Java while loop works differently on different OS

Question

I have a really strange problem with java under Debian. On windows this works:

boolean X=true;
while(X);

This code performs a loop while X is true, when i set X to false the while loop ends. The problem is that on Debian the SAME code, when i set X to false, the while loop does not stop.

If I modify the the code:

boolean X=true;
while(X) {
    System.out.println("hello");
}

This code works fine on Debian but only if I add the print statement. If I try i++ for example it doesn't work correctly, only with the print statement it works.

Why is my code handled differently on a different OS?

Are you sure that the loop actualy start? maybe if a while loop is no point it might be ignored — Anthony Raymond, May 06 '14 at 12:51
Can you post the full code you're using when you're setting X to false? — David, May 06 '14 at 12:51
Java is Java. Please be more explicit, cannot recreate your issue with what you have provided here. — Elliott Frisch, May 06 '14 at 12:52
I am trying my telepathic skills. I think your program has at least two threads (one where the while-loop spins and the other where you set the flag X to false). If a variable is read/written from more than a single thread one needs to make an effort for changes made in one thread to be visible others. If you make your X volatile or replace it with AtomicBoolean it should do the trick. Has nothing to do with Debian. — bobah, May 06 '14 at 12:54
last I checked the default "java" on linux was gcj, which isn't Java and hasn't been updated in years. It's also a bug riddled monstrosity. — jwenting, May 06 '14 at 12:57
And are you building both versions on the same OS or once on each OS? — indivisible, May 06 '14 at 12:57
java -version and javac -version would provide a lot of clarity here... — jwenting, May 06 '14 at 12:59
i use jdk 8 now, i set the X variable to false on different thread , the code is this, the real code is more big but the problem is that, i have tryed to compile the code on windows and execute it on Debian, is same.. oh sorry for my bad english i hope is understable ;-) — Salvosnake, May 06 '14 at 13:04
@user3608219 Are you saying that you created the `.class` files on Windows, then ran them on Debian? — 2rs2ts, May 06 '14 at 13:07
@bobah was right and that if you are changing the boolean in a different thread you need to make it volatile — Dan, May 06 '14 at 13:09
i mean i have tryed to compile all under debian and also i have tryed to compile under windows and execute the jar file on debian, but the same file on windows work on debian not if i not add the printing statment, i ahve tryed also to compile with print statment under widnows, when i run on debian work fine — Salvosnake, May 06 '14 at 13:10
possible duplicate of [Loop doesn't see changed value without a print statement](http://stackoverflow.com/questions/25425130/loop-doesnt-see-changed-value-without-a-print-statement) — Boann, Aug 23 '14 at 10:21

score 2 · Accepted Answer · edited May 23 '17 at 11:57

2

If non-volatile variable is accessed from other thread, the behavior may be unpredictable: it may be cached on the current thread. Try to define X as volatile:

volatile boolean X = true;

Take a look at: Do you ever use the volatile keyword in Java?

edited May 23 '17 at 11:57

Community

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answered May 06 '14 at 13:10

Dmitry Ginzburg

7,391
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score 0 · Answer 2 · answered May 06 '14 at 13:15

I found this to give interesting results depending on the processor's architecure x86 vs 64bits.

public class WhileTrueTest
{
    boolean keepGoing = true;
    volatile boolean volatileKeepGoing = true;

    public static void main( String[] args ) throws InterruptedException
    {
        new WhileTrueTest().go();
    }

    private void go() throws InterruptedException
    {
        final Thread tNormal = new InnerNormal();
        final Thread tVolatile = new InnerVolatile();
        tNormal.start();
        tVolatile.start();
        Thread.sleep( 1000 );
        keepGoing = false;
        volatileKeepGoing = false;
        Thread.sleep( 1000 );
        System.out.printf("Threads shouldn't be alive. Are they? Normal:%s Volatile:%s", 
           tNormal.isAlive(), 
           tVolatile.isAlive());
        System.out.println();
        System.exit(1);

    }

    class InnerNormal extends Thread
    {
        @Override
        public void run()
        {
            while(keepGoing) {}
        }

    }

    class InnerVolatile extends Thread
    {
        @Override
        public void run()
        {
            while(volatileKeepGoing) {}
        }
    }        
}

Java while loop works differently on different OS

2 Answers2