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，
并不是没被构建的CountryServlet类中。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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时间： 2024-08-06 20:51:44