Java中的监听器

向右看齐 2022-03-09 11:23 190阅读 0赞

**1、概念**

Servlet中的listener的作用是通过监听一些事件的发生，listener在事件发生前后可以做一些必要的处理。

**2、原理**

简单理解就是事件源生成事件并将其发送至一个或多个监听器，监听器简单地等待，直到它收到一个事件。一旦事件被接受，监听器将处理这些事件并进行一些处理。listner监听的主要事件抽象的理解包括三种：ServletContextEvent、ServletRequestEvent、 HttpSessionEvent。

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**3、八种监听器**

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> ServletContextEvent对Servlet上下文进行监听
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<table style="direction:ltr; border-collapse:collapse; border:1pt solid rgb(163,163,163)"> 
 <tbody> 
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 <td style="border-style:solid; border-color:#A3A3A3; border-width:1pt; vertical-align:top; width:1.6284in; padding:4pt 4pt 4pt 4pt"> 监听器 </td> 
 <td style="border-style:solid; border-color:#A3A3A3; border-width:1pt; vertical-align:top; width:1.7791in; padding:4pt 4pt 4pt 4pt"> 监听事件 </td> 
 <td style="border-style:solid; border-color:#A3A3A3; border-width:1pt; vertical-align:top; width:4.625in; padding:4pt 4pt 4pt 4pt"> 相关方法 </td> 
 </tr> 
 <tr> 
 <td style="border-style:solid; border-color:#A3A3A3; border-width:1pt; vertical-align:top; width:1.6284in; padding:4pt 4pt 4pt 4pt"> ServletContextListener </td> 
 <td style="border-style:solid; border-color:#A3A3A3; border-width:1pt; vertical-align:top; width:1.7791in; padding:4pt 4pt 4pt 4pt"> ServletContextEvent </td> 
 <td style="border-style:solid; border-color:#A3A3A3; border-width:1pt; vertical-align:top; width:3in; padding:4pt 4pt 4pt 4pt"> contextDestroyed(ServletContextEvent sce)&nbsp; ServletContext对象销毁时触发 （容器销毁时销毁ServletContext对象） contextInitialized(ServletContextEvent sce) &nbsp;ServletContext对象创建时触发 （容器加载时创建ServletContext对象） </td> 
 </tr> 
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 <td style="border-style:solid; border-color:#A3A3A3; border-width:1pt; vertical-align:top; width:1.6284in; padding:4pt 4pt 4pt 4pt"> ServletContextAttributeListener </td> 
 <td style="border-style:solid; border-color:#A3A3A3; border-width:1pt; vertical-align:top; width:1.7791in; padding:4pt 4pt 4pt 4pt"> ServletContextAttributeEvent （继承 ServletContextEvent） </td> 
 <td style="border-style:solid; border-color:#A3A3A3; border-width:1pt; vertical-align:top; width:4.625in; padding:4pt 4pt 4pt 4pt"> attributeAdded(ServletContextAttributeEvent event) ServletContext对象添加属性时触发 （ request.getSession().getServletContext().setAttribute("person1","Jim"); ） attributeRemoved(ServletContextAttributeEvent event) ServletContext对象移除属性时触发 （ request.getSession().getServletContext().removeAttribute("person1"); ） attributeReplaced(ServletContextAttributeEvent event) ServletContext对象重新设定属性值时触发 （ request.getSession().getServletContext().setAttribute("person1","Green"); request.getSession().getServletContext().setAttribute("person1","Green1"); ） </td> 
 </tr> 
 </tbody> 
</table>

