为什么我的threadExecutor会调用两次相同的回调？

Question

i have this code:

public <T> T executeCodeBlockWithTimeLimit(String criticalBlockTimeOutMilli, Callable<T> callable) throws
        Exception {
    ExecutorService service = Executors.newSingleThreadExecutor();
    Future<T> future = service.submit(callable);

    startStopWatch();
    T result;

    if (criticalBlockTimeOutMilli != null) {
        result = future.get(Long.parseLong(criticalBlockTimeOutMilli), TimeUnit.MILLISECONDS);
    } else {
        result = callable.call();
    }
    long timeElapsed = stopStopWatch();
    e2eResult.runTime = timeElapsed;
    service.shutdown();
    return result;
}

and in outer method:

    Callable<Void> callable = new
            Callable<Void>() {
                @Override
                public Void call() throws
                        Exception {

                    System.out.println("22222");

                    return null;
                }
            };
    timerUtils.executeCodeBlockWithTimeLimit(criticalBlockTimeOutMilli, callable);

And I see the console prints

22222
22222

why is this called twice?

Answer 1

When you get the from an with the submit method this one calls the object method and stores the result. In your code you first get then if the variable criticalBlockTimeOutMilli is you call the call() method on your Callable object. That means executing twice the object.

You should use the get() method instead because this one use the already collected result from your object (see Future#get() method ).

Answer 2

if (criticalBlockTimeOutMilli != null) {
    result = future.get(Long.parseLong(criticalBlockTimeOutMilli), TimeUnit.MILLISECONDS);
} else {
    result = future.get();
}

Answer 3

Maybe ExecutorService isn't clear for you, here's the javadoc for reference: http://docs.oracle.com/javase/7/docs/api/java/util/concurrent/ExecutorService.html#submit(java.util.concurrent.Callable)

When you call submit(Callable) the executor service will invoke your callable for you. There's no need for you to invoke it as well.

Answer 4

If criticalBlockTimeOutMilli is null, then the executor will invoke the callable once, and the

result = callable.call();

will invoke it a second time.

(?)

Answer 5

Assuming criticalBlockTimeOutMilli, is set to null, you are calling call method twice. One with submit method via executor service as below:

Future<T> future = service.submit(callable);

And then you call it via call method as below:

result = callable.call();