ThreadLocal Memory Leak

最新推荐文章于 2022-04-06 14:10:22 发布

转载最新推荐文章于 2022-04-06 14:10:22 发布 · 678 阅读

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#permgen leak #memory leak #thread local #weak reference

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本文探讨了 PermGen 泄露问题及其与 ThreadLocal 的关联性，特别是在应用服务器中部署 Web 应用时可能出现的 classloader 泄露问题。当 Web 应用未能正确清理 ThreadLocal 中的数据，可能导致永久代内存泄露，进而引发一系列问题。

PermGen exhaustions in combination with ThreadLocal are often caused by classloader leaks .

An example:

Imagine an application server which has a pool of worker threads .

They will be kept alive until application server termination.

A deployed web application uses a static ThreadLocal in one of its classes in order to store some thread-local data, an instance of another class (lets call it SomeClass ) of the web application. This is done within the worker thread (e.g. this action originates from a HTTP request ).

Important:

By definition , a reference to a ThreadLocal value is kept until the "owning" thread dies or if the ThreadLocal itself is no longer reachable.

If the web application fails to clear the reference to the ThreadLocal on shutdown , bad things will happen:

Because the worker thread will usually never die and the reference to the ThreadLocal is static, the ThreadLocal value still references the instance of SomeClass , a web application's class - even if the web application has been stopped!

As a consequence, the web application's classloader cannot be garbage collected , which means that all classes (and all static data) of the web application remain loaded (this affects the PermGen memory pool as well as the heap).

Every redeployment iteration of the web application will increase permgen (and heap) usage.

=> This is the permgen leak