CODEWITHSUNDEEP
javamultithreadingloops
LostMekkaSoft
LostMekkaSoft

Reputation: 363

Java: Running potentially blocking code

I am developing a small game, (Java, LibGdx) where the player fills cloze-style functions with predefined lines of code. The game would then compile the code and run a small test suite to verify that the function does the stuff it is supposed to.

Compiling and running the code already works, but I am faced with the problem of detecting infinite loops. Consider the following function:

// should compute the sum of [1 .. n]
public int foo(int n) {
    int i = 0;
    while (n > 0) {
        i += n;
        // this is the place where the player inserts one of many predefined lines of code
        // the right one would be: n--;
        // but the player could also insert something silly like: i++;
    }
    return i;
}

Please note that the functions actually used may be more complex and in general it is not possible to make sure that there cannot be any infinite loops.

Currently I am running the small test suite (provided for every function) in a Thread using an ExecutorService, setting a timeout to abort waiting in case the thread is stuck. The problem with this is, that the threads stuck in an endless loop will run forever in the background, which of course will at some point have a considerable impact on game performance.

// TestClass is the compiled class containing the function above and the corresponding test suite
Callable<Boolean> task = new Callable<Boolean>() {
    @Override
    public Boolean call() throws Exception {
        // call the test suite
        return new TestClass().test();
    }
};
Future<Boolean> future = executorService.submit(task);
try {
    Boolean result = future.get(100, TimeUnit.MILLISECONDS);
    System.out.println("result: " + (result == null ? "null" : result.toString()));
} catch (InterruptedException e) {
    e.printStackTrace();
} catch (ExecutionException e) {
    e.printStackTrace();
} catch (TimeoutException e) {
    e.printStackTrace();
    future.cancel(true);
}

My question is now: How can I gracefully end the threads that accidentally spin inside an endless loop?

*EDIT To clarify why in this case, preventing infinite loops is not possible/feasable: The functions, their test suite and the lines to fill the gaps are loaded from disk. There will be hundrets of functions with at least two lines of code that could be inserted. The player can drag any line into any gap. The effort needed to make sure no combination of function gap/code line produces something that loops infinitely or even runs longer than the timeout grows exponentially with the number of functions. This quickly gets to the point where nobody has the time to check all of these combinations manually. Also, in general, determining, whether a function will finish in time is pretty much impossible because of the halting problem.

Upvotes: 1

Views: 78

Answers (3)

Marko Topolnik
Marko Topolnik

Reputation: 200168

There is no such thing as "graceful termination" of a thread inside the same process. The terminated thread can leave inconsistent shared-memory state behind it.

You can either organize things so that each task is started in its own JVM, or make do with forceful termination using the deprecated Thread.stop() method.

Another option is inserting a check into the generated code, but this would require much more effort to implement properly.

Upvotes: 1

Davide Lorenzo MARINO
Davide Lorenzo MARINO

Reputation: 26926

It is not normal to have a never ending loop if it not wanted.

To solve the problem You can add a counter in the loop and if you reach a limit you can exit.

int counter = 0;
while (n > 0) {
    counter++;
    if (counter > THRESHOLD) {
       break; 
    }
    i += n;
    // this is the place where the player inserts one of many predefined lines of code
    // the right one would be: n--;
    // but the player could also insert something silly like: i++;
}

Upvotes: 0

Mario Santini
Mario Santini

Reputation: 3003

The right way is to change the design and avoids never ending loops.

For the time being, inside your loop you could check if the thread is interrupted some way by: isInterrupted() or even isAlive().

And if it is you just exit.

Upvotes: 1

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