一、Spring与Web环境集成

ApplicationContext应用上下文获取方式

初步设想代码示例：

<!--pom.xml配置文件-->
<dependencies>
	<dependency>
		<groupId>org.springframework</groupId>
		<artifactId>spring-context</artifactId>
		<version>5.0.5.RELEASE</version>
	</dependency>
	<dependency>
		<groupId>javax.servlet</groupId>
		<artifactId>javax.servlet-api</artifactId>
		<version>3.0.1</version>
		<scope>provided</scope>
	</dependency>
	<dependency>
		<groupId>javax.servlet.jsp</groupId>
		<artifactId>javax.servlet.jsp-api</artifactId>
		<version>2.2.1</version>
		<scope>provided</scope>
	</dependency>
</dependencies>

public class UserServlet extends HttpServlet {

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        ApplicationContext app = new ClassPathXmlApplicationContext("applicationContext.xml");
        UserService userService = app.getBean(UserService.class);
        userService.save();
    }
}

<!--web.xml配置文件-->
<servlet>
	<servlet-name>UserServlet</servlet-name>
	<servlet-class>com.itheima.web.UserServlet</servlet-class>
</servlet>
<servlet-mapping>
	<servlet-name>UserServlet</servlet-name>
	<url-pattern>/userServlet</url-pattern>
</servlet-mapping>

弊端：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。

改进设想代码示例：

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配置文件-->
<!--配置监听器-->
<listener>
	<listener-class>com.study.listener.MyContextLoaderListener</listener-class>
</listener>

public class MyContextLoaderListener implements ServletContextListener {
    public void contextInitialized(ServletContextEvent servletContextEvent) {
        ServletContext servletContext = servletContextEvent.getServletContext();
        ApplicationContext app = new ClassPathXmlApplicationContext("applicationContext.xml");
        //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 servletContextEvent) {
    }
}

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

优化设想代码示例：

• applicationContext.xml从web.xml配置文件文件中获取；
• ApplicationContext对象从工具类中直接获取；

<!--web.xml配置文件-->
<!--全局初始化参数-->
<context-param>
	<param-name>myContextConfigLocation</param-name>
	<param-value>applicationContext.xml</param-value>
</context-param>

public class MyContextLoaderListener implements ServletContextListener {
    public void contextInitialized(ServletContextEvent servletContextEvent) {
        ServletContext servletContext = servletContextEvent.getServletContext();
        //读取web.xml中的全局参数
        String contextConfigLocation = servletContext.getInitParameter("myContextConfigLocation");
        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 servletContextEvent) {
    }
}

public class MyWebApplicationContextUtils {
    public static ApplicationContext getWebApplicationContext(ServletContext servletContext){
        return (ApplicationContext) servletContext.getAttribute("app");
    }
}

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

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

导入Spring集成web的坐标

<!--pom.xml配置文件-->
<dependencies>
	<dependency>
		<groupId>org.springframework</groupId>
		<artifactId>spring-context</artifactId>
		<version>5.0.5.RELEASE</version>
	</dependency>
	<dependency>
		<groupId>org.springframework</groupId>
		<artifactId>spring-web</artifactId>
		<version>5.0.5.RELEASE</version>
	</dependency>
	<dependency>
		<groupId>javax.servlet</groupId>
		<artifactId>javax.servlet-api</artifactId>
		<version>3.0.1</version>
		<scope>provided</scope>
	</dependency>
	<dependency>
		<groupId>javax.servlet.jsp</groupId>
		<artifactId>javax.servlet.jsp-api</artifactId>
		<version>2.2.1</version>
		<scope>provided</scope>
	</dependency>
</dependencies>

配置ContextLoaderListener监听器

<!--web.xml配置文件-->
<!--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>

通过工具获得应用上下文对象

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