Question

说我有一些这样的代码：

public void deleteResource(UUID resourceId) {
    deleteFromDb();
    deleteFromALotOfOtherPlaces();    // Takes a long time!
}

public DescribeResourceResult describeResource(UUID resourceId) throws ResourceNotFoundException {
    return getResourceDescription(resourceId);
}

不幸的是，删除操作并不表示已完成删除。验证删除是否完成的唯一方法是调用describeResource，如果资源已删除，它将引发异常。

我想编写一个重试器，该重试器将反复调用describeResrouce，直到发生ResourceNotFoundException为止。我该怎么办？

这是我到目前为止所拥有的：

final Retryer<ResourceNotFoundException> deleteResourceRetryer = RetryerBuilder.<ResourceNotFoundException>newBuilder()
                .withWaitStrategy(WaitStrategies.fixedWait(500, TimeUnit.MILLISECONDS))
                .withStopStrategy(StopStrategies.stopAfterDelay(10, TimeUnit.SECONDS))
                .build();

// Error: Bad return type in lambda expression: DescribeResourceResult cannot be converted to ResourceNotFoundException
deleteResourceRetryer.call(() -> describeResource(resourceId));

谢谢！

Answer 1

我对Guava的Retryer不太熟悉，因此经过短暂的调查，我找不到立即可用的StopStrategy，所以我的建议是自己实施它。

static class OnResourceNotFoundExceptionStopStrategy implements StopStrategy {

    @Override
    public boolean shouldStop(Attempt attempt) {
        if (attempt.hasException() 
                     && attempt.getExceptionCause() instanceof ResourceNotFoundException) {
            return true;
        }
        return false;
    }

}

使用该策略，重试将在您抓到ResourceNotFoundException时停止。之后，修复类型和正确定义Retryer

final Retryer<DescribeResourceResult> deleteResourceRetryer = RetryerBuilder
            .<DescribeResourceResult>newBuilder()
            .retryIfResult(Predicates.notNull())
            .withWaitStrategy(WaitStrategies.fixedWait(500, TimeUnit.MILLISECONDS))
            .withStopStrategy(new OnResourceNotFoundExceptionStopStrategy())
            .build();

最后，开始重试

try {
    deleteResourceRetryer.call(() -> describeResource(resourceId));
} catch (ExecutionException e) {
    // should not happens, because you will retry if any exception rather
    // than ResourceNotFoundException raised in your describeResource method
} catch (RetryException e) {
    // should not happens, because my implementation of StopStrategy
    // (as it looks in this example) is effectively infinite, until target exception.
    // For sure you're free to override it to anything you want
}

希望有帮助！

Answer 2

使用Failsafe：

RetryPolicy<Object> retryPolicy = new RetryPolicy<>()
  .abortOn(ResourceNotFoundException.class);
  .withDelay(Duration.ofMillis(500))
  .withMaxDuration(Duration.ofSeconds(10)); 

Failsafe.with(retryPolicy).get(() -> getResourceDescription(resourceId));

调用函数直到异常发生的番石榴重试器

2 个答案: