Spring 中的重試機制，簡單、實用！
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來源：https://albenw.github.io/posts/69a9647f/
概要
Spring實現了一套重試機制，功能簡單實用。Spring Retry是從Spring Batch獨立出來的一個功能，已經廣泛應用于Spring Batch,Spring Integration, Spring for Apache Hadoop等Spring項目。
本文將講述如何使用Spring Retry及其實現原理。
背景
如果我們要做重試，要為特定的某個操作做重試功能，則要硬編碼，大概邏輯基本都是寫個循環(huán)，根據返回或異常，計數失敗次數，然后設定退出條件。這樣做，且不說每個操作都要寫這種類似的代碼，而且重試邏輯和業(yè)務邏輯混在一起，給維護和擴展帶來了麻煩。
從面向對象的角度來看，我們應該把重試的代碼獨立出來。
使用介紹
基本使用
先舉個例子：
@Configuration
@EnableRetry
public?class?Application?{
????@Bean
????public?RetryService?retryService(){
????????return?new?RetryService();
????}
????public?static?void?main(String[]?args)?throws?Exception{
????????ApplicationContext?applicationContext?=?new?AnnotationConfigApplicationContext("springretry");
????????RetryService?service1?=?applicationContext.getBean("service",?RetryService.class);
????????service1.service();
????}
}
@Service("service")
public?class?RetryService?{
????@Retryable(value?=?IllegalAccessException.class,?maxAttempts?=?5,
????????????backoff=?@Backoff(value?=?1500,?maxDelay?=?100000,?multiplier?=?1.2))
????public?void?service()?throws?IllegalAccessException?{
????????System.out.println("service?method...");
????????throw?new?IllegalAccessException("manual?exception");
????}
????@Recover
????public?void?recover(IllegalAccessException?e){
????????System.out.println("service?retry?after?Recover?=>?"?+?e.getMessage());
????}
}
@EnableRetry - 表示開啟重試機制
@Retryable - 表示這個方法需要重試，它有很豐富的參數，可以滿足你對重試的需求
@Backoff - 表示重試中的退避策略
@Recover - 兜底方法，即多次重試后還是失敗就會執(zhí)行這個方法
Spring-Retry 的功能豐富在于其重試策略和退避策略，還有兜底，監(jiān)聽器等操作。
然后每個注解里面的參數，都是很簡單的，大家看一下就知道是什么意思，怎么用了，我就不多講了。關注公眾號Java技術棧，在后臺回復：spring，可以獲取我整理的 Spring 系列教程，非常齊全。
重試策略
看一下Spring Retry自帶的一些重試策略，主要是用來判斷當方法調用異常時是否需要重試。（下文原理部分會深入分析實現）
SimpleRetryPolicy 默認最多重試3次 TimeoutRetryPolicy 默認在1秒內失敗都會重試 ExpressionRetryPolicy 符合表達式就會重試 CircuitBreakerRetryPolicy 增加了熔斷的機制，如果不在熔斷狀態(tài)，則允許重試 CompositeRetryPolicy 可以組合多個重試策略 NeverRetryPolicy 從不重試（也是一種重試策略哈） AlwaysRetryPolicy 總是重試
….等等
退避策略
看一下退避策略，退避是指怎么去做下一次的重試，在這里其實就是等待多長時間。（下文原理部分會深入分析實現）
FixedBackOffPolicy 默認固定延遲1秒后執(zhí)行下一次重試 ExponentialBackOffPolicy 指數遞增延遲執(zhí)行重試，默認初始0.1秒，系數是2，那么下次延遲0.2秒，再下次就是延遲0.4秒，如此類推，最大30秒。 ExponentialRandomBackOffPolicy 在上面那個策略上增加隨機性 UniformRandomBackOffPolicy 這個跟上面的區(qū)別就是，上面的延遲會不停遞增，這個只會在固定的區(qū)間隨機 StatelessBackOffPolicy 這個說明是無狀態(tài)的，所謂無狀態(tài)就是對上次的退避無感知，從它下面的子類也能看出來
原理
原理部分我想分開兩部分來講，一是重試機制的切入點，即它是如何使得你的代碼實現重試功能的；二是重試機制的詳細，包括重試的邏輯以及重試策略和退避策略的實現。另外，關注公眾號Java技術棧，在后臺回復：面試，可以獲取我整理的 Spring 系列面試題和答案，非常齊全。
切入點
@EnableRetry
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@EnableAspectJAutoProxy(proxyTargetClass?=?false)
@Import(RetryConfiguration.class)
@Documented
public?@interface?EnableRetry?{
?/**
??*?Indicate?whether?subclass-based?(CGLIB)?proxies?are?to?be?created?as?opposed
??*?to?standard?Java?interface-based?proxies.?The?default?is?{@code?false}.
