Re: Concurrency Questions

["Randolph S. Kahle" <RandyKahle@KahleAssociates.com>]

On Wed, 2001-10-24 at 08:04, Dale Bronk - Windscape wrote:
>   Hello,
> 
>   I have two questions.  Please assume multiprocessor machines with each
> thread running on separate cpu's.  Also, if you can point me to any
> reference that backs your opinion/fact would be great.  Am I safe on both of
> the following?
> 
>   1.  Lets say I have the following code:
> 
>   public class MyTest extends Thread
>   {
>       static int i = 0;
> 
>       // Please not NO sync's
>       run()
>       {
>           i++;
>       }
>   }
> 
>   Now let's say that I spawn 2 of these and they are on different cpu's and
> they both execute i++ at the same time.  Is there EVER a chance that the
> result of i can EVER be 1 or will it always be 2.
> 

i++ is not an atomic operation. If you do not synchronize you may/will
run into trouble.


> 
>   2.  Now here is another code snippet:
> 
>   static ArrayList myStaticList = null;
>   public ArrayList getList(boolean shouldRefresh)
>   {
>       if (shouldRefresh)
>       {
>           // Not only sync on refreshing, not getting
>           synchronized (someLockObject)
>           {
>               ArrayList newList = new ArrayList();
>               //  Code here to get new list from database and fill newList
>               myStaticList = newList;
>           }
>       }
>       return myStaticList;
>   }
> 
>   My question is that let's say that one thread (different cpu) is executing
> return myStaticList at the same time as another thread is executing
> myStaticList = newList.  Is it possible that the return myStaticList will
> EVER return a half/limbo state object or worse yet a Dr. Watson or
> something?

I think your question is whether a reference assignment is atomic. Yes,
it is.

Randy

> 
>   Help is wanted and appreciated.
> 
>   Thanks,
>   Dale
> 
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