Thomas,
On 3/29/22 02:42, Thomas Hoffmann (Speed4Trade GmbH) wrote:
Hello Mark,
-Ursprüngliche Nachricht-
Von: Mark Eggers
Gesendet: Montag, 28. März 2022 23:55
An: users@tomcat.apache.org
Betreff: Re: AW: AW: AW: Question to possible memory leak by Threadlocal
variable
Thomas:
On 3/28/2022 2:01 PM, Thomas Hoffmann (Speed4Trade GmbH) wrote:
Hello Chris,
-Ursprüngliche Nachricht-
Von: Christopher Schultz
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
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
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:
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
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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- proce
