Spring自動裝配報空指標異常
阿新 • • 發佈：2019-01-08
這幾天在學Spring的自動裝配，自己動手做一個小專案，但是被一個空指標異常卡住了。
啟動的時候彈出index.jsp,這是一個登陸頁面：
<%@ 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 跳轉到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;
}
}若使用者名稱和密碼都正確，跳轉到list.jsp,一個賬單列表：
<%@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>賬單</title>
</head>
<body>
<h1>賬單</h1>
<hr>
<table>
<tr>
<td>編號</td>
<td>金額</td>
<td>型別</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>注意加粗的那句話，這個list.jsp會呼叫UserService的listRecordByUserId(int userId)方法，具體如下：
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;
}
}
這個時候加粗的那句話就報空指標了，這是因為我在list.jsp直接new了一個UserService,直接new的話那這個UserService的例項service就不歸spring管了，spring是不會自動裝配UserService裡面hibernateManage，所以報了空指標異常。只要將list.jsp裡的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>賬單</title>
</head>
<body>
<h1>賬單</h1>
<hr>
<table>
<tr>
<td>編號</td>
<td>金額</td>
<td>型別</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，因為這個查了好久，對於大牛來說這種問題太low了，但是對於我這種還沒畢業的小白來說實在是一個挺難的問題了，在這裡記錄一下。