??*
??*?@return?whether?to?proxy?or?not?to?proxy?the?class
??*/
?boolean?proxyTargetClass()?default?false;
}
我們可以看到@EnableAspectJAutoProxy(proxyTargetClass = false)這個并不陌生，就是打開Spring AOP功能。
重點看看@Import(RetryConfiguration.class)@Import相當于注冊這個Bean
我們看看這個RetryConfiguration是個什么東西：
它是一個AbstractPointcutAdvisor，它有一個pointcut和一個advice。我們知道，在IOC過程中會根據PointcutAdvisor類來對Bean進行Pointcut的過濾，然后生成對應的AOP代理類，用advice來加強處理。
看看RetryConfiguration的初始化:
@PostConstruct
public?void?init()?{
????Set>?retryableAnnotationTypes?=?new?LinkedHashSet>(1);
????retryableAnnotationTypes.add(Retryable.class);
????//創(chuàng)建pointcut
????this.pointcut?=?buildPointcut(retryableAnnotationTypes);
????//創(chuàng)建advice
????this.advice?=?buildAdvice();
????if?(this.advice?instanceof?BeanFactoryAware)?{
????????((BeanFactoryAware)?this.advice).setBeanFactory(beanFactory);
????}
}
protected?Pointcut?buildPointcut(Set>?retryAnnotationTypes)?{
????ComposablePointcut?result?=?null;
????for?(Class?retryAnnotationType?:?retryAnnotationTypes)?{
????????Pointcut?filter?=?new?AnnotationClassOrMethodPointcut(retryAnnotationType);
????????if?(result?==?null)?{
????????????result?=?new?ComposablePointcut(filter);
????????}
????????else?{
????????????result.union(filter);
????????}
????}
????return?result;
}
上面代碼用到了AnnotationClassOrMethodPointcut，其實它最終還是用到了AnnotationMethodMatcher來根據注解進行切入點的過濾。這里就是@Retryable注解了。
//創(chuàng)建advice對象，即攔截器
protected?Advice?buildAdvice()?{
????//下面關注這個對象
?AnnotationAwareRetryOperationsInterceptor?interceptor?=?new?AnnotationAwareRetryOperationsInterceptor();
?if?(retryContextCache?!=?null)?{
??interceptor.setRetryContextCache(retryContextCache);
?}
?if?(retryListeners?!=?null)?{
??interceptor.setListeners(retryListeners);
?}
?if?(methodArgumentsKeyGenerator?!=?null)?{
??interceptor.setKeyGenerator(methodArgumentsKeyGenerator);
?}
?if?(newMethodArgumentsIdentifier?!=?null)?{
??interceptor.setNewItemIdentifier(newMethodArgumentsIdentifier);
?}
?if?(sleeper?!=?null)?{
??interceptor.setSleeper(sleeper);
?}
?return?interceptor;
}
AnnotationAwareRetryOperationsInterceptor
繼承關系
可以看出AnnotationAwareRetryOperationsInterceptor是一個MethodInterceptor，在創(chuàng)建AOP代理過程中如果目標方法符合pointcut的規(guī)則，它就會加到interceptor列表中，然后做增強，我們看看invoke方法做了什么增強。
@Override
public?Object?invoke(MethodInvocation?invocation)?throws?Throwable?{
????MethodInterceptor?delegate?=?getDelegate(invocation.getThis(),?invocation.getMethod());
????if?(delegate?!=?null)?{
????????return?delegate.invoke(invocation);
????}
????else?{
????????return?invocation.proceed();
????}
}
這里用到了委托，主要是需要根據配置委托給具體“有狀態(tài)”的interceptor還是“無狀態(tài)”的interceptor。
private?MethodInterceptor?getDelegate(Object?target,?Method?method)?{
????if?(!this.delegates.containsKey(target)?||?!this.delegates.get(target).containsKey(method))?{
????????synchronized?(this.delegates)?{
????????????if?(!this.delegates.containsKey(target))?{
????????????????this.delegates.put(target,?new?HashMap());
????????????}
????????????Map?delegatesForTarget?=?this.delegates.get(target);
????????????if?(!delegatesForTarget.containsKey(method))?{
