JVM标志CMSClassUnloadingEnabled实际上做了什么？ [英] What does JVM flag CMSClassUnloadingEnabled actually do?
问题描述
我不能为我的生活找到Java VM标志 CMSClassUnloadingEnabled 实际上做的定义，除了一些非常模糊的高级定义，如摆脱您的PermGen问题（它没有，顺便说一句。
I cannot for the life of me find a definition of what the Java VM flag CMSClassUnloadingEnabled actually does, other than some very fuzzy high-level definitions such as "gets rid of your PermGen problems" (which it doesn't, btw).
我查看了Sun的/ Oracle网站，甚至选项列表实际上并没有说出它的作用。
I have looked on Sun's/Oracle's site, and even the options list doesn't actually say what it does.
基于名称标志，我猜测CMS垃圾收集器默认情况下没有卸载类，这个标志打开它 - 但我不能确定。
Based upon the name of the flag, I'm guessing that the CMS Garbage Collector doesn't by default unload classes, and this flag turns it on - but I can't be sure.
推荐答案
更新这个答案与Java 5-7相关，Java 8修复了这个问题： https://blogs.oracle.com/poonam/about-g1-garbage-collector,-permanent-generation-and-metaspace 荣誉转到 mt.uulu
Update This answer is relevant for Java 5-7, Java 8 has this fixed: https://blogs.oracle.com/poonam/about-g1-garbage-collector,-permanent-generation-and-metaspace Kudos go to mt.uulu
对于Java 5-7：
For Java 5-7:
世界上标准的Oracle / Sun VM外观是：类是永远的。因此，一旦加载，即使没有人关心，它们也会留在记忆中。这通常没有问题，因为你没有那么多纯粹的设置类（=一次用于安装，然后再没有用过）。因此，即使他们占用1MB，谁也在乎。
The standard Oracle/Sun VM look on the world is: Classes are forever. So once loaded, they stay in memory even if no one cares anymore. This usually is no problem since you don't have that many purely "setup" classes (= used once for setup and then never again). So even if they take up 1MB, who cares.
但是最近，我们有像Groovy这样的语言，它们在运行时定义了类。每次运行脚本时，都会创建一个（或多个）新类，它们永远保留在PermGen中。如果您正在运行服务器，则意味着您有内存泄漏。
But lately, we have languages like Groovy, that define classes at runtime. Every time you run a script, one (or more) new classes are created and they stay in PermGen forever. If you're running a server, that means you have a memory leak.
如果启用 CMSClassUnloadingEnabled GC也将扫除PermGen，并删除不再使用的类。
If you enable CMSClassUnloadingEnabled the GC will sweep PermGen, too, and remove classes which are no longer used.
您还必须启用 UseConcMarkSweepGC （感谢 Sam Hasler ）。请参阅此答案： https://stackoverflow.com/a/3720052/2541
You will also have to enable UseConcMarkSweepGC (thanks to Sam Hasler). See this answer: https://stackoverflow.com/a/3720052/2541
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