e;
                            if ((e = e.next) == null) {
                                pred.next = new Node<K,V>(hash, key,
                                                          value, null);
                                break;
                            }
                        }
                    }
                    else if (f instanceof TreeBin) {
                        Node<K,V> p;
                        binCount = 2;
                        if ((p = ((TreeBin<K,V>)f).putTreeVal(hash, key,
                                                       value)) != null) {
                            oldVal = p.val;
                            if (!onlyIfAbsent)
                                p.val = value;
                        }
                    }
                }
            }
            if (binCount != 0) {
            //如果某个桶中的元素大于8个，则尝试树化
                if (binCount >= TREEIFY_THRESHOLD)
                    treeifyBin(tab, i);
                if (oldVal != null)
                    return oldVal;
                break;
            }
        }
    }
    //元素数量加1
    addCount(1L, binCount);
    return null;
}
//初次放入元素，进行初始化table
private final Node<K,V>[] initTable() {
    Node<K,V>[] tab; int sc;
    while ((tab = table) == null || tab.length == 0) {
        if ((sc = sizeCtl) < 0)
        //执行yield让出cpu资源
            Thread.yield(); // lost initialization race; just spin
            //如果把sizeCtl原子更新为-1成功，则当前线程进入初始化
        else if (U.compareAndSwapInt(this, SIZECTL, sc, -1)) {
            try {
            //再次检查，双重检验机制
                if ((tab = table) == null || tab.length == 0) {
                //默认创建的数组大小为16
                    int n = (sc > 0) ? sc : DEFAULT_CAPACITY;
                    @SuppressWarnings("unchecked")
                    Node<K,V>[] nt = (Node<K,V>[])new Node<?,?>[n];
                    table = tab = nt;
                    //设置扩容因子为0.75n
                    sc = n - (n >>> 2);
                }
            } finally {
            //将扩容门槛赋值为sizeCtl
                sizeCtl = sc;
            }
            break;
        }
    }
    return tab;
}
```
~~~