Using TDD to drive out thread-safe code

Question

What's a good way to leverage TDD to drive out thread-safe code? For example, say I have a factory method that utilizes lazy initialization to create only one instance of a class, and return it thereafter:

private TextLineEncoder textLineEncoder;
...
public ProtocolEncoder getEncoder() throws Exception {
    if(textLineEncoder == null)
        textLineEncoder = new TextLineEncoder();
    return textLineEncoder;
}

Now, I want to write a test in good TDD fashion that forces me to make this code thread-safe. Specifically, when two threads call this method at the same time, I don't want to create two instances and discard one. This is easily done, but how can I write a test that makes me do it?

I'm asking this in Java, but the answer should be more broadly applicable.

Answer 1

You could inject a "provider" (a really simple factory) that is responsible for just this line:

 textLineEncoder = new TextLineEncoder();

Then your test would inject a really slow implementation of the provider. That way the two threads in the test could more easily collide. You could go as far as have the first thread wait on a Semaphore that would be released by the second thread. Then success of the test would ensure that the waiting thread times out. By giving the first thread a head-start you can make sure that it's waiting before the second one releases.

Answer 2

It's hard, though possible - possibly harder than it's worth. Known solutions involve instrumenting the code under test. The discussion here, "Extreme Programming Challenge Fourteen" is worth sifting through.