第八节——使用ContextListener监听器简化开发

一、手工使用ContextListener监听器简化开发

1. 目的：由于每个servlet都需要applicationContext.xml来获取bean对象，所以使用监听器当项目启动就加载文件，并将app放入最大的域中供所有servlet使用
2. 在main-java-demo1-listener目录下创建ContextLoaderListener.class

public class ContextLoaderListener implements ServletContextListener {
    
    
    @Override
    public void contextInitialized(ServletContextEvent sce) {
    
    
        ApplicationContext app = new ClassPathXmlApplicationContext("applicationContext.xml");
        //将app对象存储到最大的域ServletContext中
        ServletContext servletContext = sce.getServletContext();
        servletContext.setAttribute("app",app);
    }
}

3. 编辑main-webapp-WEB-INF目录下的web.xml，将监听器配置进去

<listener>
	<listener-class>demo1.listener.ContextLoaderListener</listener-class>
</listener>

4. 编辑web目录下的UserServlet.class，从域中获取app

public class UserServlet extends HttpServlet {
    
    
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
    
    
//        ApplicationContext app = new ClassPathXmlApplicationContext("applicationContext.xml");
        ServletContext servletContext = req.getServletContext();
        ApplicationContext app = (ApplicationContext) servletContext.getAttribute("app");
        UserServiceImpl userService = (UserServiceImpl) app.getBean("userService");
        userService.show();
    }
}

5. 开启tomcat，访问http://localhost:8080/demo02_war/userServlet，运行成功

二、通过web.xml指定springconfig文件，不使用ClassPathXmlApplicationContext

1. 编辑web.xml，添加全局初始化参数

<context-param>
    <param-name>contextConfig</param-name>
    <param-value>applicationContext.xml</param-value>
  </context-param>

2. 编辑listener目录下的ContextLoaderListener.class，从域中获取全局初始化参数，并生成app

public class ContextLoaderListener implements ServletContextListener {
    
    
    @Override
    public void contextInitialized(ServletContextEvent sce) {
    
    
//        ApplicationContext app = new ClassPathXmlApplicationContext("applicationContext.xml");
        //将app对象存储到最大的域ServletContext中
        ServletContext servletContext = sce.getServletContext();
        String contextConfig = servletContext.getInitParameter("contextConfig");//此值与web.xml中param-name一致
        ApplicationContext app = new ClassPathXmlApplicationContext(contextConfig);
        servletContext.setAttribute("app",app);
    }
}

三、配置Spring自带的监听器简化开发

1. 导入监听器坐标

<dependency>
	<groupId>org.springframework</groupId>
	<artifactId>spring-web</artifactId>
	<version>5.3.10</version>
</dependency>

2. 从web.xml中配置ContextLoaderListener监听器

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

3. 将listener目录删除
4. 编辑web目录下的UserServlet，使用webapplicationContextUtils获取app对象

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