Listener监听器

   Listener监听器用于监听web应用中某些对象、信息的创建、销毁、增加，修改，删除等动作的发生，然后作出相应的响应，常用于统计在线人数和在线用户，系统加载时进行信息初始化，统计网站的访问量等等。

一、监听器的分类

  1. 按监听的对象划分，可以分为：

   - ServletContext对象监听器
   - HttpSession对象监听器
   - ServletRequest对象监听器

  2. 按监听的事件划分，可以分为：

   - 监听对象自身创建和销毁的监听器
   - 监听对象属性变更的监听器
   - 感知Session绑定事件的监听器

二、监听器接口源码

• ServletContextListener

public abstract interface ServletContextListener extends EventListener
{
    public abstract void contextInitialized(ServletContextEvent paramServletContextEvent);

    public abstract void contextDestroyed(ServletContextEvent paramServletContextEvent);
}

• ServletContextAttributeListener

public abstract interface ServletContextAttributeListener extends EventListener
{
    public abstract void attributeAdded(ServletContextAttributeEvent paramServletContextAttributeEvent);

    public abstract void attributeRemoved(ServletContextAttributeEvent paramServletContextAttributeEvent);

    public abstract void attributeReplaced(ServletContextAttributeEvent paramServletContextAttributeEvent);
}

• HttpSessionListener

public abstract interface HttpSessionListener extends EventListener
{
    public abstract void sessionCreated(HttpSessionEvent paramHttpSessionEvent);

    public abstract void sessionDestroyed(HttpSessionEvent paramHttpSessionEvent);
}

• HttpSessionAttributeListener

public abstract interface HttpSessionAttributeListener extends EventListener
{
    public abstract void attributeAdded(HttpSessionBindingEvent paramHttpSessionBindingEvent);

    public abstract void attributeRemoved(HttpSessionBindingEvent paramHttpSessionBindingEvent);

  public abstract void attributeReplaced(HttpSessionBindingEvent paramHttpSessionBindingEvent);
}

• HttpSessionActivationListener

public abstract interface HttpSessionActivationListener extends EventListener
{
    public abstract void sessionWillPassivate(HttpSessionEvent paramHttpSessionEvent);

    public abstract void sessionDidActivate(HttpSessionEvent paramHttpSessionEvent);
}

• HttpSessionBindingListener

public abstract interface HttpSessionBindingListener extends EventListener
{
    public abstract void valueBound(HttpSessionBindingEvent paramHttpSessionBindingEvent);

    public abstract void valueUnbound(HttpSessionBindingEvent paramHttpSessionBindingEvent);
}

• ServletRequestListener

public abstract interface ServletRequestListener extends EventListener
{
    public abstract void requestDestroyed(ServletRequestEvent paramServletRequestEvent);

    public abstract void requestInitialized(ServletRequestEvent paramServletRequestEvent);
}

• ServletRequestAttributeListener

public abstract interface ServletRequestAttributeListener extends EventListener
{
    public abstract void attributeAdded(ServletRequestAttributeEvent paramServletRequestAttributeEvent);

    public abstract void attributeRemoved(ServletRequestAttributeEvent paramServletRequestAttributeEvent);

  public abstract void attributeReplaced(ServletRequestAttributeEvent paramServletRequestAttributeEvent);
}

三、监听器的使用实例

1. ServletContextListener
2. ServletContextAttributeListener
3. HttpSessionListener
4. HttpSessionAttributeListener
5. HttpSessionActivationListener
6. HttpSessionBindingListener
7. ServletRequestListener
8. ServletRequestAttributeListener