I have no idea what the phrase take the use of means; what are you
trying to say?
I'd like to know if there's some statement from the tomcat team if the
usage of ThreadLocals within contextInitialized() is discouraged or even
not supported.
??? A ThreadLocal is _not_ inherited from the parent thread, although
the _value_ of each InheritableThreadLocal is copied to a spawned
thread.
I meant exactly that.
Consider the situation:
1. contextInitializer() sets value A to the ThreadLocal X in thread
main
2. childs threads get spawned from main thread, now we have more than
one ThreadLocal which references to value A
3. Reference to ThreadLocal X gets dropped, but references to value A
still exist - without being able to remove them.
Why is it not an option? Even if your code calls some 3rd-party
package to do its work, you can clean up the mess created before
returning from the initializer.
This means that everyone which uses tomcat has to deeply analyze which
ThreadLocals are created under which circumstances,
and everytime a 3rd party library is added/upgraded/replaced he has to
do this again.
Beside the risk, that something will be missed...
Tomcat can't work around every possible programming silliness.
Agreed.
But wouldn't it be much easier for everyone if tomcat would always use
a separate thread for each context initializer (even if parallel
initialization is disabled)
than to rely on that every possible programming silliness in
3rd-party code will be fixed?
(BTW, Log4j is one of those buggy libraries - see
https://issues.apache.org/bugzilla/show_bug.cgi?id=50486 for details)
- Patric
Am 26.01.2012 15:50, schrieb Caldarale, Charles R:
From: Patric Rufflar [mailto:pat...@rufflar.com]
Subject: ThreadLocals, context listeners and classloader leaks
It's unspecified in the servlet spec 2.5 if a servlet context
listener is allowed to take the use of ThreadLocals during
contextInitialized().
I have no idea what the phrase take the use of means; what are you
trying to say? Any thread can make use of ThreadLocal, but in a
thread pool environment, it's usually silly to do so, unless you're
very, very careful.
Is this (also) allowed in tomcat?
Tomcat makes little effort to prevent stupid programmer tricks.
Because ThreadLocals are inherited from its parent threads
??? A ThreadLocal is _not_ inherited from the parent thread, although
the _value_ of each InheritableThreadLocal is copied to a spawned
thread.
the inherited ThreadLocals are still there and reference to the
original values - and will most probably remain in the spawned
threads forever
Which is an example of why you must be very, very careful in your use
of ThreadLocal in a pooled environment.
Is the main thread usage for context initializers intended?
Yes. In Tomcat 7, you can have parallel initialization, but not in
Tomcat 6. Read the docs for Engine and Host.
What's the best way to deal with this situation (especially
if the avoidance of ThreadLocals in context initializers is
not an option)?
Why is it not an option? Even if your code calls some 3rd-party
package to do its work, you can clean up the mess created before
returning from the initializer.
Wouldn't it be better to create an individual thread for (each)
context initializer to avoid these kind of problems?
Tomcat can't work around every possible programming silliness.
- Chuck
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