Spring-Retry重試實(shí)現(xiàn)原理
點(diǎn)擊上方藍(lán)色“程序猿DD”，選擇“設(shè)為星標(biāo)”
回復(fù)“資源”獲取獨(dú)家整理的學(xué)習(xí)資料！
作者 |?Alben
概要
Spring實(shí)現(xiàn)了一套重試機(jī)制，功能簡(jiǎn)單實(shí)用。Spring Retry是從Spring Batch獨(dú)立出來(lái)的一個(gè)功能，已經(jīng)廣泛應(yīng)用于Spring Batch,Spring Integration, Spring for Apache Hadoop等Spring項(xiàng)目。本文將講述如何使用Spring Retry及其實(shí)現(xiàn)原理。
背景
重試，其實(shí)我們其實(shí)很多時(shí)候都需要的，為了保證容錯(cuò)性，可用性，一致性等。一般用來(lái)應(yīng)對(duì)外部系統(tǒng)的一些不可預(yù)料的返回、異常等，特別是網(wǎng)絡(luò)延遲，中斷等情況。還有在現(xiàn)在流行的微服務(wù)治理框架中，通常都有自己的重試與超時(shí)配置，比如dubbo可以設(shè)置retries=1，timeout=500調(diào)用失敗只重試1次，超過(guò)500ms調(diào)用仍未返回則調(diào)用失敗。如果我們要做重試，要為特定的某個(gè)操作做重試功能，則要硬編碼，大概邏輯基本都是寫個(gè)循環(huán)，根據(jù)返回或異常，計(jì)數(shù)失敗次數(shù)，然后設(shè)定退出條件。這樣做，且不說(shuō)每個(gè)操作都要寫這種類似的代碼，而且重試邏輯和業(yè)務(wù)邏輯混在一起，給維護(hù)和擴(kuò)展帶來(lái)了麻煩。從面向?qū)ο蟮慕嵌葋?lái)看，我們應(yīng)該把重試的代碼獨(dú)立出來(lái)。
使用介紹
基本使用
先舉個(gè)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(jī)制 @Retryable - 表示這個(gè)方法需要重試，它有很豐富的參數(shù)，可以滿足你對(duì)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(huì)執(zhí)行這個(gè)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(gè)注解里面的參數(shù)，都是很簡(jiǎn)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。
重試策略
看一下Spring Retry自帶的一些重試策略，主要是用來(lái)判斷當(dāng)方法調(diào)用異常時(shí)是否需要重試。（下文原理部分會(huì)深入分析實(shí)現(xiàn)）
SimpleRetryPolicy 默認(rèn)最多重試3次 TimeoutRetryPolicy 默認(rèn)在1秒內(nèi)失敗都會(huì)重試 ExpressionRetryPolicy 符合表達(dá)式就會(huì)重試 CircuitBreakerRetryPolicy 增加了熔斷的機(jī)制，如果不在熔斷狀態(tài)，則允許重試 CompositeRetryPolicy 可以組合多個(gè)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(shí)就是等待多長(zhǎng)時(shí)間。（下文原理部分會(huì)深入分析實(shí)現(xiàn)）
FixedBackOffPolicy 默認(rèn)固定延遲1秒后執(zhí)行下一次重試 ExponentialBackOffPolicy 指數(shù)遞增延遲執(zhí)行重試，默認(rèn)初始0.1秒，系數(shù)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(gè)策略上增加隨機(jī)性 UniformRandomBackOffPolicy 這個(gè)跟上面的區(qū)別就是，上面的延遲會(huì)不停遞增，這個(gè)只會(huì)在固定的區(qū)間隨機(jī) StatelessBackOffPolicy 這個(gè)說(shuō)明是無(wú)狀態(tài)的，所謂無(wú)狀態(tài)就是對(duì)上次的退避無(wú)感知，從它下面的子類也能看出來(lái)
原理
原理部分我想分開兩部分來(lái)講，一是重試機(jī)制的切入點(diǎn)，即它是如何使得你的代碼實(shí)現(xiàn)重試功能的；二是重試機(jī)制的詳細(xì)，包括重試的邏輯以及重試策略和退避策略的實(shí)現(xiàn)。
切入點(diǎn)
@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(gè)并不陌生，就是打開Spring AOP功能。重點(diǎn)看看@Import(RetryConfiguration.class)?@Import相當(dāng)于注冊(cè)這個(gè)Bean
我們看看這個(gè)RetryConfiguration是個(gè)什么東西?
它是一個(gè)AbstractPointcutAdvisor，它有一個(gè)pointcut和一個(gè)advice。我們知道，在IOC過(guò)程中會(huì)根據(jù)PointcutAdvisor類來(lái)對(duì)Bean進(jìn)行Pointcut的過(guò)濾，然后生成對(duì)應(yīng)的AOP代理類，用advice來(lái)加強(qiáng)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(shí)它最終還是用到了AnnotationMethodMatcher來(lái)根據(jù)注解進(jìn)行切入點(diǎn)的過(guò)濾。這里就是@Retryable注解了。
//創(chuàng)建advice對(duì)象，即攔截器
???protected?Advice?buildAdvice()?{
????//下面關(guān)注這個(gè)對(duì)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(guān)系?
