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 all > >>>> the 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. But > >>>> on 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.org > >>> > >>> Hello 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 > >>> before > >> the 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 storage > is 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 > >> be > better not to have one in the first place. > >> > >> Why not name the library? Why anonymize the object type if it's > >> org.apache.something? > >> > >> -chris > > > > Hello Chris, > > I didn't want to blame any library 😉 But as you ask for it, I send more > details. > > > > 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.bas > > e/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- > engi > > ne/src/main/java/org/apache/camel/impl/DefaultCamelContext.java > > Interesting 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.org
I 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 --------------------------------------------------------------------- To unsubscribe, e-mail: users-unsubscr...@tomcat.apache.org For additional commands, e-mail: users-h...@tomcat.apache.org
