src/org/nutz/lang/util/NutMap.java
@Override
    public NutMap setnx(String key, Object val) {
        if (!containsKey(key))
            setv(key, val);
        return this;
    }
@Override
    public NutMap setnx(String key, Object val) {
        if (!containsKey(key))
            setv(key, val);
        return this;
    }
与JDK8+的 putIfAbsent(key, val)一致, 当且仅当值不存在时设置进去,但与putIfAbsent返回值有不一样

@param key
键
@param val
值
@return 当前的NutMap实例