Question

I just wanted to write an example for an race condition:

MyParallelClass.java:

public class MyParallelClass implements java.lang.Runnable {
    public int counter = 0;

    @Override
    public void run() {
        if (test.globalVar > 0) {
            for (int i = 0; i < 1000000; i++) {
                counter++;
            }
            test.globalVar--;
        }
    }
}   

test.java:

public class test {
    public static int globalVar;

    public static void main(String[] args) {
        globalVar = 1;

        MyParallelClass a = new MyParallelClass();
        MyParallelClass b = new MyParallelClass();

        new Thread(a).start(); // Thread A
        new Thread(b).start(); // Thread B

        System.out.println(globalVar);
    }
}   

What I thought would happen:

I thought this could output either 0 if thread A was executed completely before Thread B starts.

The variable test.globalVar could also get manipulated like this:

Thread A                     -  Thread B  
checks if (globalVar > 0)
      looping ...               checks if (globalVar > 0)
      looping ...               execute all four bytecode commands of "test.globalVar--;"
      execute test.globalVar--;

so the value of test.globalVar would be -1.

So either one of the if-statements get executed or both.

What actually happened:

I got 0 and 1 as output of the main method. Why do I get 0 and 1 and not 0 and -1?

Answer 1

The

System.out.println(globalVar);

does not wait for the threads to complete. The threads may or may not be complete at that point. So the value can be 0, 1, or -1 depending on whether both the threads completed, one completed or both did not complete.

To have a better test,
– use Thread.sleep() in the threads to make sure that there is a delay
– Use different delays in the different threads to better visualize the race condition.
– You may want to print the value of the variable in the threads also. this way you have three threads racing (A, B and the main thread) and you get better visualizations.