最不友好的 界面报错：
HTTP Status 500 - type Exception report message description The server encountered an internal error () that prevented it from fulfilling this request. exception org.apache.jasper.JasperException: java.lang.NullPointerException org.apache.jasper.servlet.JspServletWrapper.handleJspException(JspServletWrapper.java:522) org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:416) org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:342) org.apache.jasper.servlet.JspServlet.service(JspServlet.java:267) javax.servlet.http.HttpServlet.service(HttpServlet.java:717) root cause java.lang.NullPointerException org.apache.jsp.computer_005flist_jsp._jspService(computer_005flist_jsp.java:111) org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:70) javax.servlet.http.HttpServlet.service(HttpServlet.java:717) org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:374) org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:342) org.apache.jasper.servlet.JspServlet.service(JspServlet.java:267) javax.servlet.http.HttpServlet.service(HttpServlet.java:717) note The full stack trace of the root cause is available in the Apache Tomcat/6.0.20 logs.
相对友好的界面：
type Exception report
message
description The server encountered an internal error () that prevented it from fulfilling this request.
exception
org.apache.jasper.JasperException: An exception occurred processing JSP page /computer_list.jsp at line 57
54: //System.out.println("request:"+request);
55: //if(computers == null){ System.out.println("computers:=="+computers);}
56: //else{
57: for(int i=0;i<computers.size();i++){
58: Computer c = computers.get(i);
59: %>
60: <tr>
Stacktrace:
org.apache.jasper.servlet.JspServletWrapper.handleJspException(JspServletWrapper.java:505)
org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:416)
org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:342)
org.apache.jasper.servlet.JspServlet.service(JspServlet.java:267)
javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
root cause
java.lang.NullPointerException
org.apache.jsp.computer_005flist_jsp._jspService(computer_005flist_jsp.java:111)
org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:70)
javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:374)
org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:342)
org.apache.jasper.servlet.JspServlet.service(JspServlet.java:267)
javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
note The full stack trace of the root cause is available in the Apache Tomcat/6.0.20 logs.
解决：
帮助一位女孩子解决以上异常
最初的判断，以为是哪个英文写错了，可查了半个小时，没结果，最后，我怀疑是不是jsp页面本来就有错。
后来，我回来我的电脑运行，发现我是从list.do重定向到computer_list.jsp
于是，最初得出的结论是：因为在servlet中进行session绑定引起的。
今晚，又重新定位这个原因，才发现，页面给的报错才是真正的原因，不能莫须有的联想判断，太主观。
在..\localhost\shoppingCart\org\apache\jsp目录下，找到MyExlipse编译的computer_005flist_jsp.java文件，111行，正是
for(int i=0;i<computers.size();i++){
错误的原因就是computers的值为null，所以不能直接调用
computers.size() 原来，JAVA程序友好报错界面，真的这么好~