????????????????Retryable?retryable?=?AnnotationUtils.findAnnotation(method,?Retryable.class);
????????????????if?(retryable?==?null)?{
????????????????????retryable?=?AnnotationUtils.findAnnotation(method.getDeclaringClass(),?Retryable.class);
????????????????}
????????????????if?(retryable?==?null)?{
????????????????????retryable?=?findAnnotationOnTarget(target,?method);
????????????????}
????????????????if?(retryable?==?null)?{
????????????????????return?delegatesForTarget.put(method,?null);
????????????????}
????????????????MethodInterceptor?delegate;
????????????????//支持自定義MethodInterceptor，而且優(yōu)先級最高
????????????????if?(StringUtils.hasText(retryable.interceptor()))?{
????????????????????delegate?=?this.beanFactory.getBean(retryable.interceptor(),?MethodInterceptor.class);
????????????????}
????????????????else?if?(retryable.stateful())?{
????????????????????//得到“有狀態(tài)”的interceptor
????????????????????delegate?=?getStatefulInterceptor(target,?method,?retryable);
????????????????}
????????????????else?{
????????????????????//得到“無狀態(tài)”的interceptor
????????????????????delegate?=?getStatelessInterceptor(target,?method,?retryable);
????????????????}
????????????????delegatesForTarget.put(method,?delegate);
????????????}
????????}
????}
????return?this.delegates.get(target).get(method);
}
getStatefulInterceptor和getStatelessInterceptor都是差不多，我們先看看比較簡單的getStatelessInterceptor。
private?MethodInterceptor?getStatelessInterceptor(Object?target,?Method?method,?Retryable?retryable)?{
????//生成一個RetryTemplate
????RetryTemplate?template?=?createTemplate(retryable.listeners());
????//生成retryPolicy
????template.setRetryPolicy(getRetryPolicy(retryable));
????//生成backoffPolicy
????template.setBackOffPolicy(getBackoffPolicy(retryable.backoff()));
????return?RetryInterceptorBuilder.stateless()
????????????.retryOperations(template)
????????????.label(retryable.label())
????????????.recoverer(getRecoverer(target,?method))
????????????.build();
}
具體生成retryPolicy和backoffPolicy的規(guī)則，我們等下再回頭來看。
RetryInterceptorBuilder其實就是為了生成RetryOperationsInterceptor。RetryOperationsInterceptor也是一個MethodInterceptor，我們來看看它的invoke方法。
分享資料：Spring Boot 學習筆記太全了！
public?Object?invoke(final?MethodInvocation?invocation)?throws?Throwable?{
????String?name;
????if?(StringUtils.hasText(label))?{
????????name?=?label;
????}?else?{
????????name?=?invocation.getMethod().toGenericString();
????}
????final?String?label?=?name;
????//定義了一個RetryCallback，其實看它的doWithRetry方法，調用了invocation的proceed()方法，是不是有點眼熟，這就是AOP的攔截鏈調用，如果沒有攔截鏈，那就是對原來方法的調用。
????RetryCallback?retryCallback?=?new?RetryCallback()?{
????????public?Object?doWithRetry(RetryContext?context)?throws?Exception?{
????????????context.setAttribute(RetryContext.NAME,?label);
????????????/*
?????????????*?If?we?don't?copy?the?invocation?carefully?it?won't?keep?a?reference?to
?????????????*?the?other?interceptors?in?the?chain.?We?don't?have?a?choice?here?but?to
?????????????*?specialise?to?ReflectiveMethodInvocation?(but?how?often?would?another
?????????????*?implementation?come?along?).
