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解析web.xml中常常需要配置几个Spring核心类

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在JavaEE的程序中,我们往往要配置web.xml文件。这个文件的重要性,我就不说了。说说它与Spring框架结合时,所需要配置的几个类吧。

1. org.springframework.web.util.IntrospectorCleanupListener. 在web.xml中,我们常常的配置如下。

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<listener>  
    <listener-class>org.springframework.web.util.IntrospectorCleanupListener</listener-class>  
</listener>

它的主要作用是负责处理因JavaBean Inteceptor的使用而引起的缓存泄露问题。Spring 的文档中,对这个类进行了详细的说明(Spring 3.x)。

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* Listener that flushes the JDK's {@link java.beans.Introspector JavaBeans Introspector}  
* cache on web app shutdown. <strong>在web.xml文件中,注册这个监听器保证恰到地释放web应用程序类加载及它所加载的</strong>  
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<strong> * 类</strong>(Register this listener in your <code>web.xml</code> to) guarantee proper release of   
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* the web application class loader and its loaded classes.  
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*  
* <p><b>If the JavaBeans Introspector has been used to analyze application classes,  
* the system-level Introspector cache will hold a hard reference to those classes.  
* Consequently, those classes and the web application class loader will not be  
* garbage-collected on web app shutdown!</b> This listener performs proper cleanup,  
* to allow for garbage collection to take effect.  
*  
* <p>Unfortunately, the only way to clean up the Introspector is to flush  
* the entire cache, as there is no way to specifically determine the  
* application's classes referenced there. This will remove cached  
* introspection results for all other applications in the server too.  
*  
* <p>Note that this listener is <i>not</i> necessary when using Spring's beans  
* infrastructure within the application, as Spring's own introspection results  
* cache will immediately flush an analyzed class from the JavaBeans Introspector  
* cache and only hold a cache within the application's own ClassLoader.  
*  
* <b>Although Spring itself does not create JDK Introspector leaks, note that this  
* listener should nevertheless be used in scenarios where the Spring framework classes  
* themselves reside in a 'common' ClassLoader (such as the system ClassLoader).</b>  
* In such a scenario, this listener will properly clean up Spring's introspection cache.  
*  
* <p>Application classes hardly ever need to use the JavaBeans Introspector  
* directly, so are normally not the cause of Introspector resource leaks.  
* Rather, many libraries and frameworks do not clean up the Introspector:  
* e.g. Struts and Quartz.(当然啦,许多库包和框架不会清理拦截器,如Struts 和 Quartz)  
* 既然这样,这样一个简单的缓存泄露会引起整个web引用程序的类加载不能被垃圾回收器回收。  
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* <p>Note that a single such Introspector leak will cause the entire web  
* app class loader to not get garbage collected! This has the consequence that  
* you will see all the application's static class resources (like singletons)  
* around after web app shutdown, which is not the fault of those classes!  
*  
* <p><b>This listener should be registered as the first one in <code>web.xml</code>,  
* before any application listeners such as Spring's ContextLoaderListener.</b>  
* This allows the listener to take full effect at the right time of the lifecycle.

正如文档中所说的,“在web.xml中,这个监听器应该在任何应用程序监听器,比如Spring’s ContextLoaderListener之前,作为第一个监听器被注册(上面文档最后一段)。在应用程序运行周期的正确时刻,让监听器充分发挥作用” 。
2. org.springframework.web.context.ContextLoaderListener. 这个类是程序引导监听器去开启和关闭Spring的Root,它注册在web.xml文件中的org.springframework.web.util.Log4jConfigListener配置之后。

3. org.springframework.web.context.request.RequestContextListener. 它显露Request给当前的线程(Current Thread). 这个类主要用作第三方servlet,例如the JSF FacesServlet。在Spring自己的web支持下, DispatcherServlet的处理是相当地有能力的.

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解析web.xml中常常需要配置几个Spring核心类:等您坐沙发呢!

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