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> ServletRequestEvent对客户端请求进行监听
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> > <table style="direction:ltr; border-collapse:collapse; border-style:solid; border-color:#A3A3A3; border-width:1pt"> 
> > <tbody> 
> > <tr> 
> > <td style="border-style:solid; border-color:#A3A3A3; border-width:1pt; vertical-align:top; width:2.0187in; padding:4pt 4pt 4pt 4pt"> 监听器 </td> 
> > <td style="border-style:solid; border-color:#A3A3A3; border-width:1pt; vertical-align:top; width:2.1909in; padding:4pt 4pt 4pt 4pt"> 针对事件 </td> 
> > <td style="border-style:solid; border-color:#A3A3A3; border-width:1pt; vertical-align:top; width:3.8222in; padding:4pt 4pt 4pt 4pt"> 相关方法 </td> 
> > </tr> 
> > <tr> 
> > <td style="border-style:solid; border-color:#A3A3A3; border-width:1pt; vertical-align:top; width:2.0187in; padding:4pt 4pt 4pt 4pt"> ServletRequest </td> 
> > <td style="border-style:solid; border-color:#A3A3A3; border-width:1pt; vertical-align:top; width:2.1909in; padding:4pt 4pt 4pt 4pt"> ListenerServletRequestEvent </td> 
> > <td style="border-style:solid; border-color:#A3A3A3; border-width:1pt; vertical-align:top; width:3.8222in; padding:4pt 4pt 4pt 4pt"> requestInitialized(ServletRequestEvent sre) 客户端请求发出时触发 requestDestroyed(ServletRequestEvent sre) 客户端请求结束时触发 &nbsp; </td> 
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> > <tr> 
> > <td style="border-style:solid; border-color:#A3A3A3; border-width:1pt; vertical-align:top; width:2.0187in; padding:4pt 4pt 4pt 4pt"> ServletRequestAttributeListener </td> 
> > <td style="border-style:solid; border-color:#A3A3A3; border-width:1pt; vertical-align:top; width:2.1909in; padding:4pt 4pt 4pt 4pt"> ServletRequestAttributeEvent （继承 ServletRequestEvent） </td> 
> > <td style="border-style:solid; border-color:#A3A3A3; border-width:1pt; vertical-align:top; width:3.8222in; padding:4pt 4pt 4pt 4pt"> attributeAdded(ServletRequestAttributeEvent srae) 客户端请求对象添加属性时触发（ request.setAttribute("person1","Jim"); ） attributeRemoved(ServletRequestAttributeEvent srae) 客户端请求对象移除属性时触发（ request.removeAttribute("person1"); ） attributeReplaced(ServletRequestAttributeEvent srae) 客户端请求对象重新设定属性值时触发（ request.setAttribute("person1","Green"); request.setAttribute("person1","Green1"); ） </td> 
> > </tr> 
> > </tbody> 
> > </table>
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> ServletRequestEvent对Http会话监听
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> > <table style="direction:ltr; border-collapse:collapse; border-style:solid; border-color:#A3A3A3; border-width:1pt"> 
> > <tbody> 
> > <tr> 
> > <td style="border-style:solid; border-color:#A3A3A3; border-width:1pt; vertical-align:top; width:2.0388in; padding:4pt 4pt 4pt 4pt"> 监听器 </td> 
> > <td style="border-style:solid; border-color:#A3A3A3; border-width:1pt; vertical-align:top; width:1.2138in; padding:4pt 4pt 4pt 4pt"> 监听事件 </td> 
> > <td style="border-style:solid; border-color:#A3A3A3; border-width:1pt; vertical-align:top; width:4.7798in; padding:4pt 4pt 4pt 4pt"> 相关方法 </td> 
> > </tr> 
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> > <td style="border-style:solid; border-color:#A3A3A3; border-width:1pt; vertical-align:top; width:2.0388in; padding:4pt 4pt 4pt 4pt"> HttpSessionListener </td> 
> > <td style="border-style:solid; border-color:#A3A3A3; border-width:1pt; vertical-align:top; width:1.2138in; padding:4pt 4pt 4pt 4pt"> HttpSessionEvent </td> 
> > <td style="border-style:solid; border-color:#A3A3A3; border-width:1pt; vertical-align:top; width:4.7798in; padding:4pt 4pt 4pt 4pt"> sessionCreated(HttpSessionEvent se) session对象创建时发生，即浏览器页面打开时 sessionDestroyed(HttpSessionEvent se) session对象销毁时触发，即浏览器关闭或session过期 &nbsp; </td> 
> > </tr> 
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> > <td style="border-style:solid; border-color:#A3A3A3; border-width:1pt; vertical-align:top; width:2.0388in; padding:4pt 4pt 4pt 4pt"> HttpSessionAttributeListener </td> 
> > <td style="border-style:solid; border-color:#A3A3A3; border-width:1pt; vertical-align:top; width:1.2138in; padding:4pt 4pt 4pt 4pt"> HttpSessionBindingEvent （继承 HttpSessionEvent） </td> 
> > <td style="border-style:solid; border-color:#A3A3A3; border-width:1pt; vertical-align:top; width:4.7798in; padding:4pt 4pt 4pt 4pt"> attributeAdded(HttpSessionBindingEvent event) session对象添加属性时触发（ session.setAttribute("person1","Jim"); ） attributeRemoved(HttpSessionBindingEvent event) 客户端请求对象移除属性时触发（ session.removeAttribute("person1"); ） attributeReplaced(HttpSessionBindingEvent event) session对象重新设定属性值时触发（ session.setAttribute("person1","Green"); session.setAttribute("person1","Green1"); ） </td> 
> > </tr> 
> > <tr> 
> > <td style="border-style:solid; border-color:#A3A3A3; border-width:1pt; vertical-align:top; width:2.0388in; padding:4pt 4pt 4pt 4pt"> HttpSessionBindingListener（ 这个listener是一个比较特殊的listener，它不需要在web.xml中进行配置。 此listener需要存放于session中的对象实现此接口，才能被响应。 ） </td> 
> > <td style="border-style:solid; border-color:#A3A3A3; border-width:1pt; vertical-align:top; width:1.2138in; padding:4pt 4pt 4pt 4pt"> HttpSessionBindingEvent （继承 HttpSessionEvent） </td> 
> > <td style="border-style:solid; border-color:#A3A3A3; border-width:1pt; vertical-align:top; width:4.7798in; padding:4pt 4pt 4pt 4pt"> valueBound(HttpSessionBindingEvent event) 此方法通过HttpSession的void setAttribute(String name, Object object)被调用。其中object需要实现HttpSessionBindingListener接口。 valueUnbound(HttpSessionBindingEvent event) 此方法通过HttpSession的void removeAttribute(String name)或者HttpSession失效时被调用。其中nam对应的object需要实现HttpSessionBindingListener接口。 </td> 
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> > <td style="border-style:solid; border-color:#A3A3A3; border-width:1pt; vertical-align:top; width:2.0388in; padding:4pt 4pt 4pt 4pt"> HttpSessionActivationListener （ 这个接口一般和Servlet容器的集群有关系，它监听的是同一个session转移至不同的JVM的事件。） </td> 
> > <td style="border-style:solid; border-color:#A3A3A3; border-width:1pt; vertical-align:top; width:1.2138in; padding:4pt 4pt 4pt 4pt"> HttpSessionEvent </td> 
> > <td style="border-style:solid; border-color:#A3A3A3; border-width:1pt; vertical-align:top; width:4.7798in; padding:4pt 4pt 4pt 4pt"> sessionDidActivate(HttpSessionEvent se) session对象已经变成有效的状态的时触发 sessionWillPassivate(HttpSessionEvent se) session对象将要变成无效状态的时触发 </td> 
> > </tr> 
> > </tbody> 
> > </table>
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[1366877245_9674.png]: https://img-my.csdn.net/uploads/201304/25/1366877245_9674.png