1.简介
Listener是Servlet的监听器，Servlet 监听器用于监听一些重要事件的发生，监听器对象在事情发生前、发生后可以做一些必要的处理。
JavaWeb里面的listener是通过观察者设计模式进行实现的。
观察者模式又叫发布订阅模式或者监听器模式。在该模式中有两个角色：观察者和被观察者（通常也叫做主题）。观察者在主题里面注册自己感兴趣的事件，当这个事件发生时，主题会通过回调接口的方式通知观察者。
举个生活中的例子：订阅报纸。任何一个家庭或个人都可以向报社订阅报纸。这里报社就是“主题”，家庭就是“观察者”。比如家庭需要订阅明天早晨的报纸，这个就是“事件”。到了第二天早晨，报纸生产出来了，这个就是“事件发生”。当事件发生时，送报员将报纸送到家庭的信箱里面，这里的信箱就是“回调接口”。
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" 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对于JavaWeb里面的监听器，Servlet规范定义了一些列的Listener接口类，通过接口类的方式将事件暴露给应用程序，应用程序如果想监听其感兴趣的事件，那么不必去直接注册对应的事件，而是编写自己的listener实现相应的接口类，并将自己的listener注册到servlet容器。当程序关心的事件发生时，servlet容器会通知listener，回调listener里面的方法。这里自定义的listener就是观察者，servlet容器就是主题。
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" 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2.分类
现在我们分析下，servlet规范为我们定义了哪些事件。更准确的说是定义了哪些监听接口。下面的介绍都是以servlet3.0规范为准。
servlet3.0为我们提供了8个监听器接口，按照它们的作用域来划分的话可以分为三类：
1.servlet上下文相关监听接口：
- ServletContextListener
- ServletContextAttributeListener
2.http session相关监听接口：
- HttpSessionListener
- HttpSessionActivationListener
- HttpSessionAttributeListener
- HttpSessionBindingListener
3.servlet request相关监听接口：
- ServletRequestListener
- ServletRequestAttributeListener
3.Java编写Listener实例
Listener编写分为两步：
- 编写自己的listener，实现特定的Listener接口
- 在web.xml里面注册自己的listener（也可以通过注解的方式，道理是一样的）
ListenerDemo.java(这里继承ServletContextListener接口）：
public class ListenerDemo implements ServletContextListener {
@Override
public void contextInitialized(ServletContextEvent servletContextEvent) {
System.out.println("----------ServletContextListener.contextInitialized-----------");
}
@Override
public void contextDestroyed(ServletContextEvent servletContextEvent) {
System.out.println("----------ServletContextListener.contextDestroyed----------");
}
}
web.xml:
<listener>
<listener-class>com.xzh.ListenerDemo</listener-class>
</listener>
效果图：
注意：结果中可以看到listener比filter先执行。
4.源码分析
4.1 ServletContextListener
public interface ServletContextListener extends EventListener {
void contextInitialized(ServletContextEvent var1);
void contextDestroyed(ServletContextEvent var1);
}
servlet上下文监听器接口，对应着两个事件：servlet上下文初始化事件和servlet上下文即将关闭事件。
当web应用初始化的时候，servlet容器会构造ServletContextEven实例，并回调contextInitialize方法。当servlet上下文即将关闭时，一般是关闭服务器之前，servlet容器会构造ServletContextEven实例，并回调contextDestroyed方法。这里需要注意的是，contextDestroyed方法的执行会在所有的servlet和filter执行完destroy方法之后。
4.2 ServletContextAttributeListener
public interface ServletContextAttributeListener extends EventListener {
void attributeAdded(ServletContextAttributeEvent var1);
void attributeRemoved(ServletContextAttributeEvent var1);
void attributeReplaced(ServletContextAttributeEvent var1);
}
当servlet上文属性发生增、删、改的时候，servlet容器构造ServletContextAttributeEvent事件对象，分别回调attributeAdded、attributeRemoved、attributeReplaced方法。
