struts框架下jfreechart的配置问题

Question

Servlet execution threw an exception 运行后的结果
我在Struts框架下，想用图表方式显示查询结果，配置好后tomcat抛出以上错误
具体的思路：jsp传入查询数据，ACTION调用model的方法得到结果集，然后作为调用一个生成图表类，存入request,之后供结果页面Jsp调用
Q1：Servlet execution threw an exception ，我已经在web.xml文件下配置好了文件如下：
<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns="http://java.sun.com/xml/ns/j2ee" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" version="2.4" xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee   http://java.sun.com/xml/ns/j2ee/web-app_2_4.xsd">
  <servlet>
    <servlet-name>action</servlet-name>
    <servlet-class>org.apache.struts.action.ActionServlet</servlet-class>
    <init-param>
      <param-name>config</param-name>
      <param-value>/WEB-INF/struts-config.xml</param-value>
    </init-param>
    <init-param>
      <param-name>debug</param-name>
      <param-value>3</param-value>
    </init-param>
    <init-param>
      <param-name>detail</param-name>
      <param-value>3</param-value>
    </init-param>
    <load-on-startup>0</load-on-startup>
  </servlet>
  <servlet-mapping>
    <servlet-name>action</servlet-name>
    <url-pattern>*.do</url-pattern>
  </servlet-mapping>
   <servlet>
<servlet-name>DisplayChart</servlet-name>
    <servlet-class>org.jfree.chart.servlet.DisplayChart</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>DisplayChart</servlet-name>
<url-pattern>/servlet/DisplayChart</url-pattern>
</servlet-mapping>
  <welcome-file-list>
  <welcome-file>login.jsp</welcome-file>
  </welcome-file-list>
</web-app>
Q2： String filename=ServletUtilities.saveChartAsJPEG（chart,600,400,info,session);
        //设置生成图片，包括图片的大小，长度是300，宽是200
        System.out.println(filename);
        System.out.println(info);
       session.setAttribute("file_name1",filename);
        String URL= request.getContextPath() + "/servlet/DisplayChart? filename=" + filename; 
        session.setAttribute("file_url1",URL);
        System.out.println(URL);
这段代码的含义