?????????????*/
????????????if?(invocation?instanceof?ProxyMethodInvocation)?{
????????????????try?{
????????????????????return?((ProxyMethodInvocation)?invocation).invocableClone().proceed();
????????????????}
????????????????catch?(Exception?e)?{
????????????????????throw?e;
????????????????}
????????????????catch?(Error?e)?{
????????????????????throw?e;
????????????????}
????????????????catch?(Throwable?e)?{
????????????????????throw?new?IllegalStateException(e);
????????????????}
????????????}
????????????else?{
????????????????throw?new?IllegalStateException(
????????????????????????"MethodInvocation?of?the?wrong?type?detected?-?this?should?not?happen?with?Spring?AOP,?"?+
????????????????????????????????"so?please?raise?an?issue?if?you?see?this?exception");
????????????}
????????}
????};
????if?(recoverer?!=?null)?{
????????ItemRecovererCallback?recoveryCallback?=?new?ItemRecovererCallback(
????????????????invocation.getArguments(),?recoverer);
????????return?this.retryOperations.execute(retryCallback,?recoveryCallback);
????}
????//最終還是進入到retryOperations的execute方法，這個retryOperations就是在之前的builder set進來的RetryTemplate。
????return?this.retryOperations.execute(retryCallback);
}
無論是RetryOperationsInterceptor還是StatefulRetryOperationsInterceptor，最終的攔截處理邏輯還是調用到RetryTemplate的execute方法，從名字也看出來，RetryTemplate作為一個模板類，里面包含了重試統(tǒng)一邏輯。
不過，我看這個RetryTemplate并不是很“模板”，因為它沒有很多可以擴展的地方。推薦閱讀：最新 Spring 系列教程。
重試邏輯及策略實現
上面介紹了Spring Retry利用了AOP代理使重試機制對業(yè)務代碼進行“入侵”。下面我們繼續(xù)看看重試的邏輯做了什么。RetryTemplate的doExecute方法。
protected??T?doExecute(RetryCallback?retryCallback,
???RecoveryCallback?recoveryCallback,?RetryState?state)
???throws?E,?ExhaustedRetryException?{
????RetryPolicy?retryPolicy?=?this.retryPolicy;
????BackOffPolicy?backOffPolicy?=?this.backOffPolicy;
????//新建一個RetryContext來保存本輪重試的上下文
????RetryContext?context?=?open(retryPolicy,?state);
????if?(this.logger.isTraceEnabled())?{
????????this.logger.trace("RetryContext?retrieved:?"?+?context);
????}
????//?Make?sure?the?context?is?available?globally?for?clients?who?need
????//?it...
????RetrySynchronizationManager.register(context);
????Throwable?lastException?=?null;
????boolean?exhausted?=?false;
????try?{
????????//如果有注冊RetryListener，則會調用它的open方法，給調用者一個通知。
????????boolean?running?=?doOpenInterceptors(retryCallback,?context);
????????if?(!running)?{
????????????throw?new?TerminatedRetryException(
????????????????????"Retry?terminated?abnormally?by?interceptor?before?first?attempt");
????????}
????????//?Get?or?Start?the?backoff?context...