这里需要注意的是attributeReplaced方法，当属性的值被替换的时候回调。这个时候如果调用ServletContextAttributeEvent.getValue()方法返回的是替换之前的属性值。
4.3 HttpSessionListener
public interface HttpSessionListener extends EventListener {
void sessionCreated(HttpSessionEvent var1);
void sessionDestroyed(HttpSessionEvent var1);
}
当session被创建和销毁的时候，servlet容器构造HttpSessionEvent事件对象，并回调sessionCreated和sessionDestroyed方法。
4.4 HttpSessionActivationListener
public interface HttpSessionActivationListener extends EventListener {
void sessionWillPassivate(HttpSessionEvent var1);
void sessionDidActivate(HttpSessionEvent var1);
}
当session将要钝化或已被激活时，servlet容器构造HttpSessionEvent事件对象，回调sessionWillPassivate和sessionDidActivate方法。
这里解释下钝化和激活：钝化是指服务器内存不够了或者session的活动超时时间到了，把最近不活动的session序列化到磁盘。激活是指某个钝化的session又被访问了，从磁盘将session反序列化到内存。
这里可以看出要想钝化和激活，首先session得可序列化和反序列化。同时我们在编程过程中，session尽量用String、Integer等简单的对象，尽量不要用list、map等集合。3.5
4.5 HttpSessionAttributeListener
public interface HttpSessionAttributeListener extends EventListener {
void attributeAdded(HttpSessionBindingEvent var1);
void attributeRemoved(HttpSessionBindingEvent var1);
void attributeReplaced(HttpSessionBindingEvent var1);
}
当session属性发生增、删、改的时候，servlet容器构造HttpSessionBindingEvent事件对象，分别回调attributeAdded、attributeRemoved、attributeReplaced方法。
这里需要注意的是attributeReplaced方法，当属性的值被替换的时候回调。这个时候如果调用ServletContextAttributeEvent.getValue()方法返回的是替换之前的属性值。
当调用session的invalidate方法或者session失效时，也会回调attributeRemoved方法。
4.6 HttpSessionBindingListener
public interface HttpSessionBindingListener extends EventListener {
void valueBound(HttpSessionBindingEvent var1);
void valueUnbound(HttpSessionBindingEvent var1);
}
这个监听器也是监听session的属性变化。当session属性发生增和删，也就是属性值绑定和属性值解绑的时候，servlet容器构造HttpSessionBindingEvent事件对象，分别回调valueBound、valueUnbound方法。
这么一看和HttpSessionAttributeListener没什么区别，其实不是这样。两者有个本质的区别就是事件触发的条件。
当session的属性有任何的变化，servlet容器都会通知HttpSessionAttributeListener。但是对于HttpSessionBindingListener，只有当绑定或解绑的属性值是监听器的实例时，servlet容器才会通知。
4.7 ServletRequestListener
public interface ServletRequestListener extends EventListener {
void requestDestroyed(ServletRequestEvent var1);
void requestInitialized(ServletRequestEvent var1);
}
当请求初始化或者销毁时，即客户端请求进入web应用（进入servlet或者第一个filter）或web应用返回响应给客户端（退出servlet或者第一个filter）。servlet容器构造ServletRequestEvent实例，回调requestInitialized和requestDestroyed方法。
4.8 ServletRequestAttributeListener
public interface ServletRequestAttributeListener extends EventListener {
void attributeAdded(ServletRequestAttributeEvent var1);
void attributeRemoved(ServletRequestAttributeEvent var1);
void attributeReplaced(ServletRequestAttributeEvent var1);
}
当请求的属性发生增、删、改的时候，servlet容器构造ServletRequestAttributeEvent事件对象，分别回调attributeAdded、attributeRemoved、attributeReplaced方法。
这里需要注意的是attributeReplaced方法，当属性的值被替换的时候回调。这个时候如果调用ServletRequestAttributeEvent.getValue()方法返回的是替换之前的属性值。
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