Java: Empty while loop

Question

I'm making a program with while loops that execute in this manner:

Main thread enters a while loop.
Nothing happens in the while loop.
Thread will stay in the while loop until the condition is satisfied.
Another thread runs a function that will satisfy said condition.

Here is an example:

while(path != null);

There's another function in the class that will set the path to null, and once that happens the main thread should exit this loop. The other function is called in another thread.

However, the main thread does not exit the loop even when path is set to null. Any suggestions?

CODE:

try 
 { 
  for (Node n:realpath) 
    { 
      Thread.sleep(100); 
      actor.walk(n); 
     }
    Thread.sleep(100); 
 } 
 catch (InterruptedException ex) 
  { 
    Logger.getLogger(VNScreen.class.getName()).log(Level.SEVERE, null, ex); 
  } 
  realpath.clear(); 
  path = null;

if(path == null)
    System.out.println("NULLED PATH");

if you post your full code, people will be more able to find your problem. — ewok
– ewok, Commented Dec 7, 2011 at 2:18
path is a private property in the class. Both functions above are also in said class. — Abraham Miguel Espiritu
– Abraham Miguel Espiritu, Commented Dec 7, 2011 at 2:20
Someone took care of it for you, @AbrahamMiguelEspiritu, but in the future it is better to edit your original question to add code. It's far more readable. — Paul
– Paul, Commented Dec 7, 2011 at 2:29
Prove that path is set to null by logging it. I am not convinced that Thread executes. — emory
– emory, Commented Dec 7, 2011 at 2:35

Ted Hopp · Accepted Answer · 2011-12-07 02:45:25Z

13

Busy waits are very expensive. I'd do it this way:

Object LOCK = new Object(); // just something to lock on

synchronized (LOCK) {
    while (path != null) {
        try { LOCK.wait(); }
        catch (InterruptedException e) {
            // treat interrupt as exit request
            break;
        }
    }
}

Then when you set path to null, just call

synchronized (LOCK) {
    LOCK.notifyAll();
}

(You can just synchronize on this if both pieces of code are in the same object.)

answered Dec 7, 2011 at 2:45

Ted Hopp

235k48 gold badges412 silver badges533 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Halil Oymacı Over a year ago

Thank you so much, do you think where this lock variable should be, in class that calls wait or in class that notifyAll?

Ted Hopp Over a year ago

@HalilİbrahimOymacı - I don't think it matters, as long as the variable is accessible to all code that needs it. It could even be a separate singleton object in its own class.

assylias · Accepted Answer · 2012-12-16 10:31:09Z

6

Rather than using a low level wait/notify approach, and to avoid busy waiting with a volatile, you can use a CountDownLatch. Because there is only one event to expect, instantiate the latch as:

CountDownLatch latch = new CountDownLatch(1);

Replace your while (path != null) with:

latch.await();

and in your main code, simply do:

path = somePath;
latch.countDown();

answered Dec 16, 2012 at 10:31

assylias

330k84 gold badges680 silver badges806 bronze badges

Comments

Hot Licks · Accepted Answer · 2011-12-07 02:31:35Z

You probably need to declare path to be volatile. If the variable is not declared to be volatile then the compiler is free to cache its value in a register, and so any change to the value will not be recognized.

But in general this style of code is very bad (except in some low-level real-mode stuff). While your while loop is looping it's consuming 100% of the CPU, so whatever other process there is to change the value of path may never get cycles to execute, and other processes that do mundane things like maintain communications links or update the mouse cursor location will not execute either.

Best is to do some sort of event/message/semaphore based communications, so that your "loop" waits for a signal before continuing. "Acceptable" is to utilize some sort of delay, such as Thread.sleep, so that you only "poll" the variable every few milliseconds, say. You could also try Thread.yield, but that's sort of a crap-shoot and would not produce reliable results across a range of environments.

seyed abolfazl hashemi · Accepted Answer · 2012-12-16 06:52:24Z

-1

the main thread does not exit the loop because because in while condition does not anything and checking condition is doing very speedy and while not sense changing variable. you have 2 way for resolve this problem: 1-in while loop waiting for a short time for example 5ms

    while(path != null){
     try{
       Thread.currentThread().sleep(50);
     }catch(){
       //do nothing
     }
    }

2-define path variable as volatile:

  volatile String path;

answered Dec 16, 2012 at 6:52

seyed abolfazl hashemi

1

1 Comment

assylias Over a year ago

The Java memory model gives no guarantee whatsoever that your first solution will work. It is even likely that it won't work if you run it with a -server argument because the condition will be optimised away and transformed in to while(true).

Collectives™ on Stack Overflow

Java: Empty while loop

4 Answers 4

2 Comments

Comments

Comments

1 Comment

Linked

Hot Network Questions

Collectives™ on Stack Overflow

4 Answers 4

2 Comments

Comments

Comments

1 Comment

Linked

Related