final V putVal(K key, V value, boolean onlyIfAbsent) {
    if (key == null || value == null) throw new NullPointerException();
    int hash = spread(key.hashCode());  //hash (h ^ (h >>> 16)) & HASH_BITS;减少hash冲突，保证均匀分布
    int binCount = 0; 	//统计节点长度
    for (Node<K,V>[] tab = table;;) {   //CAS自旋
        Node<K,V> f; int n, i, fh;
        if (tab == null || (n = tab.length) == 0)  //数组为空，则初始化
            tab = initTable();
        //(n - 1) & hash 取模运算得到元素的位置，tabAt利用Unsafe类得到当前元素是否有数据
        else if ((f = tabAt(tab, i = (n - 1) & hash)) == null) {
            //如果没有，利用CAS新增数据
            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 { // f != null 即数组当前节点有值
            V oldVal = null;
            //synchronized 加锁 使锁的力度细化到数组当前节点
            synchronized (f) {
                if (tabAt(tab, i) == f) {  //已经加锁，为什么还要判断？
                    if (fh >= 0) {   //fh = f.hash
                        binCount = 1;
                        //处理链表
                        for (Node<K,V> e = f;; ++binCount) {
                            K ek;
                            //put数组与当前节点数据相同 则根据onlyIfAbsent判断覆盖
                            if (e.hash == hash &&
                                ((ek = e.key) == key ||
                                 (ek != null && key.equals(ek)))) {
                                oldVal = e.val;
                                if (!onlyIfAbsent) //覆盖
                                    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;
                            }
                        }
                    }
                    //f是红黑树节点
                    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) {
                //节点长度 >= 8， 转红黑树
                if (binCount >= TREEIFY_THRESHOLD)
                    treeifyBin(tab, i);
                if (oldVal != null)
                    return oldVal;
                break;
            }
        }
    }
    addCount(1L, binCount);
    return null;
}

@Target({ ElementType.METHOD })
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface TxManagerAnnotation {

    String db() default "";

    int level() default Connection.TRANSACTION_REPEATABLE_READ;

}