这是小白偶尔一直null指针的错误，调试了好久，原来是自己对spring注入的不够了解
我相信有很多跟我差不多的初学者会遇上，所以特地写出来，防止有人跟我一样。哈哈，也写上去，以防自己下次还犯这样的错误。
一样，首先，举个反例
所有类
有个城市类
有个华北地区类，有个城市类的集合属性
同上，华南地区类
index.jsp页面
<%@ page language="java" import="java.util.*" pageEncoding="GB18030"%> <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"> <html> <body> <center> <h1>欢迎光临</h1><p><p><p><p> <form action="query" method="post"> <table border="0"> <tr> <td> <input type="radio" name="country" value="SC" checked> 华南旅游城市<br> <input type="radio" name="country" value="NC"> 华北旅游城市 <br> </td> </tr> <tr> <td colspan="2" align="center"> <input type = "submit" name = "submit" value = "查 询" /> </td> </tr> </table> </form> </center> </body> </html>最后有个servlet类
package com.servlet; import java.io.IOException; import java.io.PrintWriter; import java.util.List; 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 com.javaBean.City; import com.javaBean.NorthChina; import com.javaBean.SouthChina; public class CountryServlet extends HttpServlet { private NorthChina northChina; private SouthChina southChina; public NorthChina getNorthChina() { return northChina; } public void setNorthChina(NorthChina northChina) { System.out.println("已经注入了，城市个数："+northChina.getCitys().size()); this.northChina = northChina; } public SouthChina getSouthChina() { return southChina; } public void setSouthChina(SouthChina southChina) { this.southChina = southChina; } public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { response.setContentType("text/html;charset=utf-8"); String diqu=request.getParameter("country"); PrintWriter writer = response.getWriter(); List<City> citys=null; if(diqu!=null){ if("SC".equals(diqu)){ citys=southChina.getCitys(); }else if("NC".equals(diqu)){ citys=northChina.getCitys(); } } String str=""; for (int i = 0; i < citys.size(); i++) { str+=citys.get(i).getCity()+","; } writer.write(str); } public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { doGet(request, response); } } <?xml version="1.0" encoding="UTF-8"?> <beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:p="http://www.springframework.org/schema/p" xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-3.0.xsd"> <bean id="shenzhen" class="com.javaBean.City"> <property name="city" value="深圳"></property> </bean> <bean id="hongkong" class="com.javaBean.City"> <property name="city" value="香港"></property> </bean> <bean id="guilin" class="com.javaBean.City"> <property name="city" value="桂林"></property> </bean> <bean id="guangzhou" class="com.javaBean.City"> <property name="city" value="广州"></property> </bean> <bean id="beijin" class="com.javaBean.City"> <property name="city" value="北京"></property> </bean> <bean id="tianjin" class="com.javaBean.City"> <property name="city" value="天津"></property> </bean> <bean id="shanghai" class="com.javaBean.City"> <property name="city" value="上海"></property> </bean> <bean id="haerbin" class="com.javaBean.City"> <property name="city" value="哈尔滨"></property> </bean> <bean id="southChina" class="com.javaBean.SouthChina"> <property name="citys"> <list> <ref bean="guilin"/> <ref bean="shenzhen"/> <ref bean="hongkong"/> <ref bean="guangzhou"/> </list> </property> </bean> <bean id="northChina" class="com.javaBean.NorthChina"> <property name="citys"> <list> <ref bean="shanghai"/> <ref bean="haerbin"/> <ref bean="beijin"/> <ref bean="tianjin"/> </list> </property> </bean> <bean id="countryServlet" class="com.servlet.CountryServlet"> <property name="northChina" ref="northChina"></property> <property name="southChina" ref="southChina"></property> </bean> </beans>web.xml配置是正确的，开始部署到tomcat
打开页面访问
点击查询，然而错误就来了
测试方法中执行时会发现
原因竟然是我误认为启动服务器时，已经把NorthChina注入进去了，所以就在servlet调用类中的方法，报出null，
结果我百度搜索“set注入”（其实都是很多demo，并没有仔细详解），然后我就发现注入NorthChina的是spring容器中的CountryServlet，
而现在调用的CountryServlet类并非是Spring构建的，可以理解为自己new了一个。所以就只能在这两个类中加个static，就可以获取了
这样，问题就解决了(相信大神应该还有跟多好的方法，方便的话可以在评论中教导小白)。小白只是把自己在学习中遇到的问题写出来，方便自己查看学习，也可以让大家防止遇到一样的错误，嘿嘿。
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转载于:https://www.cnblogs.com/xswz/p/7062344.html
相关资源：JAVA上百实例源码以及开源项目