这几天在学SPRing的自动装配，自己动手做一个小项目，但是被一个空指针异常卡住了。
启动的时候弹出index.jsp,这是一个登陆页面：
<%@ page language="java" contentType="text/html; charset=utf-8" pageEncoding="utf-8"%><!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"><html><head><meta http-equiv="Content-Type" content="text/html; charset=utf-8"><title>消费管理系统--首页</title><style type="text/CSS"> #surebtn{ align:center; }</style><script type="text/Javascript"></script></head><body> <h1>用户登录</h1> <div> <form action="UserServlet"> 用户名：<input id="username" type="text" name="nickname"/><br/> 密码：<input id="passWord" type="password" name="password"><br/> <input type="hidden" name="action" value="login"> <input id="surebtn" type="submit" value="登录"><button onclick="#" id="regbtn">注册</button> </form> </div></body></html>跳转到UserServlet:
package com.ffy.servlet;import java.io.IOException;import javax.servlet.ServletException;import javax.servlet.http.HttpServlet;import javax.servlet.http.HttpServletRequest;import javax.servlet.http.HttpServletResponse;import org.springframework.beans.factory.annotation.Autowired;import org.springframework.stereotype.Component;import org.springframework.web.context.WebapplicationContext;import org.springframework.web.context.support.WebApplicationContextUtils;import com.ffy.bean.User;import com.ffy.service.UserService;public class UserServlet extends HttpServlet{ @Autowired private UserService userService; private static WebApplicationContext context; private String action=null; @Override protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { context=WebApplicationContextUtils.getRequiredWebApplicationContext(getServletContext()); userService=context.getBean(UserService.class); if(req.getParameter("action")!=null){ action=req.getParameter("action"); if("login".equals(action)){ if(login(req,resp)!=null){ User user=login(req,resp); req.getRequestDispatcher("/list.jsp?userId="+user.getId()).forward(req, resp); }else{ req.getRequestDispatcher("loginError.jsp").forward(req, resp); } } } } @Override protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { doGet(req, resp); } private User login(HttpServletRequest req,HttpServletResponse resp){ User user=new User(); user.setNickName(req.getParameter("nickname")); user.setPassword(req.getParameter("password")); User returnu=userService.login(user); if(returnu!=null){ return returnu; } return null; }}若用户名和密码都正确，跳转到list.jsp,一个账单列表：
<%@page import="org.springframework.web.context.support.WebApplicationContextUtils"%><%@page import="org.springframework.context.ApplicationContext"%><%@page import="javax.enterprise.context.spi.Context"%><%@page import="org.springframework.beans.factory.annotation.Autowired"%><%@page import="com.ffy.bean.Record"%><%@ page language="java" contentType="text/html; charset=utf-8" pageEncoding="utf-8"%><%@ page import="java.util.List" %><%@ page import="com.ffy.service.UserService" %><!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"><html><head><meta http-equiv="Content-Type" content="text/html; charset=utf-8"><%**UserService service=new UserService();** list=service.listRecordByUserId(Integer.parseInt(request.getParameter("userId")));%><title>账单</title></head><body> <h1>账单</h1> <hr> <table> <tr> <td>编号</td> <td>金额</td> <td>类型</td> <td>日期</td> <td>描述</td> </tr> <% for(Record r:list){ %> <tr> <td><%=r.getId() %></td> <td><%=r.getPrice() %></td> <td><%=r.getType().getName() %></td> <td><%=r.getDate() %></td> <td><%=r.getDescription() %></td> </tr> <% } %> </table></body></html>注意加粗的那句话，这个list.jsp会调用UserService的listRecordByUserId(int userId)方法，具体如下：
package com.ffy.service;import java.util.List;import org.hibernate.session;import org.hibernate.query.Query;import org.springframework.beans.factory.annotation.Autowired;import org.springframework.stereotype.Service;import org.springframework.transaction.annotation.Transactional;import com.ffy.bean.Record;import com.ffy.bean.User;@Service@Transactionalpublic class UserService { @Autowired private HibernateManage hibernateManage; public HibernateManage getHibernateManage() { return hibernateManage; } public void setHibernateManage(HibernateManage hibernateManage) { this.hibernateManage = hibernateManage; } public User login(User user){ System.out.println("login........"); Session session=hibernateManage.getSession(); Query<User>query=session.createQuery("from User where nickname=:nickname and password=:password",User.class); query.setParameter("nickname",user.getNickName()); query.setParameter("password", user.getPassword()); if(query.getResultList().size()>0){ return query.getResultList().get(0); } return null; } public List<Record> listRecordByUserId(int userId){ if(hibernateManage==null){ System.out.println("null-----------------------"); } **Session session=hibernateManage.getSession();** Query<Record> q=session.createQuery("from Record r,User u where r.id=r.user.id",Record.class); List<Record> list=q.getResultList(); return list; }}这个时候加粗的那句话就报空指针了，这是因为我在list.jsp直接new了一个UserService,直接new的话那这个UserService的实例service就不归spring管了，spring是不会自动装配UserService里面hibernateManage，所以报了空指针异常。只要将list.jsp里的UserService service=new UserService();改成ApplicationContext context=WebApplicationContextUtils.getWebApplicationContext(request.getSession().getServletContext()); UserService service=context.getBean(UserService.class);就可以了。
<%@page import="org.springframework.web.context.support.WebApplicationContextUtils"%><%@page import="org.springframework.context.ApplicationContext"%><%@page import="javax.enterprise.context.spi.Context"%><%@page import="org.springframework.beans.factory.annotation.Autowired"%><%@page import="com.ffy.bean.Record"%><%@ page language="java" contentType="text/html; charset=utf-8" pageEncoding="utf-8"%><%@ page import="java.util.List" %><%@ page import="com.ffy.service.UserService" %><!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"><html><head><meta http-equiv="Content-Type" content="text/html; charset=utf-8"><% ApplicationContext context=WebApplicationContextUtils.getWebApplicationContext(request.getSession().getServletContext()); UserService service=context.getBean(UserService.class); List<Record> list=service.listRecordByUserId(Integer.parseInt(request.getParameter("userId")));%><title>账单</title></head><body> <h1>账单</h1> <hr> <table> <tr> <td>编号</td> <td>金额</td> <td>类型</td> <td>日期</td> <td>描述</td> </tr> <% for(Record r:list){ %> <tr> <td><%=r.getId() %></td> <td><%=r.getPrice() %></td> <td><%=r.getType().getName() %></td> <td><%=r.getDate() %></td> <td><%=r.getDescription() %></td> </tr> <% } %> </table></body></html>唉，因为这个查了好久，对于大牛来说这种问题太low了，但是对于我这种还没毕业的小白来说实在是一个挺难的问题了，在这里记录一下。
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