Re: using ConcurrentHashMaps

Robert Klemme <>
Fri, 20 Apr 2007 19:47:11 +0200
On 20.04.2007 19:35, Vaibhav wrote:

On Apr 20, 11:49 am, Tom Hawtin <> wrote:

Vaibhav wrote:

thats sounds good. I have another question along the same line.. under
which scenario we should not remove synchronize block while migrating
to CHM? If there is such a block.. is it still ok to use
synchronize(m) or just stay with SHM in that case..

Synchronised blocks are not much use when it comes to ConcurrentHashMap,
as it doesn't use synchronized itself.

Iterators (of ConcurrentHashMap) happen to behave reasonably in the face
concurrent modification. Otherwise atomic operations need to be single
operations, hence the extra methods in the ConcurrentMap interface.

Tom Hawtin

Can you throw som e light on how usage of local variables is effected
in a multi-threaded environment.. all examples I found refer to
instance variables and do not say anything about local vars.

Local variables are thread local. However, if you have a local variable
of a reference type (i.e. everything that is not a int, long etc.) and
the reference is shared among threads all sorts of things can happen
depending on the usage patterns.

I get the impression that you first should get the basics right. I
suggest you get yourself a copy of Doug Lea's Book about concurrent
programming in Java and read up on the basic concepts or at least read
some primer on the web about the matter.

Kind regards


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