Re: Proposed new Java feature

Robert Klemme <>
Mon, 28 May 2012 12:13:02 +0200
On 05/27/2012 08:14 PM, Mike Schilling wrote:

"Robert Klemme"<> wrote in message

2. ThreadLocals interact badly with ThreadPools, because the ThreadLocals
keep their value when the tyhread is put back into the pool. This can
to leaks and even potential security issues.

I would actually consider this good interaction with thread pools: the
local stays around for as long as the thread lives. If you introduce
security issues this way than you are probably not using thread locals
properly. There are two things that you generally need to consider with
thread locals which both result from the fact that the life time of a
thread local value extends across a current method call (i.e. earlier and

OK. Now, coinsider these two cases (for, say, a webserver):

1. I create a new thread to handle each new request.
2. I optimize (1) by using a thread pool to minimize thread creation.

I want those two to behave identically (other than performance). To acheive
that, I need to be able to kill all the ThreadLocals when putting the
Threads back into the pool for later reuse. Otherwise

No, you don't. If you employ proper cache size management schemes (e.g.
via a LRU) then you do not get rid of the ThreadLocal after each
request. If for any reason this is not possible and you want to get rid
of state after the request then it should be done per individual
ThreadLocal and not globally for all ThreadLocals.

A. The ThreadLocals for threads in the pool cause packratting.

Then you do not employ proper cache size management. You always need to
be aware of the life time difference between a method call and a
ThreadLocal and code appropriately. Not doing this is not following the
contract of ThreadLocal.

B. A reused thread contains context created during its previous use. This
may be context that the user correspnding to the request currently being
handled by the thread should not be able to see.

Then you are not using ThreadLocal properly, i.e. have omitted proper
cleanup (e.g. because you either did not do it or did not do it in a
safe way, i.e. finally block).

Kind regards


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