基本tomcat servlet错误

package mypackage;

import java.io.*;

import javax.servlet.http.*;
import javax.servlet.*;

public class Hello extends HttpServlet {
  public void doGet (HttpServletRequest req,
                     HttpServletResponse res)
    throws ServletException, IOException
  {
    PrintWriter out = res.getWriter();

    out.println("Hello, world!");
    out.close();
  }
}

web.xml文件如下:

<?xml version="1.0" encoding="ISO-8859-1"?>
<web-app 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"
    version="2.4">

    <display-name>Hello, World Application</display-name>
    <description>
    This is a simple web application with a source code organization
    based on the recommendations of the Application Developer's Guide.
    </description>

    <servlet>
        <servlet-name>HelloServlet</servlet-name>
        <servlet-class>mypackage.Hello</servlet-class>
    </servlet>

    <servlet-mapping>
        <servlet-name>HelloServlet</servlet-name>
        <url-pattern>/hello</url-pattern>
    </servlet-mapping>

</web-app>

然后我将执行ant all、ant reload和/etc/init.d/tomcat restart

但我在浏览器上看到这个错误:

http状态404-/hello/

类型状态报告

留言/你好/

说明请求的资源(/hello/)不可用。 阿帕奇Tomcat/6.0.26

怎么了?