Re: synchronize vs gate

From:

Robert Klemme <shortcutter@googlemail.com>

Newsgroups:

comp.lang.java.programmer

Date:

Mon, 04 Jun 2007 16:30:07 +0200

Message-ID:

<5cilv4F30mrrsU2@mid.individual.net>

On 04.06.2007 02:09, Tom Hawtin wrote:

christopher@dailycrossword.com wrote:

I have a singleton in a web service that provides a collection and
self-updates it on a periodic basis:

doSelfUpdate()

  create temporary collection (time consuming)
  syncronize
    update temporary collection from current collection (fast)
    create temporary reference 'old' to current collection
    point current collection reference to temporary collection
  end syncronize
  do stuff with 'old' collection (time consuming)
  done

getCollection()
  synchronize
  just waiting on monitor
  end synchronize
  return collection
  done

The obvious thing to do (from Java 1.5 spec, and 1.4 implementation) is
to make the reference to the collection volatile. Keep the lock in
doSelfUpdate, but not getCollection. Alternatively just use
java.util.concurrent.ConcurrentHashmap.

Yet another alternative would be to use a ReadWriteLock that is held
during update of the reference only and use an immutable collection.
This of course works best if it is made sure through other means that
there is just one updater at a time - although two concurrent updaters
might only cost additional CPU. The logic should still work, you only
get two (or n) updates of the reference in a short period of time. If
the update process itself needs additional synchronization that can be
either provided by another Lock or different logic. Difficult to
speculate without knowing more detail...

Kind regards

robert