概要

Spring實(shí)現(xiàn)了一套重試機(jī)制，功能簡單實(shí)用。Spring Retry是從Spring Batch獨(dú)立出來的一個(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)展帶來了麻煩。從面向?qū)ο蟮慕嵌葋砜?#xff0c;我們應(yīng)該把重試的代碼獨(dú)立出來。

使用介紹

基本使用

先舉個(gè)例子：

@Configuration @EnableRetry public class Application {@Beanpublic 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");}@Recoverpublic 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)說明是無狀態(tài)的，所謂無狀態(tài)就是對(duì)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(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過程中會(huì)根據(jù)PointcutAdvisor類來對(duì)Bean進(jìn)行Pointcut的過濾，然后生成對(duì)應(yīng)的AOP代理類，用advice來加強(qiáng)處理。看看RetryConfiguration的初始化:

@PostConstructpublic void init() {Set<Class<? extends Annotation>> retryableAnnotationTypes = new LinkedHashSet<Class<? extends Annotation>>(1);retryableAnnotationTypes.add(Retryable.class);//創(chuàng)建pointcutthis.pointcut = buildPointcut(retryableAnnotationTypes);//創(chuàng)建advicethis.advice = buildAdvice();if (this.advice instanceof BeanFactoryAware) {((BeanFactoryAware) this.advice).setBeanFactory(beanFactory);}}protected Pointcut buildPointcut(Set<Class<? extends Annotation>> retryAnnotationTypes) {ComposablePointcut result = null;for (Class<? extends Annotation> 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來根據(jù)注解進(jìn)行切入點(diǎn)的過濾。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)。

@Overridepublic 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<Method, MethodInterceptor>());}Map<Method, MethodInterceptor> 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interceptordelegate = getStatefulInterceptor(target, method, retryable);}else {//得到“無狀態(tài)”的interceptordelegate = 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(gè)RetryTemplateRetryTemplate template = createTemplate(retryable.listeners());//生成retryPolicytemplate.setRetryPolicy(getRetryPolicy(retryable));//生成backoffPolicytemplate.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(shí)就是為了生成RetryOperationsInterceptor。RetryOperationsInterceptor也是一個(gè)MethodInterceptor，我們來看看它的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，如果沒有攔截鏈，那就是對(duì)原來方法的調(diào)用。RetryCallback<Object, Throwable> retryCallback = new RetryCallback<Object, Throwable>() {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(diào)用到RetryTemplate的execute方法，從名字也看出來，RetryTemplate作為一個(gè)模板類，里面包含了重試統(tǒng)一邏輯。不過，我看這個(gè)RetryTemplate并不是很“模板”，因?yàn)樗鼪]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, E extends Throwable> T doExecute(RetryCallback<T, E> retryCallback,RecoveryCallback<T> recoveryCallback, RetryState state)throws E, ExhaustedRetryException {RetryPolicy retryPolicy = this.retryPolicy;BackOffPolicy backOffPolicy = this.backOffPolicy;//新建一個(gè)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(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(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 retryif (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.<E>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.<E>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是怎么來的。我們回頭看看getStatelessInterceptor方法中的getRetryPolicy和getRetryPolicy方法。

private RetryPolicy getRetryPolicy(Annotation retryable) {Map<String, Object> attrs = AnnotationUtils.getAnnotationAttributes(retryable);@SuppressWarnings("unchecked")Class<? extends Throwable>[] includes = (Class<? extends Throwable>[]) attrs.get("value");String exceptionExpression = (String) attrs.get("exceptionExpression");boolean hasExpression = StringUtils.hasText(exceptionExpression);if (includes.length == 0) {@SuppressWarnings("unchecked")Class<? extends Throwable>[] value = (Class<? extends Throwable>[]) attrs.get("include");includes = value;}@SuppressWarnings("unchecked")Class<? extends Throwable>[] excludes = (Class<? extends Throwable>[]) 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<Class<? extends Throwable>, Boolean> policyMap = new HashMap<Class<? extends Throwable>, Boolean>();for (Class<? extends Throwable> type : includes) {policyMap.put(type, true);}for (Class<? extends Throwable> 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(jié)好了。就是通過@Retryable注解中的參數(shù)，來判斷具體使用文章開頭說到的哪個(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@Backoff注解中的參數(shù)，來判斷具體使用文章開頭說到的哪個(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

@Overridepublic 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(shù)return (t == null || retryForException(t)) && context.getRetryCount() < maxAttempts;}

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

我覺得有必要說說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)策略都是單例來的。我剛開始直覺是對(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，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AOP機(jī)制來實(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

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