jsp调用applet生成jfreechart图像的异常

异常信息：


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.NullPointerException


jsp：


   

<jsp:plugin type="applet" code="CrosshairDemo.class"
    codebase="."    width="500" height="400" archive="jcommon-1.0.0.jar,jfreechart-1.0.0.jar">
    <jsp:params>
        <jsp:param name="width" value="560" />
        <jsp:param name="height" value="400" />
    </jsp:params>
 
</jsp:plugin>


java类：


public class CrosshairDemo extends JApplet {


public CrosshairDemo() {


DemoPanel d = new DemoPanel();
Container cp = getContentPane();
 
    cp.add(d.content);

}

public static class DemoPanel extends JPanel implements
ChartChangeListener, ChartProg