spring中set注入的一些小细节错误

2022-06-08 01:30:10

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>