Spring自动装配报空指针异常

野性酷女 2022-07-12 16:40 292阅读 0赞

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
    @Transactional
    public 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。