近期学习web开发时，就遇到这个令人头疼的问题。
百度谷歌了N遍，最终在博客http://blog.csdn.net/liu578182160/article/details/17266879中找到了问题的根源。
开发环境：win7 32位旗舰版，jdk1.7_45，jre7，eclipse4.3.1 JavaEE版，tomcat 7.0.42，struts2.3.16.3
问题出如今web.xml配置
原web.xml：
<?xml version="1.0" encoding="UTF-8"?>
<web-app id="struts2Demo" version="2.4"
xmlns="http://java.sun.com/xml/ns/j2ee"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee
http://java.sun.com/xml/ns/j2ee/web-app_2_4.xsd">
<filter>
<filter-name>struts2</filter-name>
<filter-class>
org.apache.struts2.dispatcher.ng.filter.StrutsPrepareAndExecuteFilter
</filter-class>
</filter>
<filter-mapping>
<filter-name>struts2</filter-name>
<url-pattern>/*</url-pattern>
</filter-mapping>
</web-app>改动后的web.xml：
<?xml version="1.0" encoding="UTF-8"?>
<web-app version="3.0"
xmlns="http://java.sun.com/xml/ns/javaee"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://java.sun.com/xml/ns/javaee
http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd">
<display-name></display-name>
<filter>
<filter-name>struts2</filter-name>
<filter-class>org.apache.struts2.dispatcher.ng.filter.StrutsPrepareAndExecuteFilter</filter-class>
</filter>
<filter-mapping>
<filter-name>struts2</filter-name>
<url-pattern>/*</url-pattern>
</filter-mapping>
<welcome-file-list>
<welcome-file>login.jsp</welcome-file>
</welcome-file-list>
</web-app>关键的差别就在于web-app的version
(似乎不关这版本号问题，刚试了2.4的web-app，能够执行成功。)
可能这不是问题的关键，项目改动的地方还有
原login.jsp：
<%@ page language="java" import="java.util.*" pageEncoding="utf-8"%>
<%
String path = request.getContextPath();
String basePath = request.getScheme()+"://"+request.getServerName()+":"+request.getServerPort()+path+"/";
%>
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<base href="<%=basePath%>">
<title>My JSP 'login.jsp' starting page</title>
<meta http-equiv="pragma" content="no-cache">
<meta http-equiv="cache-control" content="no-cache">
<meta http-equiv="expires" content="0">
<meta http-equiv="keywords" content="keyword1,keyword2,keyword3">
<meta http-equiv="description" content="This is my page">
<!--
<link rel="stylesheet" type="text/css" href="styles.css">
-->
</head>
<body>
<form action="login.action" method="post"><!-- login就是struts.xml里的action的name -->
用户名<input type="text" name="username" /> <!-- username相应LoginAction的username通过set方法传入 -->
密码： <input type="password" name="password" /> <!-- password相应LoginAction的password通过set方法传入 -->
<input type="submit" name="Submit" value="Submit"/>
</form>
</body>
</html>
改动后的login.jsp：
<%@ page language="java" contentType="text/html; charset=UTF-8"
pageEncoding="UTF-8"%>
<%@taglib prefix="s" uri="/struts-tags" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title><s:text name="loginPage"/></title>
</head>
<body>
<s:form action="login">
<s:textfield name="username" key="user"/>
<s:textfield name="password" key="pass"/>
<s:submit key="login" value="Submit"/>
</s:form>
</body>
</html>