报错:HTTP Status 500 --优快云博客

这篇博客记录了一次解决HTTP 500错误的经历。作者首先排除了英文拼写错误的可能性，然后通过在本地环境中复现问题，发现错误源于从list.do重定向到computer_list.jsp时，在Servlet中进行session绑定的操作。最终，通过查看编译后的jsp文件，发现在特定行号存在错误，强调了解决问题时要依据实际报错信息，避免主观推测。

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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~