Spring与Web环境集成

应用上下文对象是通过 new ClassPathXmlApplicationContext(Spring配置文件) 方式获取的，但是每次从容器中获取Bean时都要编写 new ClassPathXmlApplicationContext(Spring配置文件)，这样的弊端是配置文件加载多次，应用上下文对象创建多次。

在Web项目中，可以使用ServletContextLIstener监听Web应用的启动，我们可以在Web应用启动时，就加载Spring的配置文件，创建应用上下文对象ApplicationContext，再将其存储到最大的域servletContext域中，这样就可以在任意位置获取应用上下文ApplicationContext对象。

1、创建监听器

 package com.ntect.listener;
 ​
 import org.springframework.context.ApplicationContext;
 import org.springframework.context.support.ClassPathXmlApplicationContext;
 ​
 import javax.servlet.ServletContext;
 import javax.servlet.ServletContextEvent;
 import javax.servlet.ServletContextListener;
 ​
 public class ContextLoaderListener implements ServletContextListener {
     public void contextInitialized(ServletContextEvent sce) {
 ​
         ServletContext servletContext = sce.getServletContext();
         //读取web.xml中的全局参数
         String contextConfigLocation = servletContext.getInitParameter("contextConfigLocation");
         ApplicationContext app = new ClassPathXmlApplicationContext(contextConfigLocation);
         //将Spring的应用上下文对象存储到ServletContext域中
 ​
         servletContext.setAttribute("app",app);
         System.out.println("spring容器创建完毕....");
     }
 ​
     public void contextDestroyed(ServletContextEvent sce) {
 ​
     }
 } 

2、创建获取applicationContext对象的工具类

package com.ntect.listener;
 ​
 import org.springframework.context.ApplicationContext;
 ​
 import javax.servlet.ServletContext;
 ​
 public class WebApplicationContextUtils {
 ​
     public static ApplicationContext getWebApplicationContext(ServletContext servletContext) {
         return (ApplicationContext)servletContext.getAttribute("app");
     }
 }
 

3、创建servlet的测试类

 package com.ntect.web;
 ​
 import com.ntect.listener.WebApplicationContextUtils;
 import com.ntect.service.UserService;
 import org.springframework.context.ApplicationContext;
 import org.springframework.context.support.ClassPathXmlApplicationContext;
 ​
 import javax.servlet.ServletContext;
 import javax.servlet.ServletException;
 import javax.servlet.http.HttpServlet;
 import javax.servlet.http.HttpServletRequest;
 import javax.servlet.http.HttpServletResponse;
 import java.io.IOException;
 ​
 public class UserServlet extends HttpServlet {
 ​
     @Override
     protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
         //ApplicationContext app = new ClassPathXmlApplicationContext("applicationContext.xml");
         ServletContext servletContext = this.getServletContext();
         //ApplicationContext app = (ApplicationContext) servletContext.getAttribute("app");
         ApplicationContext app = WebApplicationContextUtils.getWebApplicationContext(servletContext);
         UserService userService = app.getBean(UserService.class);
         userService.save();
     }
 }

4、配置web.xml文件

 <?xml version="1.0" encoding="UTF-8"?>
 <web-app xmlns="http://xmlns.jcp.org/xml/ns/javaee"
          xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
          xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_4_0.xsd"
          version="4.0">
 ​
 <!--    全局初始化参数-->
     <context-param>
         <param-name>contextConfigLocation</param-name>
         <param-value>applicationContext.xml</param-value>
     </context-param>
 ​
 <!--    配置监听器-->
     <listener>
         <listener-class>com.ntect.listener.ContextLoaderListener</listener-class>
     </listener>
     
 <!--    配置测试用的servlet-->
     <servlet>
         <servlet-name>UserServlet</servlet-name>
         <servlet-class>com.ntect.web.UserServlet</servlet-class>
     </servlet>
 ​
     <servlet-mapping>
         <servlet-name>UserServlet</servlet-name>
         <url-pattern>/userServlet</url-pattern>
     </servlet-mapping>
 ​
 </web-app>

5、测试

浏览器访问

 http://localhost:8080/userServlet

控制台输出

上面地分析只是为了方便理解，不用手动实现，Spring提供了一个监听器ContextLoaderListener就是对上述功能的封装，该监听器内部加载Spring配置文件，创建应用上下文对象，并存储到ServletContext域中，提供了一个客户端工具WebApplicationContextUtils获取应用上下文对象。

需要做两件事：

1、在web.xml中配置ContextLoaderListener监听器（导入spring-web坐标）

2、使用WebApplicationContextUtils获得应用上下文对象ApplicationContext

 <dependency>
     <groupId>org.springframework</groupId>
     <artifactId>spring-web</artifactId>
     <version>5.3.16</version>
</dependency>

<!--    全局初始化参数-->
     <context-param>
         <param-name>contextConfigLocation</param-name>
         <param-value>classpath:applicationContext.xml</param-value>
     </context-param>
 ​
 <!--    配置监听器-->
     <listener>
         <listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
     </listener>

ApplicationContext app = WebApplicationContextUtils.getWebApplicationContext(servletContext);
UserService userService = app.getBean(UserService.class);