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All,
I've got a servlet that needs to log every request (potentially big
requests) to files on the disk. In order to do that in a
reasonably-tidy way, we write each file into a directory with the
current date in the path, something like this:
.../logs/2011-11-23/request-XYX.log
To do this, we have a SimpleDateFormat object that we use to ensure we
target the right directory. Since SimpleDateFormat isn't threadsafe,
we have two choices: synchronize or use ThreadLocal. We have opted for
the latter: ThreadLocal.
Our servlet defines the ThreadLocal to be protected (because this is a
base class for several servlets that all do similar things) and
transient (because we just don't need it to be serialized) and
override the initialValue method, like this:
protected transient ThreadLocal<SimpleDateFormat> dayFormat = new
ThreadLocal<SimpleDateFormat>() {
public SimpleDateFormat initialValue()
{
return new SimpleDateFormat("yyyy-MM-dd");
}
};
In the servlet's destroy method, we dutifully call dayFormat.remove().
Tomcat complains that we are leaving sloppy ThreadLocals around on
shutdown. Duh: Servlet.destroy is called by a single thread and won't
actually remove the ThreadLocal in any meaningful way.
So, my question is whether or not there is a good way to clean-out the
ThreadLocals from our webapp?
Given the declaration above, we are creating a new class which will be
loaded by our webapp's ClassLoader and therefore pinning that
ClassLoader in memory definitely causing a memory leak across reploy
cycles.
One way to avoid this would be to have a library at the server-level
that only contains this simple ThreadLocat<SimpleDateFormat>
definition, but that seems like kind of an awkward solution.
Removing the ThreadLocal after every request of course means that the
use of ThreadLocal is entirely useless.
Should I stop worrying about the overhead of creating a
SimpleDateFormat? Should I look for a threadsafe implementation of
SimpleDateFormat (maybe in commons-lang or something)? Should I
synchronize access to the object?
Any suggestions would be very helpful.
Thanks,
- -chris
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