????????BackOffContext?backOffContext?=?null;
????????Object?resource?=?context.getAttribute("backOffContext");
????????if?(resource?instanceof?BackOffContext)?{
????????????backOffContext?=?(BackOffContext)?resource;
????????}
????????if?(backOffContext?==?null)?{
????????????backOffContext?=?backOffPolicy.start(context);
????????????if?(backOffContext?!=?null)?{
????????????????context.setAttribute("backOffContext",?backOffContext);
????????????}
????????}
????????//判斷能否重試，就是調用RetryPolicy的canRetry方法來判斷。
????????//這個循環(huán)會直到原方法不拋出異常，或不需要再重試
????????while?(canRetry(retryPolicy,?context)?&&?!context.isExhaustedOnly())?{
????????????try?{
????????????????if?(this.logger.isDebugEnabled())?{
????????????????????this.logger.debug("Retry:?count="?+?context.getRetryCount());
????????????????}
????????????????//清除上次記錄的異常
????????????????lastException?=?null;
????????????????//doWithRetry方法，一般來說就是原方法
????????????????return?retryCallback.doWithRetry(context);
????????????}
????????????catch?(Throwable?e)?{
????????????????//原方法拋出了異常
????????????????lastException?=?e;
????????????????try?{
????????????????????//記錄異常信息
????????????????????registerThrowable(retryPolicy,?state,?context,?e);
????????????????}
????????????????catch?(Exception?ex)?{
????????????????????throw?new?TerminatedRetryException("Could?not?register?throwable",
????????????????????????????ex);
????????????????}
????????????????finally?{
????????????????????//調用RetryListener的onError方法
????????????????????doOnErrorInterceptors(retryCallback,?context,?e);
????????????????}
????????????????//再次判斷能否重試
????????????????if?(canRetry(retryPolicy,?context)?&&?!context.isExhaustedOnly())?{
????????????????????try?{
????????????????????????//如果可以重試則走退避策略
????????????????????????backOffPolicy.backOff(backOffContext);
????????????????????}
????????????????????catch?(BackOffInterruptedException?ex)?{
????????????????????????lastException?=?e;
????????????????????????//?back?off?was?prevented?by?another?thread?-?fail?the?retry
????????????????????????if?(this.logger.isDebugEnabled())?{
????????????????????????????this.logger
????????????????????????????????????.debug("Abort?retry?because?interrupted:?count="
????????????????????????????????????????????+?context.getRetryCount());
????????????????????????}
????????????????????????throw?ex;
????????????????????}
????????????????}
????????????????if?(this.logger.isDebugEnabled())?{
????????????????????this.logger.debug(
????????????????????????????"Checking?for?rethrow:?count="?+?context.getRetryCount());
????????????????}
????????????????if?(shouldRethrow(retryPolicy,?context,?state))?{
????????????????????if?(this.logger.isDebugEnabled())?{
????????????????????????this.logger.debug("Rethrow?in?retry?for?policy:?count="
????????????????????????????????+?context.getRetryCount());
????????????????????}
????????????????????throw?RetryTemplate.wrapIfNecessary(e);
????????????????}
????????????}
????????????/*
?????????????*?A?stateful?attempt?that?can?retry?may?rethrow?the?exception?before?now,
?????????????*?but?if?we?get?this?far?in?a?stateful?retry?there's?a?reason?for?it,
?????????????*?like?a?circuit?breaker?or?a?rollback?classifier.
