RE: Concurrency Questions

["Fuqua, Andrew (ISSAtlanta)" <AFuqua@iss.net>]

Doug Lea, Concurrent Programming in Java, 2nd ed.

 

pg6:

- always lock during updates to object fields

- always lock during access of possibly updated object fields

- never lock when invoking methods on other objects

 

pg 93:

"Access and updates to the memory cells corresponding to fields of any type except long or double are guaranteed to be atomic. This includes fields serving as references to other objects. Additionally, atomicity extends to volatile long and double. ... However as seen below, atomicity alone does not guarantee that you will get the value most recently written by any thread. For this reason, atomicity guarantees per se normally have little impact on concurrent program design."

 

pg 95:

"The model also allows inconsistent visibility in the absence of synchronization. For example, it is possible to obtain a fresh value for one field of an object, but a stale value for another. Similarly, it is possible to read a fresh, updated value of a reference variable, but a stale value of one of the fields of the object now being referenced." !!

 

THEREFORE...

 

In #1, yes, i CAN be 1.

 

In #2, you should never get a Dr. Watson. The question is, can the 1st thread get visibility to the new arraylist before the initialization is complete? With a little uncertainty on my part, I believe the answer to that is yes, the newlist reference could leak out before its initialization is complete.

 

The only guarantee for safety in these situations is for thread 2 to get the very same lock that thread 1 just released.

 

I strongly recommend anyone working with threads to read Lea's book and also "The Java Language Specification", Gosling, Joy, Steele.

 

regards

 

-----Original Message-----
From: Dale Bronk - Windscape [mailto:dbronk@windscape.com]
Sent: Wednesday, October 24, 2001 11:04 AM
To: ajug-members@ajug.org
Subject: Concurrency Questions

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.

 

 

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?

 

Help is wanted and appreciated.

 

Thanks,

Dale