Re: using ConcurrentHashMaps

From:

Robert Klemme <shortcutter@googlemail.com>

Newsgroups:

comp.lang.java.programmer

Date:

Sat, 21 Apr 2007 11:07:56 +0200

Message-ID:

<58u2jdF2g96ikU1@mid.individual.net>

On 21.04.2007 02:33, Tom Hawtin wrote:

Robert Klemme wrote:

On 20.04.2007 20:01, Tom Hawtin wrote:

Vaibhav wrote:

I came across usage of
Collections.synchronizedMap(new ConcurrentHashMap())
Can this be useful?

I think that's just confused...

You could conceivably make use of a ConcurrentHashMap passed through
synchronizedMap if you have another reference to the original.
However, I would use synchronized explicitly in those cases.

I cannot think of a scenario where that would be useful. Do you have
an example? I mean, usually you would use CHM in order to get faster
and more efficient synchronization as with synchronized, wouldn't you?

Consider the case when you want a lock on all of your writes, but not
your reads. Perhaps because constructing objects for writes interferes,
or you have to return a map that client code has some (partial)
expectation that it will be a synchronised map. You can use the wrapped
map for writes and the raw map for reads. However, that'd probably be
considerably confusing.

I see. That makes sense. Although in that case I'd rather use a
reentrant lock as this has shown to be faster than synchronized in my tests.

OTOH, I guess there are some corner cases. For instances, making use
of unsynchronised iterators.

I am not sure what you mean by this.

In the original posting there is a synchronised loop. Map the map a
synchronised concurrent map, and you can lose that synchronised whilst
still being able to use the lock for multiple operations that should be
atomic (with regards to everything but that iterator).

Yeah, basically a similar scenario as above: you use the CHM for "basic"
thread safety and add synchronization mechanisms for special scenarios /
methods / operations.

Thanks for clarifying!

Kind regards

robert