JavaWeb中的listener简述

概念：web的三大组件之一。
* 事件监听机制
* 事件 ：一件事情
* 事件源 ：事件发生的地方
* 监听器 ：一个对象
* 注册监听：将事件、事件源、监听器绑定在一起。 当事件源上发生某个事件后，执行监听器代码

• ServletContextListener:监听ServletContext对象的创建和销毁
  • 方法：
    • void contextDestroyed(ServletContextEvent sce) ：ServletContext对象被销毁之前会调用该方法
    • void contextInitialized(ServletContextEvent sce) ：ServletContext对象创建后会调用该方法
  • 步骤：
    1. 定义一个类，实现ServletContextListener接口
    2. 复写方法
    3. 配置
       1. web.xml
          2. 注解：
          @WebListener

 <listener>
        <listener-class>com.code_g.listener.ListenerDemo</listener-class>
    </listener>
    <context-param>
        <param-name>applicationContext</param-name>
        <param-value>WEB-INF/classes/applicationContext.xml</param-value>
    </context-param>

import javax.servlet.ServletContext;
import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;
import javax.servlet.annotation.WebListener;
import javax.servlet.http.HttpSessionAttributeListener;
import javax.servlet.http.HttpSessionEvent;
import javax.servlet.http.HttpSessionListener;
import javax.servlet.http.HttpSessionBindingEvent;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.IOException;

@WebListener
public class ListenerDemo implements ServletContextListener,
        HttpSessionListener, HttpSessionAttributeListener {

    // Public constructor is required by servlet spec
    public ListenerDemo() {
    }

    // -------------------------------------------------------
    // ServletContextListener implementation
    // -------------------------------------------------------
    public void contextInitialized(ServletContextEvent sce) {
        System.out.println("ServletContext对象创建...");

        ServletContext servletContext = sce.getServletContext();
        String context = servletContext.getInitParameter("applicationContext");
        String realPath = servletContext.getRealPath(context);
        try {
            FileInputStream fileInputStream = new FileInputStream(realPath);
            System.out.println(fileInputStream);
            try {
                fileInputStream.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        }
    }

    public void contextDestroyed(ServletContextEvent sce) {
        System.out.println("ServletContext对象销毁...");
    }

    // -------------------------------------------------------
    // HttpSessionListener implementation
    // -------------------------------------------------------
    public void sessionCreated(HttpSessionEvent se) {
        /* Session is created. */
    }

    public void sessionDestroyed(HttpSessionEvent se) {
        /* Session is destroyed. */
    }

    // -------------------------------------------------------
    // HttpSessionAttributeListener implementation
    // -------------------------------------------------------

    public void attributeAdded(HttpSessionBindingEvent sbe) {
      /* This method is called when an attribute 
         is added to a session.
      */
    }

    public void attributeRemoved(HttpSessionBindingEvent sbe) {
      /* This method is called when an attribute
         is removed from a session.
      */
    }

    public void attributeReplaced(HttpSessionBindingEvent sbe) {
      /* This method is invoked when an attribute
         is replaced in a session.
      */
    }
}