Thomas: On 3/28/2022 2:01 PM, Thomas Hoffmann (Speed4Trade GmbH) wrote:
Hello Chris,-----Ursprüngliche Nachricht----- Von: Christopher Schultz <ch...@christopherschultz.net> Gesendet: Montag, 28. März 2022 18:48 An: users@tomcat.apache.org Betreff: Re: AW: AW: Question to possible memory leak by Threadlocal variable Thomas, On 3/25/22 16:59, Thomas Hoffmann (Speed4Trade GmbH) wrote:-----Ursprüngliche Nachricht----- Von: Christopher Schultz <ch...@christopherschultz.net> Gesendet: Freitag, 25. März 2022 14:05 An: users@tomcat.apache.org Betreff: Re: AW: Question to possible memory leak by Threadlocal variable Thomas, On 3/24/22 05:49, Thomas Hoffmann (Speed4Trade GmbH) wrote:-----Ursprüngliche Nachricht----- Von: Mark Thomas <ma...@apache.org> Gesendet: Donnerstag, 24. März 2022 09:32 An: users@tomcat.apache.org Betreff: Re: Question to possible memory leak by Threadlocal variable On 24/03/2022 07:57, Thomas Hoffmann (Speed4Trade GmbH) wrote: <snip/>Is it correct, that every spawned thread must call tl.remove() to cleanup allthe references to prevent the logged warning (and not only the main thread)? Yes. Or the threads need to exit.Second question is: How might it cause a memory leak? The threads are terminated and hold a reference to this static variable. Buton the other side, that class A is also eligible for garbage collection after undeployment.So both, the thread class and the class A are ready to get garbage collected. Maybe I missed something (?)It sounds as if the clean-up is happening too late. Tomcat expects clean-up to be completed once contextDestroyed() has returned for all ServLetContextListeners. If the clean-up is happening asynchronously(e.g.the call is made to stop the threads but doesn't wait until the threads have stopped) you could see this message. In this case it sounds as if you aren't going to get a memory leak but Tomcat can't tell that at the point it checks. Mark ------------------------------------------------------------------- -- To unsubscribe, e-mail: users-unsubscr...@tomcat.apache.org For additional commands, e-mail: users-h...@tomcat.apache.orgHello Mark, thanks for the information. The shutdown of the framework is currently placed within the destroy()method of a servlet (with load on startup).At least the debugger shows that servlet-->destroy() is executed beforethe method checkThreadLocalMapForLeaks() runs.I will take a look, whether the threads already exited.Tomcat only checks its own request-processing threads for ThreadLocals, so any threads created by the application or that library are unrelated to the warning you are seeing. Any library which saves ThreadLocals from request-processing threads is going to have this problem if the objects are of types loaded by the webapp ClassLoader. There are a few ways to mitigate this, but they are ugly and it would be better if the library didn't use ThreadLocal storage, or if it would use vanilla classes from java.* and not its own types. You say that those objects are eligible for GC after the library shuts down, but that's not true: anything you stick in ThreadLocal storageis being held ...by the ThreadLocal storage and won't be GC'd. If an object can't be collected, the java.lang.Class defining it can't be collected, and therefore the ClassLoader which loaded it (the webapp ClassLoader) can't be free'd. We call this a "pinned ClassLoader" and it still contains all of the java.lang.Class instances that the ClassLoader ever loaded during its lifetime. If you reload repeatedly, you'll see un-collectable ClassLoader instances piling up in memory which is *definitely* a leak. The good news for you is that Tomcat has noticed the problem and will, over time, retire and replace each of the affected Threads in its request- processing thread pool. As those Thread objects are garbage-collected, the TheradLocal storage for each is also collected, etc. and *eventually* your leak will be resolved. But it would bebetter not to have one in the first place.Why not name the library? Why anonymize the object type if it's org.apache.something? -chrisHello Chris, I didn't want to blame any library 😉 But as you ask for it, I send moredetails.Regarding the ThreadLocal thing: I thought that the threadlocal variables are stored within the Thread-class in the member variable "ThreadLocal.ThreadLocalMap threadLocals": https://github.com/AdoptOpenJDK/openjdk-jdk11/blob/master/src/java.base/share/classes/java/lang/Thread.java So I thought, when the thread dies, these variables will also be released and automatically removed from the ThreadLocal variable / instance (?)This is correct, but if the ThreadLocal is being stored in the request- processing thread, then when your web application is redeployed, the request processing threads outlive that event. Maybe you thought your application gets a private set of threads for its own use, but that's not the case: Tomcat pools threads across all applications deployed on the server. You can play some games to segregate some applications from others, but it's a lot of work for not much gain IMO. Since the threads outlive the application, you can see the problem, now.I considered the ThreadLocal class as just the manager of the thread's member variable "threadLocals".Basically, yes.Regarding the library: The full log-message is: 12-Mar-2022 15:01:16.302 SCHWERWIEGEND [Thread-15]org.apache.catalina.loader.WebappClassLoaderBase.checkThreadLocalMapF orLeaks The web application [ROOT] created a ThreadLocal with key of type [java.lang.ThreadLocal.SuppliedThreadLocal] (value [java.lang.ThreadLocal$SuppliedThreadLocal@2121cbad]) and a value of type [org.apache.camel.impl.DefaultCamelContext.OptionHolder] (value [org.apache.camel.impl.DefaultCamelContext$OptionHolder@338d0413]) but failed to remove it when the web application was stopped. Threads are going to be renewed over time to try and avoid a probable memory leak. >The blamed class is this version: https://github.com/apache/camel/blob/camel-3.14.x/core/camel-core-engine/src/main/java/org/apache/camel/impl/DefaultCamelContext.javaInteresting that Camel is storing a ThreadLocal. Maybe there is a better way to use Camel in the context of a web application?Within our app we have a startup servlet with: Servlet-> init: context = new DefaultCamelContext(); Servlet -> destroy: context.stop(); The stop-method will call the doStop() method of this class (via the class hierarchy DefaultCamelContext --> SimpleCamelContext --> AbstractCamelContext). > After the destroy-method is executed, all spawned threads of Camel are stopped / vanished. There is also no log entry, that some orphaned threads exist when undeploying the app. So I don’t know, what's the mistake within this class. What would be the right way to clean up the ThreadLocal variable? Just stopping the threads didn’t seem to clean it up properly.The saved ThreadLocal was done from within one of the request-processing threads that Tomcat owns. This wasn't a thread spawned by the library, which is likely to already be cleaned-up when stop() is completed, as expected. It looks like Camel may be capturing some values and storing them in ThreadLocal when it doesn't make sense (in a web application) to do so. Are you able to instrument your application to see when those ThreadLocals are set? -chris --------------------------------------------------------------------- To unsubscribe, e-mail: users-unsubscr...@tomcat.apache.org For additional commands, e-mail: users-h...@tomcat.apache.orgI think I understand now, how the memory leak is created. So lets say Tomcat has three worker Threads W1, W2 and W3. If every one of them is using the CamelContext, then all of them will inherit this ThreadLocal-Value within their worker threads. I will debug into the library and try to confirm this. Thanks to your explanation, the leak report makes sense to me now. Right now I don’t have a clue, how all the workers might release that ThreadLocal variable. I will let you know, what the debugger says. Thanks! Thomas
This was just released: https://camel.apache.org/releases/release-3.16.0/ And according to the release notes: https://issues.apache.org/jira/browse/CAMEL-17712 was addressed. Is this useful? . . . just my two cents /mde/
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