Scheduled executor: poll for result at fix rate and exit if timeout or result valid

问题

Problem: I have a requirement to call a dao method at fix rate say every 10 sec, then I need to check if the result is valid if yes exit, else keep on calling that method every 10 sec till I get a valid result or defined time out (say 2 min) is over.

Approaches: I want to keep the task and scheduler logic separate, and write a task in such a way that it can be used by different classes having similar requirement.

One way I can think is to define a new poller task

public abstract class PollerTask<T> implements Runnable {

    abstract public boolean isValid(T result);

    abstract public T task();

    private T result;

    private volatile boolean complete;

    public boolean isComplete() {
        return complete;
    }

    public T getResult() {
        return result;
    }

    @Override
    final public void run() {
        result = task();
        if (complete = isValid(result)) {
            //may be stop scheduler ??
        }

    }
}

User need to simply provide implementation of task and isValid;

Then we can define a separate class that takes pooling freq and timeout and creates a scheduled executor and submit this task

public class PollerTaskExecutor {

    private int pollingFreq;
    private int timeout;
    private ScheduledExecutorService executor;
    private ScheduledExecutorService terminator;
    private ExecutorService condition;
    private volatile boolean done;
    private ScheduledFuture future;

    public PollerTaskExecutor(int pollingFreq, int timeout) {
        this.pollingFreq = pollingFreq;
        this.timeout = timeout;
        executor = Executors.newSingleThreadScheduledExecutor();
        terminator = Executors.newSingleThreadScheduledExecutor();
        condition = Executors.newSingleThreadExecutor();
    }

    public void submitTaskForPolling(final PollerTask pollerTask) {
        future = executor.scheduleAtFixedRate(pollerTask, 0, pollingFreq, TimeUnit.SECONDS);
        terminator.schedule(new Runnable() {
            @Override
            public void run() {
                complete();
            }
        }, timeout, TimeUnit.SECONDS);
        condition.execute(new Runnable() {
            @Override
            public void run() {
                if (pollerTask.isComplete()) {
                    complete();
                }
            }
        });

    }

    public boolean isDone() {
        return done;
    }

    public void complete() {
        future.cancel(false);
        executor.shutdown();
        terminator.shutdown();
        condition.shutdown();
        done = true;

    }

now user can wait till pollerExecutor.isDone returns true and get the result. I had to use three executors for following purposes:

executor to run task at fix interval
executor to stop all when time out is over
executor to stop all if valid result is obtained before time out.

Can someone please suggest a better approach, this seems to be complicated for such a trivial task ?

回答1:

Make it a self-scheduling task. In pseudo code:

public class PollingTaskRunner {

...
CountDownLatch doneWait = new CountDownLatch(1);
volatile boolean done;

PollingTaskRunner(Runnable pollingTask, int frequency, int period) {
    ...
    endTime = now + period;
    executor.schedule(this, 0);
}

run() {

    try {
        pollingTask.run();
    } catch (Exception e) {
        ...
    }
    if (pollingTask.isComplete() || now + frequency > endTime) {
        done = true;
        doneWait.countDown();
        executor.shutdown();
    } else {
        executor.schedule(this, frequency);
    }
}

await() {
    doneWait.await();
}

isDone() {
    return done;
}
}

It is not that complicated but add plenty of debug statements the first time you run/test this so you know what is going on. Once it is running as intended, it is easy to re-use the pattern.

回答2:

A slightly simpler method, you don't need a separate executor service for the terminator, you could simply push the terminator task into the same executor.

Even simpler. Have PollerTask place it's result in a BlockingQueue. Then have the PollingTaskRunner do a timed poll on that BlockingQueue. Whenever control is returned from the poll call ScheduledFuture.cancel because the task either succeeded or timed out.

来源：https://stackoverflow.com/questions/23226563/scheduled-executor-poll-for-result-at-fix-rate-and-exit-if-timeout-or-result-va

标签

java

multithreading

scheduledexecutorservice