求教core java 10.7节

就是这小节用 java WEB START发布第9章的计算器应用程序.....为什么我还是失败了....我已经照它的方法做了..打包了Calculator.jar.
在\webapps\calculator目录下有Calculator.jar,Calculator.jnlp,还有个什么WEB-INF子目录里有个web.xml我照着书上打进去了
<?xml version="1.0" encoding="utf-8"?>
<jnlp spec="1.0+" codebase="http://localhost:8080/calculator/" href="Calculator.jnlp">
  <information>
    <title>Calculator Demo Application</title>
    <vendor>Cay S. Horstmann</vendor>
    <description>A Calculator</description>
    <offline-allowed/>
  </information>  
  <resources>
    <j2se version="1.5+"/>
    <jar href="Calculator.jar"/>
  </resources>
  <application-desc/>
</jnlp> 
<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE web-app PUBLIC "-//Sun Microsystems,Inc.//DTD Web Application 2.3//EN"
   "http://java.sun.com//dtd/web-app_2_3.dtd">
<web-app>
</web-app>
可是最后还是失败 说
启动/运行应用程序时发生错误。标题：Calculator Demo Application
供应商：Cay S. Horstmann
类别：启动文件错误未指定应用程序的主类JNLPException[category: 启动文件错误 : Exception: null : LaunchDesc: 
<jnlp spec="1.0+" codebase="http://localhost:8080/calculator/" href="http://localhost:8080/calculator/Calculator.jnlp">
  <information>
    <title>Calculator Demo Application</title>
    <vendor>Cay S. Horstmann</vendor>
    <homepage href="null"/>
    <description>A Calculator</description>
    <offline-allowed/>
  </information>
  <resources>
    <j2se version="1.5+"/>
    <jar href="http://localhost:8080/calculator/Calculator.jar" download="eager" main="false"/>
  </resources>
  <application-desc/>
</jnlp> ]
at com.sun.javaws.LaunchDownload.getMainClassName(Unknown Source)
at com.sun.javaws.Launcher.continueLaunch(Unknown Source)
at com.sun.javaws.Launcher.handleApplicationDesc(Unknown Source)
at com.sun.javaws.Launcher.handleLaunchFile(Unknown Source)
at com.sun.javaws.Launcher.run(Unknown Source)
at java.lang.Thread.run(Unknown Source)
高手帮我分析一下  谢谢了