关于struts2 动态action 的问题

下面是我的struts2.xml的配置，有一个action ：

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE struts PUBLIC
"-//Apache Software Foundation//DTD Struts Configuration 2.3//EN"
"http://struts.

<struts>
    <constant name="struts.enable.DynamicMethodInvocation" value="false" />
    <constant name="struts.devMode" value="true" />

<package name="default" namespace="/" extends="struts-default">

        <action name="first" class="FirstAction" >
            <result>
                /Hello.jsp
            </result>
            <result name="add">
                /add.jsp
            </result>
            <result name="update">
                /update.jsp
            </result>
        </action>

    </package>
   
</struts>

然后是我定义的action 类：
import com.opensymphony.xwork2.ActionSupport;
public class FirstAction extends ActionSupport {
        @Override
        public String execute() throws Exception {
             return SUCCESS;
        }
   
        public String update() throws Exception {
                return "update";
        }
   
        public String add() throws Exception {
             return "add";
        }

}

问题来了，我在地址栏敲入http://localhost:8080/struts_test_1.0/first 的时候能够正常打开Hello.jsp页面，但是我敲入http://localhost:8080/struts_test_1.0/first!add 或者敲入者http://localhost:8080/struts_test_1.0/first!update 的时候就出错了，提醒如下：

There is no Action mapped for namespace [/] and action name [first!add] associated with context path [/struts_test_1.0].

   
为什么会这样？怎么解决？？求助