在web.xml中注册IntrospectorCleanupListener监听器以解决struts等框架可能产生的内存泄露问题

转载于：http://blog.csdn.net/yakoo5/article/details/8240925

增加方式如下：

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

org.springframework.web.util.IntrospectorCleanupListener源代码中对其的解释如下：

        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.

        在Web应用程序关闭时IntrospectorCleanupListener将会刷新JDK的JavaBeans的Introspector缓存。在 你的web.xml中注册这个listener来确保Web应用程序的类加载器以及其加载的类正确的释放资源。

        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.      

       如果JavaBeans的Introspector已被用来分析应用程序类，系统级的Introspector缓存将持有这些类的一 个硬引用。因此，这些类和Web应用程序的类加载器在Web应用程序关闭时将不会被垃圾收集器回收！而 IntrospectorCleanupListener则会对其进行适当的清理，已使其能够被垃圾收集器回收。

       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.

       不幸的是，唯一能够清理Introspector的方法是刷新整个Introspector缓存，没有其他办法来确切指定应用程序所引用的类。这将删除所有其他应用程序在服务器的缓存的Introspector结果。

       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.

       请注意，在使用Spring内部的bean机制时，不需要使用此监听器，因为Spring自己的introspection results cache将会立即刷新被分析过的JavaBeans Introspector cache，而仅仅会在应用程序自己的ClassLoader里面持有一个cache。虽然Spring本身不产生泄漏，注意，即使在Spring框架的 类本身驻留在一个“共同”类加载器（如系统的ClassLoader）的情况下，也仍然应该使用使用 IntrospectorCleanupListener。在这种情况下，这个IntrospectorCleanupListener将会妥善清理 Spring的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.

       应用程序类，几乎不需要直接使用JavaBeans Introspector，所以，通常都不是Introspector resource造成内存泄露。相反，许多库和框架，不清理Introspector，例如： Struts和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!

        需要注意的是一个简单Introspector泄漏将会导致整个Web应用程序的类加载器不会被回收！这样做的结果，将会是在web应用程序关闭时，该应 用程序所有的静态类资源（比如：单实例对象）都没有得到释放。而导致内存泄露的根本原因其实并不是这些未被回收的类！

       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. 

       IntrospectorCleanupListener应该注册为web.xml中的第一个Listener，在任何其他 Listener之前注册，比如在Spring's ContextLoaderListener注册之前，才能确保IntrospectorCleanupListener在Web应用的生命周期适当时机 生效。