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From Christopher Schultz <>
Subject Re: WebappClassLoader weak/soft reachable
Date Tue, 06 Nov 2012 21:02:49 GMT
Hash: SHA1


On 11/6/12 2:59 PM, Konstantin Kolinko wrote:
> 2012/11/6 Christopher Schultz <>:
>> On 11/6/12 10:37 AM, Konstantin Kolinko wrote:
>>> 2012/11/6 Christopher Schultz <>:
>>> 2) I know of a bug in JVM when a certain kind of leak goes 
>>> undetected. I mean the leak through a shared Exception
>>> instance and its native stacktrace. See
>>> (Linked from
>> I don't create any of my own Exception objects that get shared.
>> Could this be something introduced by Tomcat itself, or more
>> likely some dependent library of mine?
>> Since I can't readily trace the WebappClassLoader to its GC
>> roots, do you have any suggestions for identifying the specific
>> cause in my case?
> IIRC, I found that issue by looking for Exception objects that
> were present in the memory. I was using Eclipse MAT.  IIRC I
> filtered the living objects by their class name.

Okay, the good news is that there are very few objects with the word
"Exception" in their name. Better still, many of them are classes like
ExceptionHandler and thus not likely to be exceptions themselves.

These are the actual Exception objects I can see:

3 instances of org.apache.tomcat.util.buf.UDecoder$DecodeException

1 instance of

(I am using a Tomcat-configured JNDI DataSource)

1 instance of

The few other Exception objects that I can see, I inspected and they
said "no GC paths found" even when including soft/weak/finalizer

Does that help at all? I can provide details about the GC paths for
the above Tomcat-related exceptions if that helps.

Tomcat 7.0.32

I realized when disclosing the Tomcat version that I had been running
off Tomcat 7.0.x-trunk, so I have repeated my tests on Tomcat 7.0.32
with a bit more information:

1. Tomcat start
2. List applications in manager - no active sessions in main webapp
3. Reload target webapp
4. Find leaks: none reported
5. Reload webapp again
6. Find leaks again: none reported

So far, so good.

7. Access protected page in target webapp (session is created),
   no authentication, yet (Tomcat redirects to login page, which
   invokes some Apache Struts routing code and a simple Velocity
8. Reload & find leaks: found a leak

Performed 3 full GCs from within Yourkit, then took a memory snapshot
for inspection. I see the aforementioned Exception objects in memory,
but they all have o.a.c.loader.StandardClassLoader as their parent

Yourkit lists the following interesting memory tidbits:

Objects unreachable from GC roots, but not yet collected: 68358
Objects reachable from GC roots via weak and/or soft references only:
Objects pending finalization (finalizer queue objects unreachable via
strong references): 288

It still looks like there is quite a bit of garbage laying around,
even after 3 full GC requests (with a noticeable drop in memory usage
when having performed my 3-GCs-in-a-row above, so the JVM isn't
ignoring GC requests).

Let's keep going. There's a session in memory.

9. Stop Tomcat (sessions are persisted to disk)
10. Start Tomcat (sessions are loaded from disk)
11. List Applications, confirm 1 active session
12. Inspect session. It contains the following objects:
  A ClientPreferences object for our own application (basically a
  wrapper around Map<String,String>)

13. Reload, find leaks: found a leak

So, it doesn't even require that I actually access a resource within
my webapp: it is enough to have an active session.

14. Expire all sessions
15. Reload, find leaks: found *2* leaks
16. Stop Tomcat (no sessions persisted: all expired in step #14)
17. Start Tomcat
18. Reload, find leaks: none found

So something my session loading triggers is causing the leak it seems.

I'll take a closer look at the ClientPrefs class to see if there's
anything odd going on in there.

- -chris
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