可以看出AnnotationAwareRetryOperationsInterceptor是一個(gè)MethodInterceptor，在創(chuàng)建AOP代理過(guò)程中如果目標(biāo)方法符合pointcut的規(guī)則，它就會(huì)加到interceptor列表中，然后做增強(qiáng)，我們看看invoke方法做了什么增強(qiáng)。
@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(jù)配置委托給具體“有狀態(tài)”的interceptor還是“無(wú)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(jí)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(wú)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(jiǎn)單的getStatelessInterceptor。
private?MethodInterceptor?getStatelessInterceptor(Object?target,?Method?method,?Retryable?retryable)?{
??//生成一個(gè)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(lái)看。RetryInterceptorBuilder其實(shí)就是為了生成RetryOperationsInterceptor。RetryOperationsInterceptor也是一個(gè)MethodInterceptor，我們來(lái)看看它的invoke方法。
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(gè)RetryCallback，其實(shí)看它的doWithRetry方法，調(diào)用了invocation的proceed()方法，是不是有點(diǎn)眼熟，這就是AOP的攔截鏈調(diào)用，如果沒(méi)有攔截鏈，那就是對(duì)原來(lái)方法的調(diào)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(jìn)入到retryOperations的execute方法，這個(gè)retryOperations就是在之前的builder set進(jìn)來(lái)的RetryTemplate。
??return?this.retryOperations.execute(retryCallback);
?}
無(wú)論是RetryOperationsInterceptor還是StatefulRetryOperationsInterceptor，最終的攔截處理邏輯還是調(diào)用到RetryTemplate的execute方法，從名字也看出來(lái)，RetryTemplate作為一個(gè)模板類，里面包含了重試統(tǒng)一邏輯。不過(guò)，我看這個(gè)RetryTemplate并不是很“模板”，因?yàn)樗鼪](méi)有很多可以擴(kuò)展的地方。
重試邏輯及策略實(shí)現(xiàn)
上面介紹了Spring Retry利用了AOP代理使重試機(jī)制對(duì)業(yè)務(wù)代碼進(jìn)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(gè)RetryContext來(lái)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(cè)RetryListener，則會(huì)調(diào)用它的open方法，給調(diào)用者一個(gè)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(diào)用RetryPolicy的canRetry方法來(lái)判斷。
???//這個(gè)循環(huán)會(huì)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(lái)說(shuō)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(diào)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(jié)束后如果有兜底R(shí)ecovery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(guān)閉邏輯
???close(retryPolicy,?context,?state,?lastException?==?null?||?exhausted);
???//調(diào)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(jiǎn)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(gè)Policy是怎么來(lái)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(jiǎn)單做一下總結(jié)好了。就是通過(guò)@Retryable注解中的參數(shù)，來(lái)判斷具體使用文章開頭說(shuō)到的哪個(gè)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(guò)@Backoff注解中的參數(shù)，來(lái)判斷具體使用文章開頭說(shuō)到的哪個(gè)退避策略，是FixedBackOffPolicy還是UniformRandomBackOffPolicy等。
那么每個(gè)RetryPolicy都會(huì)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(guò)了重試的次數(shù)
??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(shí)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(jué)得有必要說(shuō)說(shuō)RetryContext，先看看它的繼承關(guān)系。
可以看出對(duì)每一個(gè)策略都有對(duì)應(yīng)的Context。
在Spring Retry里，其實(shí)每一個(gè)策略都是單例來(lái)的。我剛開始直覺(jué)是對(duì)每一個(gè)需要重試的方法都會(huì)new一個(gè)策略，這樣重試策略之間才不會(huì)產(chǎn)生沖突，但是一想就知道這樣就可能多出了很多策略對(duì)象出來(lái)，增加了使用者的負(fù)擔(dān)，這不是一個(gè)好的設(shè)計(jì)。Spring Retry采用了一個(gè)更加輕量級(jí)的做法，就是針對(duì)每一個(gè)需要重試的方法只new一個(gè)上下文Context對(duì)象，然后在重試時(shí)，把這個(gè)Context傳到策略里，策略再根據(jù)這個(gè)Context做重試，而且Spring Retry還對(duì)這個(gè)Context做了cache。這樣就相當(dāng)于對(duì)重試的上下文做了優(yōu)化。
總結(jié)
Spring Retry通過(guò)AOP機(jī)制來(lái)實(shí)現(xiàn)對(duì)業(yè)務(wù)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。
參考資料
http://www.10tiao.com/html/164/201705/2652898434/1.html https://www.jianshu.com/p/58e753ca0151 https://paper.tuisec.win/detail/90bd660fad92183
DD自研的滬牌代拍業(yè)務(wù)，點(diǎn)擊直達(dá)
【往期推薦】
2020-12-04
2020-12-04
2020-12-03
2020-12-03
2020-12-02
2020-12-01
掃一掃，關(guān)注我
一起學(xué)習(xí)，一起進(jìn)步
每周贈(zèng)書，福利不斷
﹀
﹀
﹀
深度內(nèi)容
推薦加入
