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volatile keyword in java 1.3

Posted on 2008-10-02
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Last Modified: 2012-05-05
Hi,

Does the volatile keyword make a primitive safe to read/write from multiple threads without guarding it? I don't know where I read that but - something like:

public class Test
{
    public volatile m_flag;

    new Thread() {
        public void run() {
            while(...) {
                m_flag = false;
                print(m_flag);
            }
        }
    }.start();

    new Thread() {
        public void run() {
            while(...) {
                m_flag = true;
                print(m_flag);
            }
        }
    }.start();
}

I mean is setting and getting m_flag atomic now, so no thread is reading or writing it at the same time? I can put a guard object around it but it seems annoying to do for a simple boolean primitive,

Thanks
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Question by:DJ_AM_Juicebox
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Expert Comment

by:mnrz
ID: 22627536
no I think  this is not safe, you had better to change it into this line:

private volatile java.util.concurrent.atomic.AtomicBoolean m_flag = false;

atomic types make your variable safe in read/write concurrently
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Expert Comment

by:mnrz
ID: 22627545

private volatile java.util.concurrent.atomic.AtomicBoolean m_flag = false;

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Expert Comment

by:mbodewes
ID: 22627896
mnrz: java.util.concurrent is not available to 1.3 as far as I know. Has this changed?
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mbodewes earned 500 total points
ID: 22627973
No, this does not work, the change may happen in between the setting and the printing. So the print statement may print about anything. There is nothing locking the volatile variable between these two states. You might want to synchronize the code parts setting and printing the variable instead.
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Expert Comment

by:mnrz
ID: 22631793
yes in version 1.3 you need to synchronize it
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Author Comment

by:DJ_AM_Juicebox
ID: 22650055
ok thanks
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