HTTP Status 404 - /HelloWorld/HelloWorld.action,该怎么解决_Java Web开发

HTTP Status 404 - /HelloWorld/HelloWorld.action
在eclipse中创建一个Dynamic Web Project。具体内容如下：
HelloWorld.java


public class HelloWorld
{
	private String username;
	public void setUsername(String username)
	{
		this.username = username;		
	}
	public String getUsername()
	{
		return username;
	}
	public String execute()
	{
		if(getUsername().equals(""))
		{
			System.out.println("no String input!");
			return "error"; 
		}
		else
		{
			System.out.println(getUsername());
			return "success";
		}
	}
}

web.xml


<?xml version="1.0" encoding="UTF-8"?>
<web-app id="WebApp_9" 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">

    <display-name>Struts Blank</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>index.jsp</welcome-file>
    </welcome-file-list>

</web-app>

struts.xml


<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE struts PUBLIC
    "-//Apache Software Foundation//DTD Struts Configuration 2.0//EN"
    "http://struts.apache.org/dtds/struts-2.0.dtd">

<struts>

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

        <action name="index" class="HelloWorld">
            <result name="success">success.jsp</result>
            <result name="error">error.jsp</result>
        </action>
    </package>

</struts>

index.jsp


<%@ page language="java" contentType="text/html; charset=ISO-8859-1"
    pageEncoding="ISO-8859-1"%>
<!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=ISO-8859-1">
<title>Insert title here</title>
</head>
<body>
Hello World!
<form method="post" action="HelloWorld.action">
	<input type="text" name="username" />
	<input type="submit" name="submit" />
</form>
</body>
</html>

运行后，出现下面的错误：
HTTP Status 404 - /HelloWorld/HelloWorld.action

--------------------------------------------------------------------------------

type Status report

message /HelloWorld/HelloWorld.action

description The requested resource (/HelloWorld/HelloWorld.action) is not available.

--------------------------------------------------------------------------------

Apache Tomcat/7.0.21

不知是何故？？
------解决方案--------------------
<form method="post" action="index.action">