tomcat5.5 reload 出现方法区溢出的问题

        最近在用eclipse调试一个老项目，server用的tomcat5.5， reload了几次应用就报错，方法区溢出：

Exception in thread "main" java.lang.OutOfMemoryError: PermGen space。

用visualvm查看heap dump，发现项目中的第三方jar包的类都被重新加载了好几次，最后方法区空间不足，一直进行fullgc，却无法回收重复的类。
  用tomcat7相同环境下运行正常，reload几次后，方法区空间不足时，会回收所有原先WebappClassLoader重复加载的类，。
2个tomcat jvm运行模式和参数都一致，都是使用默认的垃圾回收机制，后来tomcat5.5试着用了CMS垃圾收集器还是报原先的错误：-XX:+UseConcMarkSweepGC -XX:+CMSClassUnloadingEnabled -XX:+CMSPermGenSweepingEnabled
应该是tomcat5.5 reload代码实现有问题

tomcat7的reload源码，应该tomcat7在reload时清理引用做了写改进吧，WebappClassLoader处理内存泄露 clearReferences():

    /**     * Clear references.     */    protected void clearReferences() {        // De-register any remaining JDBC drivers        clearReferencesJdbc();        // Stop any threads the web application started        clearReferencesThreads();        // Check for leaks triggered by ThreadLocals loaded by this class loader        checkThreadLocalsForLeaks();        // Clear RMI Targets loaded by this class loader        clearReferencesRmiTargets();        // Null out any static or final fields from loaded classes,        // as a workaround for apparent garbage collection bugs        if (clearReferencesStatic) {            clearReferencesStaticFinal();        }         // Clear the IntrospectionUtils cache.        IntrospectionUtils.clear();        // Clear the classloader reference in common-logging        if (clearReferencesLogFactoryRelease) {            org.apache.juli.logging.LogFactory.release(this);        }        // Clear the resource bundle cache        // This shouldn't be necessary, the cache uses weak references but        // it has caused leaks. Oddly, using the leak detection code in        // standard host allows the class loader to be GC'd. This has been seen        // on Sun but not IBM JREs. Maybe a bug in Sun's GC impl?        clearReferencesResourceBundles();        // Clear the classloader reference in the VM's bean introspector        java.beans.Introspector.flushCaches();    }

查了相关的资料：

 一般程序库可能存在内存泄漏的地方有：

 1.JDBC驱动注册

2.一些日志框架

3.在ThreadLocal中保存对象，但是并不去删除它 

4.启动了线程，但没有停止它 

而Java API存在内存泄漏的地方包括：

1.使用javax.imageio API (the Google Web Toolkit can trigger this) 

2.使用java.beans.Introspector.flushCaches() (Tomcat does this to prevent memory leaks caused by this caching) 

3.使用XML解析器(the root cause is unknown due to a bug in the JRE) 

4.使用RMI远程方法调用(somewhat ironically, causes a leak related to the garbage collector)  

 5.从Jar文件中读取资源