IntrospectorCleanupListener问题

spring中有一个比较少人注意的Listener——org.springframework.web.util.IntrospectorCleanupListener,这个类的说明如下：
它是一个在web应用关闭的时候,清除JavaBeans Introspector缓存的监听器.在web.xml中注册这个listener.可以保证在web 应用关闭的时候释放与掉这个web 应用相关的class loader 和由它加载的类
如果你使用了JavaBeans Introspector来分析应用中的类,系统级Introspector 缓冲中会保留这些类的hard引用。结果在你的web应用关闭的时候,这些类以及web 应用相关的class loader没有被垃圾收集器回收.
不幸的是,清除Introspector的唯一方式是刷新整个缓存。这是因为我们没法判断哪些是属于你的应用的引用.所以删除被缓冲的introspection会导致把这台server上的所有应用的introspection（内省）结果都删掉.
需要注意的是,spring 托管的bean不需要使用这个监听器.因为spring它自己的introspection所使用的缓冲在分析完一个类之后会被马上从javaBeans Introspector缓冲中清除掉(上面提到的代码说明了这一点）。
一 般的应用基本不会直接用到JavaBean的内省方法，所以一般不用考虑遇到此类内省资源泄露，但是，很多的类库或者框架（比如struts, Quartz）没有清除Introspector。请注意，这个监听器需要注册在web.xml中的所有应 用监听器之前（比如ContentLoaderListener之前）。

spring中对它的描述如下：
    它是一个在web应用封闭的时辰，清除JavaBeans Introspector的器.在web.xml中注册这个listener.可以包管在web 应用封闭的时辰开释与掉这个web 应用相干的class loader 和由它经管的类。
    若是你应用了JavaBeans Introspector来解析应用中的类，Introspector 缓冲中会保存这些类的引用.成果在你的应用封闭的时辰，这些类以及web 应用相干的class loader没有被垃圾收受接管.
    不幸的是，清除Introspector的独一体式格式是刷新全部缓冲.这是因为我们没法断定哪些是属于你的应用的引用.所以删除被缓冲的introspection会导致把这台电脑上的所有应用的introspection都删掉.
    须要重视的是，spring 托管的bean不须要应用这个器.因为spring它本身的introspection所应用的缓冲在解析完一个类之后会被即速从javaBeans Introspector缓冲中清除掉.

Listener that flushes the JDK's JavaBeans Introspector cache on web app shutdown. Register this listener in your web.xml to guarantee proper release of the web application class loader and its loaded classes.

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! This listener performs proper cleanup, to allow for garbage collection to take effect.

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.

Note that this listener is not 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. 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). In such a scenario, this listener will properly clean up Spring's introspection cache.

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.

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!

This listener should be registered as the first one in web.xml, before any application listeners such as Spring's ContextLoaderListener. This allows the listener to take full effect at the right time of the lifecycle.

用法很简单，就是在web.xml中加入:

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

Refer:

http://anoxia.blogbus.com/logs/34360203.html