spring中set注入的一些小细节错误

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spring中set注入的一些小细节错误

2024-10-10 00:29:01 212人阅读

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

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有个城市类

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有个华北地区类，有个城市类的集合属性

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同上，华南地区类

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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>