?????????????*/
????????????if?(state?!=?null?&&?context.hasAttribute(GLOBAL_STATE))?{
????????????????break;
????????????}
????????}
????????if?(state?==?null?&&?this.logger.isDebugEnabled())?{
????????????this.logger.debug(
????????????????????"Retry?failed?last?attempt:?count="?+?context.getRetryCount());
????????}
????????exhausted?=?true;
????????//重試結束后如果有兜底Recovery方法則執(zhí)行，否則拋異常
????????return?handleRetryExhausted(recoveryCallback,?context,?state);
????}
????catch?(Throwable?e)?{
????????throw?RetryTemplate.wrapIfNecessary(e);
????}
????finally?{
????????//處理一些關閉邏輯
????????close(retryPolicy,?context,?state,?lastException?==?null?||?exhausted);
????????//調用RetryListener的close方法
????????doCloseInterceptors(retryCallback,?context,?lastException);
????????RetrySynchronizationManager.clear();
????}
}
主要核心重試邏輯就是上面的代碼了，看上去還是挺簡單的。
在上面，我們漏掉了RetryPolicy的canRetry方法和BackOffPolicy的backOff方法，以及這兩個Policy是怎么來的。我們回頭看看getStatelessInterceptor方法中的getRetryPolicy和getRetryPolicy方法。
private?RetryPolicy?getRetryPolicy(Annotation?retryable)?{
????Map?attrs?=?AnnotationUtils.getAnnotationAttributes(retryable);
????@SuppressWarnings("unchecked")
????Class[]?includes?=?(Class[])?attrs.get("value");
????String?exceptionExpression?=?(String)?attrs.get("exceptionExpression");
????boolean?hasExpression?=?StringUtils.hasText(exceptionExpression);
????if?(includes.length?==?0)?{
????????@SuppressWarnings("unchecked")
????????Class[]?value?=?(Class[])?attrs.get("include");
????????includes?=?value;
????}
????@SuppressWarnings("unchecked")
????Class[]?excludes?=?(Class[])?attrs.get("exclude");
????Integer?maxAttempts?=?(Integer)?attrs.get("maxAttempts");
????String?maxAttemptsExpression?=?(String)?attrs.get("maxAttemptsExpression");
????if?(StringUtils.hasText(maxAttemptsExpression))?{
????????maxAttempts?=?PARSER.parseExpression(resolve(maxAttemptsExpression),?PARSER_CONTEXT)
????????????????.getValue(this.evaluationContext,?Integer.class);
????}
????if?(includes.length?==?0?&&?excludes.length?==?0)?{
????????SimpleRetryPolicy?simple?=?hasExpression???new?ExpressionRetryPolicy(resolve(exceptionExpression))
????????????????????????????????????????????????????????.withBeanFactory(this.beanFactory)
?????????????????????????????????????????????????:?new?SimpleRetryPolicy();
????????simple.setMaxAttempts(maxAttempts);
????????return?simple;
????}
????Map,?Boolean>?policyMap?=?new?HashMap,?Boolean>();
????for?(Class?type?:?includes)?{
????????policyMap.put(type,?true);
????}
????for?(Class?type?:?excludes)?{
????????policyMap.put(type,?false);
????}
????boolean?retryNotExcluded?=?includes.length?==?0;
????if?(hasExpression)?{
????????return?new?ExpressionRetryPolicy(maxAttempts,?policyMap,?true,?exceptionExpression,?retryNotExcluded)
????????????????.withBeanFactory(this.beanFactory);
????}
????else?{
????????return?new?SimpleRetryPolicy(maxAttempts,?policyMap,?true,?retryNotExcluded);
????}
}
嗯～，代碼不難，這里簡單做一下總結好了。就是通過@Retryable注解中的參數，來判斷具體使用文章開頭說到的哪個重試策略，是SimpleRetryPolicy還是ExpressionRetryPolicy等。
private?BackOffPolicy?getBackoffPolicy(Backoff?backoff)?{
