Conditional statement at the beginning of a while loop doesn't get checked

Question

I've noticed this bug (if it is a bug) and decided to make a test environment. Does anyone know if this is an actual problem or is it just me?

import java.util.Random;

public class Start {

    public static boolean value;

    public static void main(String[] args){
        Thread thread = new Thread(){
            @Override
            public void run(){
                random();
            }
        };
        thread.start();
        while(true){
            if(value) System.out.println("Done");
        }
    }

    public static void random(){
        while(true){
            value = new Random().nextInt(5) == 0;
        }
    }

}

I would expect this code to be spamming out "Done"s, but instead I get a bunch at the beginning and then no additions. When I modify the code as so:

while(true){
System.out.print(""); //Modification
    if(value) System.out.println("Done");
}

It starts spamming the "Done"s. Anyone know what's up? NOTE: I have tested this in an Eclipse environment and a compiled jar while using jdk 1.8.0 v.25 and jre 1.8.0 v.51.

You're racing. Declare value as volatile, but that's still not going to make this code correct. — Falmarri, Aug 04 '15 at 23:59
You may want to have a look at this http://stackoverflow.com/questions/30965206/why-do-i-get-an-infinite-while-loop-even-if-i-modify-the-lock-variable/30965392#30965392 — Jean-François Savard, Aug 05 '15 at 00:00
Declaring it as volatile fixed it. Running as expected. Thanks. — Noah, Aug 05 '15 at 00:01
@Noah I would still recommend to read the post I linked to understand why the `System.out.println` call changes the behaviour... — Jean-François Savard, Aug 05 '15 at 00:02

score 3 · Answer 1 · edited Aug 05 '15 at 00:19

3

I suspect that you hit a compiler optimization called hoisting - it recognizes that you overwrite value without having these changes "visible" to other threads and that's why it optimizes at some point and stops printing.

This behavior will change to what is likely the "wanted" behavior once you'll declare value as volatile - then updates to it become visible to other threads and then the printings will keep on forever.

edited Aug 05 '15 at 00:19

chrylis -cautiouslyoptimistic-

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answered Aug 05 '15 at 00:00

Nir Alfasi

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2

the printing changes the behaviour because the underlying stream is synchronized thus forces the threads to synchronize. – Jean-François Savard Aug 05 '15 at 00:05

Conditional statement at the beginning of a while loop doesn't get checked

1 Answers1