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From Simon Kitching <>
Subject RE: Logging: SimpleLog not thread-safe
Date Sat, 14 Oct 2006 08:04:59 GMT

On Fri, 2006-10-13 at 00:10 -0400, Kenneth Xu wrote:
> Yes, it'll be GC'ed when thread is. And initialized when first use in a new
> thread. Here is the test code:

But if an application has long-running threads then the object won't be
recycled until the thread dies. So an app with 100 threads has 100
SimpleDateFormat objects long-term. 

And as James noted, when using frameworks like Application Servers,
threads could be pooled in unexpected ways.

I also suspect that in order to fetch data from a ThreadLocal, the JVM
effectively performs a get on a synchronised map, ie that ThreadLocal is
not much more efficient than having a synchronised static DateFormat on
SimpleLog (nb: I have no proof of this, just a hunch).

I think the easiest & most reliable solution is to simply create a
SimpleDateFormat object in the method that needs it. Note that this is
only done after it has been determined that a message *will* be output,
which in most cases means that there will be disk io that will have far
more impact on the system than creating a simple object. Optimising the
path in commons-logging that determines *if* a message is to be logged
is important; optimising the actual logging operation is much less

Of course I'm open to persuasion on this (eg performance stats)..



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