@WebListener注解简介说明

@Weblistener注解简介

`@WebListener`是Java Servlet 3.0引入一个标准注解(Annotation)
   用于声明一个类为Web应用的监听器(Listener)
   它可以监听Web应用的生命周期事件和HTTP会话、
     请求等状态变化

替代传统`web.xml`中 `<listener>`配置方式
  使开发者可以更简洁地定义监听逻辑

`@WebListener`注解主要功能

`@WebListener`注解定义(简化版)

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.Runtime)
public @interface WebListener {
    String value() default "";
}
 
- `value()`：可选描述信息，通常不使用

例 1：监听 Web 应用启动和关闭(`ServletContextListener`)

@WebListener
public class AppContextListener implements ServletContextListener {

    @Override
    public void contextInitialized(ServletContextEvent sce) {
        System.out.println("Web 应用启动中...");
        ServletContext context = sce.getServletContext();
        context.setAttribute("appStartTime", System.currentTimeMillis());
    }

    @Override
    public void contextDestroyed(ServletContextEvent sce) {
        System.out.println("Web 应用正在关闭...");
    }
}

例 2：监听用户会话创建与销毁(`HttpSessionListener`)

@WebListener
public class SessionCounterListener implements HttpSessionListener {

    private static int activeSessions = 0;

    @Override
    public void sessionCreated(HttpSessionEvent se) {
        activeSessions++;
        System.out.println("新会话创建，当前在线人数：" + activeSessions);
    }

    @Override
    public void sessionDestroyed(HttpSessionEvent se) {
        activeSessions--;
        System.out.println("会话销毁，当前在线人数：" + activeSessions);
    }
}

例 3：监听请求开始与结束(`ServletRequestListener`)

@WebListener
public class RequestLoggerListener implements ServletRequestListener {

    @Override
    public void requestInitialized(ServletRequestEvent sre) {
        HttpServletRequest request = (HttpServletRequest) sre.getServletRequest();
        System.out.println("请求开始: " + request.getRequestURI());
    }

    @Override
    public void requestDestroyed(ServletRequestEvent sre) {
        HttpServletRequest request = (HttpServletRequest) sre.getServletRequest();
        System.out.println("请求结束: " + request.getRequestURI());
    }
}

例 4：监听会话属性变化(`HttpSessionAttributeListener`)

@WebListener
public class SessionAttributeListener implements HttpSessionAttributeListener {

    @Override
    public void attributeAdded(HttpSessionBindingEvent event) {
        System.out.println("会话属性添加: " + event.getName() + "=" + event.getValue());
    }

    @Override
    public void attributeRemoved(HttpSessionBindingEvent event) {
        System.out.println("会话属性移除: " + event.getName());
    }

    @Override
    public void attributereplaced(HttpSessionBindingEvent event) {
        System.out.println("会话属性替换: " + event.getName());
    }
}

`@WebListener`注解与 `web.xml` 对比

使用`web.xml`传统写法

 
<listener>
    <listener-class>com.example.AppContextListener</listener-class>
</listener>
<listener>
    <listener-class>com.example.SessionCounterListener</listener-class>
</listener>

常见的监听器类型