Proving the following code not thread safe

Question

How can I quickly prove that the following class is not thread-safe (as it uses Lazy Initialization and not using synchronization) by writing some code ? In other words, if I am testing the following class for thread safety, how can I fail it?

public class LazyInitRace {
  private ExpensiveObject instance = null;

  public ExpensiveObject getInstance() {
     if (instance == null)
        instance = new ExpensiveObject();
    return instance;
  }
}

Answer 1

By definition, race conditions cannot be tested deterministically, unless you control the thread scheduler (which you don't). The closest thing you can do is either to add a configurable delay in the getInstance() method, or write code where the problem might manifest and run it thousands of times in a loop.

BTW, none of this really constitutes "proof". Formal Verification would, but is very, very hard to do, even for relatively small amounts of code.