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1. ApplicationContext应用上下文获取方式

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得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，在将其存储到最大的域servletContext域中，这样就可以在任意位置从域中获得应用上下文ApplicationContext对象了。
web.xml配置全局初始化参数

<!--全局初始化参数--><context-param><param-name>contextConfigLocation</param-name><param-value>applicationContext.xml</param-value></context-param><listener><listener-class>com.zhxd.listener.ContextLoaderListener</listener-class>
</listener>

• 创建ServletContextListener

package com.zhxd.listener;import org.springframework.context.support.ClassPathXmlApplicationContext;import javax.servlet.ServletContext;
import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;
import javax.servlet.annotation.WebListener;public class ContextLoaderListener implements ServletContextListener {public void contextInitialized(ServletContextEvent servletContextEvent) {ServletContext servletContext = servletContextEvent.getServletContext();//ClassPathXmlApplicationContext app = new ClassPathXmlApplicationContext("applicationContext.xml");//隐藏Spring配置文件String contextConfigLocation = servletContext.getInitParameter("contextConfigLocation");ClassPathXmlApplicationContext app = new ClassPathXmlApplicationContext(contextConfigLocation);servletContext.setAttribute("app",app);System.out.println("容器创建完毕...");}public void contextDestroyed(ServletContextEvent servletContextEvent) {}
}

• UserServlet.java

public class UserServlet extends HttpServlet {@Overrideprotected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {ServletContext servletContext = req.getServletContext();ApplicationContext app = (ApplicationContext)servletContext.getAttribute("app");UserService userService = app.getBean(UserService.class);userService.save();}
}

优化上述程序，把上下文变量名字（app）隐藏：

• 定义获取上下文对象工具类

package com.zhxd.listener;import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;import javax.servlet.ServletContext;
import javax.servlet.ServletContextListener;public class WebApplicationContextUtils {public static ApplicationContext getApp(ServletContext servletContext) {return (ApplicationContext) servletContext.getAttribute("app");}
}

• 修改UserServlet.java

public class UserServlet extends HttpServlet {@Overrideprotected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {ServletContext servletContext = req.getServletContext();ApplicationContext app = WebApplicationContextUtils.getApp(servletContext);UserService userService = app.getBean(UserService.class);userService.save();}
}

2.Spring提供获取应用上下文的工具

上面的分析不用手动实现，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供使用者获得应用上下文对象。

所以我们需要做的只有两件事：
① 在web.xml中配置ContextLoaderListener监听器（导入spring-web坐标）

<dependency><groupId>org.springframework</groupId><artifactId>spring-web</artifactId><version>5.0.5.RELEASE</version>
</dependency>

② 使用WebApplicationContextUtils获得应用上下文对象ApplicationContext
配置web.xml

 <context-param><param-name>contextConfigLocation</param-name><param-value>classpath:applicationContext.xml</param-value></context-param><listener><listener-class>org.springframework.web.context.ContextLoaderListener</listener-class></listener>

package com.zhxd.web;import com.zhxd.service.UserService;
import org.springframework.web.context.WebApplicationContext;
import org.springframework.web.context.support.WebApplicationContextUtils;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 {@Overrideprotected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {ServletContext servletContext = req.getServletContext();WebApplicationContext app = WebApplicationContextUtils.getWebApplicationContext(servletContext);UserService userService = app.getBean(UserService.class);userService.save();}
}