##What is this? The guava-retrying module provides a general purpose method for retrying arbitrary Java code with specific stop, retry, and exception handling capabilities that are enhanced by Guava's predicate matching.

This is a fork of the excellent RetryerBuilder code posted here by Jean-Baptiste Nizet (JB). I've added a Gradle build for pushing it up to my little corner of Maven Central so that others can easily pull it into their existing projects with minimal effort. It also includes exponential and Fibonacci backoff WaitStrategies that might be useful for situations where more well-behaved service polling is preferred.

##Maven

    <dependency>
      <groupId>com.github.rholder</groupId>
      <artifactId>guava-retrying</artifactId>
      <version>2.0.0</version>
    </dependency>

##Gradle

    compile "com.github.rholder:guava-retrying:2.0.0"

##Quickstart A minimal sample of some of the functionality would look like:

Callable<Boolean> callable = new Callable<Boolean>() {
    public Boolean call() throws Exception {
        return true; // do something useful here
    }
};

Retryer<Boolean> retryer = RetryerBuilder.<Boolean>newBuilder()
        .retryIfResult(Predicates.<Boolean>isNull())
        .retryIfExceptionOfType(IOException.class)
        .retryIfRuntimeException()
        .withStopStrategy(StopStrategies.stopAfterAttempt(3))
        .build();
try {
    retryer.call(callable);
} catch (RetryException e) {
    e.printStackTrace();
} catch (ExecutionException e) {
    e.printStackTrace();
}

This will retry whenever the result of the Callable is null, if an IOException is thrown, or if any other RuntimeException is thrown from the call() method. It will stop after attempting to retry 3 times and throw a RetryException that contains information about the last failed attempt. If any other Exception pops out of the call() method it's wrapped and rethrown in an ExecutionException.

##Exponential Backoff

Create a Retryer that retries forever, waiting after every failed retry in increasing exponential backoff intervals until at most 5 minutes. After 5 minutes, retry from then on in 5 minute intervals.

Retryer<Boolean> retryer = RetryerBuilder.<Boolean>newBuilder()
        .retryIfExceptionOfType(IOException.class)
        .retryIfRuntimeException()
        .withWaitStrategy(WaitStrategies.exponentialWait(100, 5, TimeUnit.MINUTES))
        .withStopStrategy(StopStrategies.neverStop())
        .build();

You can read more about exponential backoff and the historic role it played in the development of TCP/IP in Congestion Avoidance and Control.

##Fibonacci Backoff

Create a Retryer that retries forever, waiting after every failed retry in increasing Fibonacci backoff intervals until at most 2 minutes. After 2 minutes, retry from then on in 2 minute intervals.

Retryer<Boolean> retryer = RetryerBuilder.<Boolean>newBuilder()
        .retryIfExceptionOfType(IOException.class)
        .retryIfRuntimeException()
        .withWaitStrategy(WaitStrategies.fibonacciWait(100, 2, TimeUnit.MINUTES))
        .withStopStrategy(StopStrategies.neverStop())
        .build();

Similar to the ExponentialWaitStrategy, the FibonacciWaitStrategy follows a pattern of waiting an increasing amount of time after each failed attempt.

Instead of an exponential function it's (obviously) using a Fibonacci sequence to calculate the wait time.

Depending on the problem at hand, the FibonacciWaitStrategy might perform better and lead to better throughput than the ExponentialWaitStrategy - at least according to A Performance Comparison of Different Backoff Algorithms under Different Rebroadcast Probabilities for MANETs.

The implementation of FibonacciWaitStrategy is using an iterative version of the Fibonacci because a (naive) recursive version will lead to a StackOverflowError at a certain point (although very unlikely with useful parameters for retrying).

Inspiration for this implementation came from Efficient retry/backoff mechanisms.

##Documentation Javadoc can be found here.

##Building from source The guava-retrying module uses a Gradle-based build system. In the instructions below, ./gradlew is invoked from the root of the source tree and serves as a cross-platform, self-contained bootstrap mechanism for the build. The only prerequisites are Git and JDK 1.6+.

check out sources

git clone git://github.com/rholder/guava-retrying.git

compile and test, build all jars

./gradlew build

install all jars into your local Maven cache

./gradlew install

##License The guava-retrying module is released under version 2.0 of the Apache License.

##Contributors

• Jean-Baptiste Nizet (JB)
• Jason Dunkelberger (dirkraft)
• Diwaker Gupta (diwakergupta)
• Jochen Schalanda (joschi)
• Shajahan Palayil (shasts)
• Olivier Grégoire (fror)
• Andrei Savu (andreisavu)
• (tchdp)
• (squalloser)
• Yaroslav Matveychuk (yaroslavm)
• Stephan Schroevers (Stephan202)
• Chad (voiceinsideyou)
• Kevin Conaway (kevinconaway)
• Alberto Scotto (alb-i986)