Value does not update in while loop unless printed out [duplicate]

Answer 1

Without seeing any other code there are a number of things that can be going on here. If you give us a short reproduce able example that may help.

As far as 'a number of things' It is possible that the the compiler can re-order the write of size outside of the while loop for instance

int size = t.getSize();
while(working){
   if(size >= 10){ 

   }
}

But again that is just speculation at this point.

Answer 2

If the only thing changing between your working and non working code is the println statement, then you almost certainly have a threading issue. The System.out.println() statement adds a small pause which may coincidentally cause it to behave, but is not solving the issue. You could do something like:

try {
    Thread.sleep(10);
} catch (InterruptedException ex) {
}

...in place of this and check for the same behaviour, if indeed it is the same then this pretty much confirms the threading issue. However, as pointed out below println() also does a few other things such as causing a memory barrier, so this isn't a foolproof test. Another, perhaps better check could be to temporarily swap out ArrayList for Vector, which is thread safe - though this is a legacy collection so not recommended for use in the final code.

If this is the case, it sounds like you're not synchronising on ArrayList calls properly - ArrayList is not a thread safe collection. Whenever you read or write to the list, do it inside a synchronized block like so, synchronizing on the list:

synchronized(list) {
    list.whatever();
}

...which will ensure that only one thread can access the ArrayList at once, hopefully solving the threading issue.

Answer 3

Your reader is starving the writer so it never gets a chance to run, never adding anything to your list.

By adding a I/O call (system.out.print or Thread.sleep) you put the reader thread in a blocking state which allows other one to run.

Generally, loops which consumes 100% CPU like this is bad. At least add a short sleep/yield somewhere in the loop.

Answer 4

ArrayList is not synchronized, or otherwise prepared for use in two threads at once. In JLS terms, there is no happens-before relationship between the addition of elements in one thread and a size() call in another thread.

The cleanest solution would be to use a synchronized List implementation. You can either use Vector, or create a synchronized wrapper around your ArrayList using Collections.synchronizedList().