初学struts，按照书上app02a写的程序，不知道为什么会报错。

1.web.xml

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns="http://xmlns.jcp.org/xml/ns/javaee"
    xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_3_1.xsd"
    id="WebApp_ID" version="3.1">
    <display-name>struts</display-name>
    <welcome-file-list>
        <welcome-file>index.html</welcome-file>
        <welcome-file>index.htm</welcome-file>
        <welcome-file>index.jsp</welcome-file>
        <welcome-file>default.html</welcome-file>
        <welcome-file>default.htm</welcome-file>
        <welcome-file>default.jsp</welcome-file>
    </welcome-file-list>

    <filter>
        <filter-name>struts2</filter-name>
        <filter-class>org.apache.struts2.dispatcher.ng.filter.StrutsPrepareFilter</filter-class>
    </filter>

    <filter-mapping>
        <filter-name>struts2</filter-name>
        <url-pattern>/*</url-pattern>
    </filter-mapping>
    <security-constraint>
        <web-resource-collection>
            <web-resource-name>JSPs</web-resource-name>
            <url-pattern>/jsp/*</url-pattern>
        </web-resource-collection>
        <auth-constraint></auth-constraint>
    </security-constraint>
    <login-config>
        <auth-method>BASIC</auth-method>
    </login-config>
</web-app>

2.struts.xml

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

<struts>

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

    <package name="com.imjian.struts" namespace="/" extends="struts-default">
        <action name="Product_input">
            <result>/jsp/ProductForm.jsp</result>
        </action>
        <action name="Product_save" class="com.imjian.struts.Product">
            <result>/jsp/ProductDetails.jsp</result>
        </action>
    </package>
</struts>

3.Product.java

package com.imjian.struts;

import java.io.Serializable;

public class Product implements Serializable {
    private String productName;
    private String description;
    private String price;

    public String getProductName() {
        return productName;
    }

    public void setProductName(String productName) {
        this.productName = productName;
    }

    public String getDescription() {
        return description;
    }

    public void setDescription(String description) {
        this.description = description;
    }

    public String getPrice() {
        return price;
    }

    public void setPrice(String price) {
        this.price = price;
    }

    public String execute(){
        return "seccess";
    }
}

启动tomcat的时候没有报错；
当我输入http://localhost:8080/struts/Product_input.action时，会得到：