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POProvoking a race condition in Java
 

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  1. POProvoking a race condition in Java
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    <p>I got to write a unit test which provokes a race condition so I can test if I probably fixed the problem later on. The problem is that the race condition only occurs very rarely, maybe because my computer has only two cores.</p> <p>The code is something like the following:</p> <pre><code>class MyDateTime { String getColonTime() { // datetime is some kind of lazy caching variable declared somewhere(does not matter) if (datetime == null) { initDateTime(); //Uses lazy to initlialize variable, takes some time } // Colon time stores hh:mm as string if (datetime.colonTime == null) { StringBuilder sb = new StringBuilder(); //Now do some steps to build the hh:mm string //... //set colon time datetime.colonTime = sb.toString(); } return datetime.colonTime; } } </code></pre> <p>Explanation: initDateTime assigns a new instance to dateTime, therefor, datetime.colonTime is null afterwards (as we want to initialize it lazy, as I stated before). Now if Thread A enters the method and then the scheduler stops it just before it can run initDateTime(). Thread B now runst getColonTime(), sees that datetime is still null and initialzes it. datetime.colonTime is null so the second if block is executed and datetime.colonTime gets the value of the StringBuilder. If then the scheduler stops the thread between this line and the return statement and resumes thread A, the following happens: As A was stopped just before initDateTime is called, A now calls initDateTime(), which will kind of <strong>reset</strong> the datetime object, setting datetime.colonTime to null again. Thread A then will enter the second if block, but the scheduler will interrupt A before datetime.colonTime = sb.toString(); is called. As a conclusion, dateTime.colonTime is still null. Now the scheduler resumes B and the method returns null.</p> <p>I tried to provoke the race condition by having a number of threads calling getColonTime() to a single (final) instance of MyDateTime, but it only fails in some extreeemly rare cases :( Any hints how to write a JUnit "test"?</p>
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    1. COYou could try first to use debugger to provoke race condition. I.e. start a thread, catch it on some breakpoint (like between if's) and start another - and so on. After you get the idea how RC happens (seems you have none of that now) you could write a successful unit test
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    2. COI'm not seeing how you could get to the `return datetime.colonTime;` and get null back. Are you sure it isn't a problem with how you are building the hh:mm string? Maybe add that code to your question so we can look at that.
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    3. COI added some extra explanation why it can happen. Have to admit it's not very obvious
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