Java Listener 监听器作用

Java 中 Listener监听器作用

1.1    Listener对象分类
Servlet的Listener监听器包括三种类型：
1、ServletContextListener    监听ServletContext对象

public void contextInitialized(ServletContextEvent event);
public void contextDestoryed(ServletContextEvent event);

public void contextInitialized(ServletContextEvent event);
public void contextDestoryed(ServletContextEvent event);

2、HttpSessionListener 监听Session对象

public void sessionCreated(HttpSessionEvent event);
public void sessionDestoryed(HttpSessionEvent event);

public void sessionCreated(HttpSessionEvent event);
public void sessionDestoryed(HttpSessionEvent event);

1、    HttpRequestListener 监听Request对象

public void requestinitialized(ServletRequestEvent event);
public void requestDestoryed(ServletRequestEvent event);

public void requestinitialized(ServletRequestEvent event);
public void requestDestoryed(ServletRequestEvent event);

Web.xml文件配置

<listener>
    <listener-class>listener.MyListener</listener-class>
</listener>

<listener>
        <listener-class>listener.MyListener</listener-class>
    </listener>

1.2    Listener按属性分类
按照监听事件类型划分分为如下类型：
1、    用于监听域对象自身的创建和销毁的事件监听器。
2、    用于监听域对象的属性的增加和删除的事件监听器。
3、    用于监听绑定到HttpSession域中的某个对象的状态的事件监听器。
1.2.1    监听对象的属性事件监听器接口

1、    attributeAdd当被监听域对象中增加属性时会调用该系列监听器

public void attributeAdded(ServletContextAttributeEvent event);
public void attributeAdded(HttpSessionBindingEvent event);
public void attributeAdded(ServletRequestAttributeEvent event);

public void attributeAdded(ServletContextAttributeEvent event);
public void attributeAdded(HttpSessionBindingEvent event);
public void attributeAdded(ServletRequestAttributeEvent event);

2、    attributeRemoved当被监听域对象中删除属性时会调用该系列监听器

public void attributeRemoved(ServletContextAttributeEvent event);
public void attributeRemoved(HttpSessionBindingEvent event);
public void attributeRemoved(ServletRequestAttributeEvent event);

public void attributeRemoved(ServletContextAttributeEvent event);
public void attributeRemoved(HttpSessionBindingEvent event);
public void attributeRemoved(ServletRequestAttributeEvent event);

3、    attributeReplaced当被监听域对象中属性变更时时会调用该系列监听器

public void attributeReplaced(ServletContextAttributeEvent event);
public void attributeReplaced(HttpSessionBindingEvent event);
public void attributeReplaced(ServletRequestAttributeEvent event);

public void attributeReplaced(ServletContextAttributeEvent event);
public void attributeReplaced(HttpSessionBindingEvent event);
public void attributeReplaced(ServletRequestAttributeEvent event);

注：使用属性监听器需要继承如下接口，实现以上方法

ServletContextAttributeListener,

HttpSessionAttributeListener,
ServletRequestAttributeListener

ServletContextAttributeListener,

HttpSessionAttributeListener, 
ServletRequestAttributeListener

1.2.2    感知Session绑定的事件监听器
保存到Session域中的对象可以有多种状态：
1、    绑定到Session域中
2、    从Session域中解除绑定
3、    Session对象持久化到存储设备
4、    Session对象从一个存储设备中恢复
可以使用

HttpSessionBindingListener、HttpSessionActivationListener

HttpSessionBindingListener、HttpSessionActivationListener

两个监听接口实现JavaBean对象的绑定，从而了解JavaBean对象在Session域的状态。

HttpSessionBindingListener
public void valueBound(HttpSessionBindingEvent event);
public void valueUnbound(HttpSessionBindingEvent event);
HttpSessionActivationListener
public void sessionWillPassivate(HttpSessionEvent event);

HttpSessionBindingListener
public void valueBound(HttpSessionBindingEvent event);
public void valueUnbound(HttpSessionBindingEvent event);
HttpSessionActivationListener
public void sessionWillPassivate(HttpSessionEvent event);

注：当Session对象持久化到文件系统时，激活上面方法。

public void sessionDidActivate(HttpSessionEvent event);

public void sessionDidActivate(HttpSessionEvent event);

注：当Session对象从文件系统恢复时，激活上面方法。