Design pattern for “retrying” logic that failed?

Question

I\'m writing some reconnect logic to periodically attempt to establish a connection to a remote endpoint which went down. Essentially, the code looks like this:

Answer 1

I really like this Java 8 code from this blog and you don't need any extra library on your classpath.

You only need to pass a function to the retry class.

@Slf4j
public class RetryCommand<T> {

    private int maxRetries;

    RetryCommand(int maxRetries)
    {
        this.maxRetries = maxRetries;
    }

    // Takes a function and executes it, if fails, passes the function to the retry command
    public T run(Supplier<T> function) {
        try {
            return function.get();
        } catch (Exception e) {
            log.error("FAILED - Command failed, will be retried " + maxRetries + " times.");
            return retry(function);
        }
    }

    private T retry(Supplier<T> function) throws RuntimeException {

        int retryCounter = 0;
        while (retryCounter < maxRetries) {
            try {
                return function.get();
            } catch (Exception ex) {
                retryCounter++;
                log.error("FAILED - Command failed on retry " + retryCounter + " of " + maxRetries, ex);
                if (retryCounter >= maxRetries) {
                    log.error("Max retries exceeded.");
                    break;
                }
            }
        }
        throw new RuntimeException("Command failed on all of " + maxRetries + " retries");
    }
}

And to use it:

new RetryCommand<>(5).run(() -> client.getThatThing(id));

Answer 2

Shameless plug: I have implemented some classes to allow retrying operations. The library is not made available yet, but you may fork it on github. And a fork exists.

It allows building a Retryer with various flexible strategies. For example:

Retryer retryer = 
    RetryerBuilder.newBuilder()
                  .withWaitStrategy(WaitStrategies.fixedWait(1, TimeUnit.SECOND))
                  .withStopStrategy(StopStrategies.stopAfterAttempt(3))
                  .retryIfExceptionOfType(IOException.class)
                  .build();

And you can then execute a callable (or several ones) with the Retryer:

retryer.call(new Callable<Void>() {
    public Void call() throws IOException {
        connection = newConnection();
        return null;
    }
}

Answer 3

You can also create a wrapper function that just does a loop over the intended operation and when is done just break out of the loop.

public static void main(String[] args) {
    retryMySpecialOperation(7);
}

private static void retryMySpecialOperation(int retries) {
    for (int i = 1; i <= retries; i++) {
        try {
            specialOperation();
            break;
        }
        catch (Exception e) {
            System.out.println(String.format("Failed operation. Retry %d", i));
        }
    }
}

private static void specialOperation() throws Exception {
    if ((int) (Math.random()*100) % 2 == 0) {
        throw new Exception("Operation failed");
    }
    System.out.println("Operation successful");
}

Answer 4

I'm using AOP and Java annotations. There is a ready-made mechanism in jcabi-aspects (I'm a developer):

@RetryOnFailure(attempts = 3, delay = 1, unit = TimeUnit.SECONDS)
public void establishConnection() {
  this.connection = newConnection();
}

ps. You can also try RetryScalar from Cactoos.

Answer 5

One library worth checkout out is Sarge, which automatically performs retries according to a defined plan.

Answer 6

there is nothing special in retrying at all - take this class as example http://www.docjar.com/html/api/org/springframework/jms/listener/DefaultMessageListenerContainer.java.html As you can see even spring developers still writing code for retry-ing - line 791... there is no such special pattern AFAIK..

What i can advice to deal with resources is to take apache commons pool library - check this http://commons.apache.org/pool/apidocs/org/apache/commons/pool/impl/GenericObjectPool.html and visit http://commons.apache.org/pool