在平常開發中,咱們常常會遇到須要調用外部服務和接口的場景。外部服務對於調用者來講通常都是不可靠的,尤爲是在網絡環境比較差的狀況下,網絡抖動很容易致使請求超時等異常狀況,這時候就須要使用失敗重試策略從新調用 API 接口來獲取。重試策略在服務治理方面也有很普遍的使用,經過定時檢測,來查看服務是否存活(
Active)。java
Guava Retrying 是一個靈活方便的重試組件,包含了多種的重試策略,並且擴展起來很是容易。git
用做者的話來講:github
This is a small extension to Google’s Guava library to allow for the creation of configurable retrying strategies for an arbitrary function call, such as something that talks to a remote service with flaky uptime.網絡
使用 Guava-retrying 你能夠自定義來執行重試,同時也能夠監控每次重試的結果和行爲,最重要的基於 Guava 風格的重試方式真的很方便。app
如下會簡單列出 guava-retrying 的使用方式:dom
Callable<Integer> task = new Callable<Integer>() { @Override public Integer call() throws Exception { return 2; } }; Retryer<Integer> retryer = RetryerBuilder.<Integer>newBuilder() .retryIfResult(Predicates.<Integer>isNull()) .retryIfResult(Predicates.equalTo(2)) .retryIfExceptionOfType(IOException.class) .withStopStrategy(StopStrategies.stopAfterAttempt(3)) .withWaitStrategy(WaitStrategies.fixedWait(300, TimeUnit.MILLISECONDS)) .build(); try { retryer.call(task); } catch (ExecutionException e) { e.printStackTrace(); } catch (RetryException e) { e.printStackTrace(); }
@Override public Integer call() throws Exception { return 2; } }; Retryer<Integer> retryer = RetryerBuilder.<Integer>newBuilder() .retryIfException() .withStopStrategy(StopStrategies.stopAfterDelay(30,TimeUnit.SECONDS)) .withWaitStrategy(WaitStrategies.incrementingWait(3, TimeUnit.SECONDS,1,TimeUnit.SECONDS)) .withAttemptTimeLimiter(AttemptTimeLimiters.<Integer>fixedTimeLimit(3,TimeUnit.SECONDS)) .withRetryListener(new RetryListener() { @Override public <V> void onRetry(Attempt<V> attempt) { if (attempt.hasException()){ attempt.getExceptionCause().printStackTrace(); } } }) .build(); try { retryer.call(task); } catch (ExecutionException e) { e.printStackTrace(); } catch (RetryException e) { e.printStackTrace(); }
long startTime = System.nanoTime(); for (int attemptNumber = 1; ; attemptNumber++) { Attempt<V> attempt; try { // 執行成功 V result = attemptTimeLimiter.call(callable); attempt = new ResultAttempt<V>(result, attemptNumber, TimeUnit.NANOSECONDS.toMillis(System.nanoTime() - startTime)); } catch (Throwable t) { // 執行失敗 attempt = new ExceptionAttempt<V>(t, attemptNumber, TimeUnit.NANOSECONDS.toMillis(System.nanoTime() - startTime)); } // 監聽器處理 for (RetryListener listener : listeners) { listener.onRetry(attempt); } // 是否符合終止策略 if (!rejectionPredicate.apply(attempt)) { return attempt.get(); } // 是否符合中止策略 if (stopStrategy.shouldStop(attempt)) { throw new RetryException(attemptNumber, attempt); } else { // 計算下次重試間隔時間 long sleepTime = waitStrategy.computeSleepTime(attempt); try { blockStrategy.block(sleepTime); } catch (InterruptedException e) { Thread.currentThread().interrupt(); throw new RetryException(attemptNumber, attempt); } } }
<dependency> <groupId>com.github.rholder</groupId> <artifactId>guava-retrying</artifactId> <version>2.0.0</version> </dependency>
默認的guava中也有包含。異步
Attempt:一次執行任務;ide
AttemptTimeLimiter:單次任務執行時間限制(若是單次任務執行超時,則終止執行當前任務);ui
BlockStrategies:任務阻塞策略(通俗的講就是當前任務執行完,下次任務還沒開始這段時間作什麼……),默認策略爲:BlockStrategies.THREAD_SLEEP_STRATEGY 也就是調用 Thread.sleep(sleepTime);日誌
RetryException:重試異常;
RetryListener:自定義重試監聽器,能夠用於異步記錄錯誤日誌;
StopStrategy:中止重試策略,提供三種:
WaitStrategy:等待時長策略(控制時間間隔),返回結果爲下次執行時長: