japplet调用jfreechart生成图片的异常

访问jsp页面报的异常如下：java.lang.NullPointerException
at sun.plugin2.applet.Plugin2Manager.findAppletJDKLevel(Unknown Source)
at sun.plugin2.applet.Plugin2Manager.createApplet(Unknown Source)
at sun.plugin2.applet.Plugin2Manager$AppletExecutionRunnable.run(Unknown Source)
at java.lang.Thread.run(Unknown Source)
异常：java.lang.NullPointerExceptionjsp页面代码如下：
<body><CENTER>           <jsp:plugin type="applet" code="CrosshairDemo.class" 
    codebase="."    width="500" height="400" archive="jcommon-1.0.0.jar,jfreechart-1.0.0.jar"> 
  
    <jsp:fallback> 
        <p>无法载入 Applet </p> 
    </jsp:fallback> 
</jsp:plugin> 
</CENTER> </body>CrosshairDemo类单独运行生成图片没问题，用japplet嵌入到jsp页面就报了这个异常，请问高手们是什么原因