jsp运行时抛出的异常，帮忙分析一下是什么错误

Question

exception org.apache.jasper.JasperException: An exception occurred processing JSP page /index.jsp at line 21: <%@ page language="java" contentType="text/html; charset=gb2312"%>
2: <jsp:forward page="/IndexServlet"/>
Stacktrace:
org.apache.jasper.servlet.JspServletWrapper.handleJspException(JspServletWrapper.java:510)
org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:419)
org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:313)
org.apache.jasper.servlet.JspServlet.service(JspServlet.java:260)
javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
root cause java.lang.NullPointerException
com.yxq.dao.FriendDao.queryFriend(FriendDao.java:47)
com.yxq.servlet.IndexServlet.doPost(IndexServlet.java:52)
com.yxq.servlet.IndexServlet.doGet(IndexServlet.java:25)
javax.servlet.http.HttpServlet.service(HttpServlet.java:617)
javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
org.apache.jasper.runtime.PageContextImpl.doForward(PageContextImpl.java:706)
org.apache.jasper.runtime.PageContextImpl.forward(PageContextImpl.java:677)
org.apache.jsp.index_jsp._jspService(index_jsp.java:56)
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:377)
org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:313)
org.apache.jasper.servlet.JspServlet.service(JspServlet.java:260)
javax.servlet.http.HttpServlet.service(HttpServlet.java:717)大侠们帮忙看一下是什么问题，小弟感激不尽

Answer 1

com.yxq.dao.FriendDao.queryFriend(FriendDao.java:47)
第47行有错。

Answer 2

root cause  java.lang.NullPointerException
com.yxq.dao.FriendDao.queryFriend(FriendDao.java:47)
com.yxq.servlet.IndexServlet.doPost(IndexServlet.java:52)
com.yxq.servlet.IndexServlet.doGet(IndexServlet.java:25)javax.servlet.http.HttpServlet.service(HttpServlet.java:617)
javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
org.apache.jasper.runtime.PageContextImpl.doForward(PageContextImpl.java:706)
org.apache.jasper.runtime.PageContextImpl.forward(PageContextImpl.java:677)
org.apache.jsp.index_jsp._jspService(index_jsp.java:56)
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:377)
org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:313)
org.apache.jasper.servlet.JspServlet.service(JspServlet.java:260)
javax.servlet.http.HttpServlet.service(HttpServlet.java:717)跟着红色的一路找下去，应该能发现错误根源，报的是空指针异常，猜测是参数问题！你用out.print标记，看看实际情况和你预料的是否相同！

Answer 3

com.yxq.dao.FriendDao.queryFriend(FriendDao.java:47)
com.yxq.servlet.IndexServlet.doPost(IndexServlet.java:52)
com.yxq.servlet.IndexServlet.doGet(IndexServlet.java:25不会是少了jar包，少了jar包一般是class not found

Answer 4

遇到问题尽量先下断点走DEBUG一步一步跑一遍，这样你就才会有提高，在不断地自己找错误获得经验不然你无法得到任何提高

Answer 5

java.lang.NullPointerException
com.yxq.dao.FriendDao.queryFriend(FriendDao.java:47)
这里空指针了，打断点看看

Answer 6

java.lang.NullPointerException
com.yxq.dao.FriendDao.queryFriend(FriendDao.java:47)
com.yxq.servlet.IndexServlet.doPost(IndexServlet.java:52)
com.yxq.servlet.IndexServlet.doGet(IndexServlet.java:25)可以猜想到异常源就在FriendDao或者IndexServlet源文件中如果是在IndexServlet调用FriendDao，异常就在FriendDao中可能是由方法的参数引发的，也可能是数据库的对象引发的，比如Connection、Statement、Query……建议LZ把这两个源代码贴出来