????long?min?=?backoff.delay()?==?0???backoff.value()?:?backoff.delay();
????if?(StringUtils.hasText(backoff.delayExpression()))?{
????????min?=?PARSER.parseExpression(resolve(backoff.delayExpression()),?PARSER_CONTEXT)
????????????????.getValue(this.evaluationContext,?Long.class);
????}
????long?max?=?backoff.maxDelay();
????if?(StringUtils.hasText(backoff.maxDelayExpression()))?{
????????max?=?PARSER.parseExpression(resolve(backoff.maxDelayExpression()),?PARSER_CONTEXT)
????????????????.getValue(this.evaluationContext,?Long.class);
????}
????double?multiplier?=?backoff.multiplier();
????if?(StringUtils.hasText(backoff.multiplierExpression()))?{
????????multiplier?=?PARSER.parseExpression(resolve(backoff.multiplierExpression()),?PARSER_CONTEXT)
????????????????.getValue(this.evaluationContext,?Double.class);
????}
????if?(multiplier?>?0)?{
????????ExponentialBackOffPolicy?policy?=?new?ExponentialBackOffPolicy();
????????if?(backoff.random())?{
????????????policy?=?new?ExponentialRandomBackOffPolicy();
????????}
????????policy.setInitialInterval(min);
????????policy.setMultiplier(multiplier);
????????policy.setMaxInterval(max?>?min???max?:?ExponentialBackOffPolicy.DEFAULT_MAX_INTERVAL);
????????if?(this.sleeper?!=?null)?{
????????????policy.setSleeper(this.sleeper);
????????}
????????return?policy;
????}
????if?(max?>?min)?{
????????UniformRandomBackOffPolicy?policy?=?new?UniformRandomBackOffPolicy();
????????policy.setMinBackOffPeriod(min);
????????policy.setMaxBackOffPeriod(max);
????????if?(this.sleeper?!=?null)?{
????????????policy.setSleeper(this.sleeper);
????????}
????????return?policy;
????}
????FixedBackOffPolicy?policy?=?new?FixedBackOffPolicy();
????policy.setBackOffPeriod(min);
????if?(this.sleeper?!=?null)?{
????????policy.setSleeper(this.sleeper);
????}
????return?policy;
}
嗯～，一樣的味道。就是通過@Backoff注解中的參數，來判斷具體使用文章開頭說到的哪個退避策略，是FixedBackOffPolicy還是UniformRandomBackOffPolicy等。
那么每個RetryPolicy都會重寫canRetry方法，然后在RetryTemplate判斷是否需要重試。我們看看SimpleRetryPolicy的
@Override
public?boolean?canRetry(RetryContext?context)?{
????Throwable?t?=?context.getLastThrowable();
????//判斷拋出的異常是否符合重試的異常
????//還有，是否超過了重試的次數
????return?(t?==?null?||?retryForException(t))?&&?context.getRetryCount()?}
同樣，我們看看FixedBackOffPolicy的退避方法。
protected?void?doBackOff()?throws?BackOffInterruptedException?{
????try?{
????????//就是sleep固定的時間
????????sleeper.sleep(backOffPeriod);
????}
????catch?(InterruptedException?e)?{
????????throw?new?BackOffInterruptedException("Thread?interrupted?while?sleeping",?e);
????}
}
至此，重試的主要原理以及邏輯大概就是這樣了。
RetryContext
我覺得有必要說說RetryContext，先看看它的繼承關系。
可以看出對每一個策略都有對應的Context。
在Spring Retry里，其實每一個策略都是單例來的。我剛開始直覺是對每一個需要重試的方法都會new一個策略，這樣重試策略之間才不會產生沖突，但是一想就知道這樣就可能多出了很多策略對象出來，增加了使用者的負擔，這不是一個好的設計。
Spring Retry采用了一個更加輕量級的做法，就是針對每一個需要重試的方法只new一個上下文Context對象，然后在重試時，把這個Context傳到策略里，策略再根據這個Context做重試，而且Spring Retry還對這個Context做了cache。這樣就相當于對重試的上下文做了優(yōu)化。
總結
Spring Retry通過AOP機制來實現對業(yè)務代碼的重試”入侵“，RetryTemplate中包含了核心的重試邏輯，還提供了豐富的重試策略和退避策略。
最后，關注公眾號Java技術棧，在后臺回復：面試，可以獲取我整理的 Spring 系列面試題和答案，非常齊全。
參考資料：
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