Java中如何使用Listener

定义

用于监听Web应用的内部事件的实现类。可以监听用户session的开始与结束，用户请求的到达等等，当事件发生时，会回调监听器的内部方法

使用Listener步骤

1. 通过实现具体接口创建实现类（可实现多个监听器接口）

2. 配置实现类成为监听器，有两种配置方式：
   

   1. 直接用@WebListener注解修饰实现类
   2. 通过web.xml方式配置，代码如下：

    com.zrgk.listener.MyListener

常用Web事件监听器接口

@WebListener
public class MyServetContextListener implements ServletContextListener{

    //web应用关闭时调用该方法
    @Override
    public void contextDestroyed(ServletContextEvent event) {
        ServletContext application = event.getServletContext();
        String userName = application.getInitParameter("userName"); 
        System.out.println("关闭web应用的用户名字为："+userName);
    }

    //web应用启动时调用该方法
    @Override
    public void contextInitialized(ServletContextEvent event) {
        ServletContext application = event.getServletContext();
        String userName = application.getInitParameter("userName");     
        System.out.println("启动web应用的用户名字为："+userName);
    }

}

1. ServletContextListener

1. 该接口用于监听Web应用的启动与关闭
2. 该接口的两个方法：
   
   1. contextInitialized(ServletContextEvent event); // 启动web应用时调用
   2. contextDestroyed(ServletContextEvent event); // 关闭web应用时调用
3. 如何获得application对象：
   ServletContext application = event.getServletContext();
4. 示例

   @WebListener
   public class MyServetContextListener implements ServletContextListener{
   
       //web应用关闭时调用该方法
       @Override
       public void contextDestroyed(ServletContextEvent event) {
           ServletContext application = event.getServletContext();
           String userName = application.getInitParameter("userName"); 
           System.out.println("关闭web应用的用户名字为："+userName);
       }
   
       //web应用启动时调用该方法
       @Override
       public void contextInitialized(ServletContextEvent event) {
           ServletContext application = event.getServletContext();
           String userName = application.getInitParameter("userName");     
           System.out.println("启动web应用的用户名字为："+userName);
       }
   
   }

   
   

   2. ServletContextAttributeListener

   1. 该接口 用于监听ServletContext范围（application）内属性的改变。
   2. 该接口的两个方法：
      
      1. attributeAdded(ServletContextAttributeEvent event); // 当把一个属性存进application时触发
      2. attributeRemoved(ServletContextAttributeEvent event); // 当把一个属性从application删除时触发
      3. attributeReplaced(ServletContextAttributeEvent event); // 当替换application内的某个属性值时触发
   3. 如何获得application对象：
      ServletContext application = event.getServletContext();
   4. 示例

   @WebListener
   public class MyServletContextAttributeListener implements ServletContextAttributeListener{
   
       //向application范围内添加一个属性时触发
       @Override
       public void attributeAdded(ServletContextAttributeEvent event) {
           String name = event.getName();//向application范围添加的属性名
           Object val = event.getValue();      //向application添加的属性对应的属性值
           System.out.println("向application范围内添加了属性名为："+name+"，属性值为："+val+"的属性");
   
       }
   
       //删除属性时触发
       @Override
       public void attributeRemoved(ServletContextAttributeEvent event) {
           // ...      
       }
   
       //替换属性值时触发
       @Override
       public void attributeReplaced(ServletContextAttributeEvent event) {
           // ...      
       }
   
   }

   
   

   3. ServletRequestListener与ServletRequestAttributeListener

   1. ServletRequestListener用于监听用户请求，而ServletRequestAttributeListener用于监听request范围内属性的变化。
   2. ServletRequestListener两个需要实现的方法
      
      1. requestInitialized(ServletRequestEvent event); //用户请求到达、被初始化时触发
      2. requestDestroyed(ServletRequestEvent event); // 用户请求结束、被销毁时触发
   3. ServletRequestAttributeListener两个需要实现的方法
      
      1. attributeAdded(ServletRequestAttributeEvent event); // 向request范围内添加属性时触发
      2. attributeRemoved(ServletRequestAttributeEvent event); // 从request范围内删除某个属性时触发
      3. attributeReplaced(ServletRequestAttributeEvent event); // 替换request范围内某个属性值时触发
   4. 获取reqeust对象
      HttpServletRequest req = (HttpServletRequest)event.getServletRequest();
   5. 代码片

   @WebListener
   public class MyRequestListener implements ServletRequestListener,ServletRequestAttributeListener{
   
       //用户请求结束、被销毁时触发
       @Override
       public void requestDestroyed(ServletRequestEvent event) {
           HttpServletRequest req = (HttpServletRequest) event.getServletRequest();
           String ip = req.getRemoteAddr();
           System.out.println("IP为:"+ip+"的用户发送到"+req.getRequestURI()+"的请求结束");
       }
   
       //用户请求到达、被初始化时触发
       @Override
       public void requestInitialized(ServletRequestEvent event) {
           HttpServletRequest req = (HttpServletRequest) event.getServletRequest();
           String ip = req.getRemoteAddr();
           System.out.println("IP为:"+ip+"的用户发送到"+req.getRequestURI()+"的请求被初始化");
       }
   
       //向request范围内添加属性时触发
       @Override
       public void attributeAdded(ServletRequestAttributeEvent event) {
           String name = event.getName();
           Object val = event.getValue();
           System.out.println("向request范围内添加了名为："+name+"，值为："+val+"的属性");
       }
   
       //删除request范围内某个属性时触发
       @Override
       public void attributeRemoved(ServletRequestAttributeEvent event) {
           //...       
       }
   
       //替换request范围内某个属性值时触发
       @Override
       public void attributeReplaced(ServletRequestAttributeEvent event) {
           // ...      
       }
   }

   
   

   4. HttpSessionListener与HttpSessionAttributeListener

   1. HttpSessionListener监听用户session的开始与结束，而HttpSessionAttributeListener监听HttpSession范围（session）内的属性的改变。
   2. HttpSessionListener要实现的方法：
      
      1. sessionCreated(HttpSessionEvent event); // 用户与服务器的会话开始、创建时触发
      2. sessionDestroyed(HttpSessionEvent event); // 用户与服务器的会话结束时触发
   3. HttpSessionAttributeListener要实现的方法：
      
      1. attributeAdded(HttpSessionBindingEvent event) ; // 向session范围内添加属性时触发
      2. attributeRemoved(HttpSessionBindingEvent event); // 删除session范围内某个属性时触发
      3. attributeReplaced(HttpSessionBindingEvent event); // 替换session范围内某个属性值时触发
   4. 如何得到session对象
      HttpSession session = event.getSession();
   5. 代码片

   @WebListener
   public class MySessionListener implements HttpSessionListener,HttpSessionAttributeListener {
   
       //建立session会话时触发
       @Override
       public void sessionCreated(HttpSessionEvent event) {
           HttpSession session = event.getSession();
           String sessionId = session.getId();
           System.out.println("建立了会话，会话ID为："+sessionId);
       }
   
       @Override
       public void sessionDestroyed(HttpSessionEvent event) {
           // ...      
       }
   
       //向session范围内添加属性时触发
       @Override
       public void attributeAdded(HttpSessionBindingEvent event) {
           String name = event.getName();
           Object val = event.getValue();
           System.out.println("向session范围内添加了名为："+name+",值为："+val+"的属性");
       }
   
       @Override
       public void attributeRemoved(HttpSessionBindingEvent event) {
           // ...      
       }
   
       @Override
       public void attributeReplaced(HttpSessionBindingEvent event) {
           // ...      
       }
   
   }