springmvc + hibernate 集成的时候出现PermGen（永久代）内存泄露问题

我把spring 和hibernate集成，如果我修改后台代码，那么tomcat就会重新发布，然后就出现这个问题了。我查了一下stackoverflow ,有说改永久代的大小的，也有说要停止服务器重启，不要重新发布。请问大家都是怎么搞的，然后PermGen我了解是存放类信息的地方，虽然垃圾收集器一般不清理这块，不过信息这么小怎么会爆掉呢，我现在代码里定义的类也没有超过20个，即时是第三方包也不至于吧，我有试过把PermGen的最大调至512m后还是会出现这个问题。

SEVERE: The web application [/NetMall] appears to have started a thread named [MySQL Statement Cancellation Timer] but has failed to stop it. This is very likely to create a memory leak.
Exception in thread "ContainerBackgroundProcessor[StandardEngine[Catalina]]" java.lang.OutOfMemoryError: PermGen space
    at java.lang.ClassLoader.defineClass1(Native Method)
    at java.lang.ClassLoader.defineClass(ClassLoader.java:792)
    at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:142)
    at org.apache.catalina.loader.WebappClassLoader.findClassInternal(WebappClassLoader.java:2818)
    at org.apache.catalina.loader.WebappClassLoader.findClass(WebappClassLoader.java:1148)
    at org.apache.catalina.loader.WebappClassLoader.loadClass(WebappClassLoader.java:1643)
    at org.apache.catalina.loader.WebappClassLoader.loadClass(WebappClassLoader.java:1521)
    at org.apache.catalina.startup.ContextConfig.checkHandlesTypes(ContextConfig.java:1956)
    at org.apache.catalina.startup.ContextConfig.processAnnotationsStream(ContextConfig.java:1919)
    at org.apache.catalina.startup.ContextConfig.processAnnotationsJar(ContextConfig.java:1806)
    at org.apache.catalina.startup.ContextConfig.processAnnotationsUrl(ContextConfig.java:1765)
    at org.apache.catalina.startup.ContextConfig.processAnnotations(ContextConfig.java:1751)
    at org.apache.catalina.startup.ContextConfig.webConfig(ContextConfig.java:1255)
    at org.apache.catalina.startup.ContextConfig.configureStart(ContextConfig.java:882)
    at org.apache.catalina.startup.ContextConfig.lifecycleEvent(ContextConfig.java:317)
    at org.apache.catalina.util.LifecycleSupport.fireLifecycleEvent(LifecycleSupport.java:119)
    at org.apache.catalina.util.LifecycleBase.fireLifecycleEvent(LifecycleBase.java:89)
    at org.apache.catalina.core.StandardContext.startInternal(StandardContext.java:5081)
    at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:145)
    at org.apache.catalina.core.StandardContext.reload(StandardContext.java:3832)
    at org.apache.catalina.loader.WebappLoader.backgroundProcess(WebappLoader.java:424)
    at org.apache.catalina.core.ContainerBase.backgroundProcess(ContainerBase.java:1205)
    at org.apache.catalina.core.ContainerBase$ContainerBackgroundProcessor.processChildren(ContainerBase.java:1391)
    at org.apache.catalina.core.ContainerBase$ContainerBackgroundProcessor.processChildren(ContainerBase.java:1401)
    at org.apache.catalina.core.ContainerBase$ContainerBackgroundProcessor.processChildren(ContainerBase.java:1401)
    at org.apache.catalina.core.ContainerBase$ContainerBackgroundProcessor.run(ContainerBase.java:1380)
    at java.lang.Thread.run(Thread.java:724)