Spring Retry

概要

Spring实现了一套重试机制，功能简单实用。Spring Retry是从Spring Batch独立出来的一个功能，已经广泛应用于Spring Batch，Spring Integration，Spring for Apache Hadoop等Spring项目。这里讲述如何使用Spring Retry及其实现原理。

背景

重试其实其实很多时候都需要的，为了保证容错性，可用性，一致性等。一般用来应对外部系统的一些不可预料的返回、异常等，特别是网络延迟，中断等情况。还有在现在流行的微服务治理框架中，通常都有自己的重试与超时配置，比如dubbo可以设置retries=1，timeout=500调用失败只重试1次，超过500ms调用仍未返回则调用失败。如果要做重试，要为特定的某个操作做重试功能，则要硬编码，大概逻辑基本都是写个循环，根据返回或异常，计数失败次数，然后设定退出条件。这样做，且不说每个操作都要写这种类似的代码，而且重试逻辑和业务逻辑混在一起，给维护和扩展带来了麻烦。从面向对象的角度来看，应该把重试的代码独立出来。

使用介绍

基本使用

先举个例子：

  1. @Configuration  
  2. @EnableRetry  
  3. public class Application {  
  5.     @Bean  
  6.     public RetryService retryService(){  
  7.         return new RetryService();  
  8.     }  
  10.     public static void main(String[] args) throws Exception{  
  11.         ApplicationContext applicationContext = new AnnotationConfigApplicationContext("springretry");  
  12.         RetryService service1 = applicationContext.getBean("service", RetryService.class);  
  13.         service1.service();  
  14.     }  
  15. }  
  17. @Service("service")  
  18. public class RetryService {  
  20.     @Retryable(value = IllegalAccessException.class, maxAttempts = 5,  
  21.                backoff= @Backoff(value = 1500, maxDelay = 100000, multiplier = 1.2))  
  22.     public void service() throws IllegalAccessException {  
  23.         System.out.println("service method...");  
  24.         throw new IllegalAccessException("manual exception");  
  25.     }  
  27.     @Recover  
  28.     public void recover(IllegalAccessException e){  
  29.         System.out.println("service retry after Recover => " + e.getMessage());  
  30.     }  
  31. }

@EnableRetry - 表示开启重试机制 @Retryable - 表示这个方法需要重试，它有很丰富的参数，可以满足对重试的需求 @Backoff - 表示重试中的退避策略 @Recover - 兜底方法，即多次重试后还是失败就会执行这个方法Spring-Retry 的功能丰富在于其重试策略和退避策略，还有兜底，监听器等操作。

重试策略

看一下Spring Retry自带的一些重试策略，主要是用来判断当方法调用异常时是否需要重试。

  • SimpleRetryPolicy 默认最多重试3次
  • TimeoutRetryPolicy 默认在1秒内失败都会重试
  • ExpressionRetryPolicy 符合表达式就会重试
  • CircuitBreakerRetryPolicy 增加了熔断的机制，如果不在熔断状态，则允许重试
  • CompositeRetryPolicy 可以组合多个重试策略
  • NeverRetryPolicy 从不重试（也是一种重试策略）

  • AlwaysRetryPolicy 总是重试

    退避策略

    看一下退避策略，退避是指怎么去做下一次的重试，在这里其实就是等待多长时间。Spring-Retry重试机制实现原理 - 图1

  • FixedBackOffPolicy 默认固定延迟1秒后执行下一次重试

  • ExponentialBackOffPolicy 指数递增延迟执行重试，默认初始0.1秒，系数是2，那么下次延迟0.2秒，再下次就是延迟0.4秒，如此类推，最大30秒。
  • ExponentialRandomBackOffPolicy 在上面那个策略上增加随机性
  • UniformRandomBackOffPolicy 这个跟上面的区别就是，上面的延迟会不停递增，这个只会在固定的区间随机

  • StatelessBackOffPolicy 这个说明是无状态的，所谓无状态就是对上次的退避无感知，从它下面的子类也能看出来

    原理

    原理部分，一是重试机制的切入点，即它是如何使得代码实现重试功能的；二是重试机制的详细，包括重试的逻辑以及重试策略和退避策略的实现。

    切入点

    @EnableRetry

    ```java @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;

}

  1. 可以看到`@EnableAspectJAutoProxy(proxyTargetClass = false)`这个并不陌生，就是打开Spring AOP功能。重点看看`@Import(RetryConfiguration.class)`，`@Import`相当于注册这个Bean，看看这个`RetryConfiguration`![](https://cdn.nlark.com/yuque/0/2020/webp/396745/1608081416536-a0a57d0d-bdfa-4379-800e-885b3595577d.webp#crop=0&crop=0&crop=1&crop=1&height=328&id=yoIU3&originHeight=328&originWidth=998&originalType=binary&ratio=1&rotation=0&showTitle=false&size=0&status=done&style=shadow&title=&width=998)它是一个`AbstractPointcutAdvisor`，它有一个pointcut和一个advice。在IOC过程中会根据`PointcutAdvisor`类来对Bean进行`Pointcut`的过滤，然后生成对应的AOP代理类，用advice来加强处理。看看`RetryConfiguration`的初始化:
  2. ```java
  3. @PostConstruct  
  4. public void init() {  
  5.     Set<Class<? extends Annotation>> retryableAnnotationTypes = new LinkedHashSet<Class<? extends Annotation>>(1);  
  6.     retryableAnnotationTypes.add(Retryable.class);  
  7.     //创建pointcut  
  8.     this.pointcut = buildPointcut(retryableAnnotationTypes);  
  9.     //创建advice  
  10.     this.advice = buildAdvice();  
  11.     if (this.advice instanceof BeanFactoryAware) {  
  12.         ((BeanFactoryAware) this.advice).setBeanFactory(beanFactory);  
  13.     }  
  14. }  
  16. protected Pointcut buildPointcut(Set<Class<? extends Annotation>> retryAnnotationTypes) {  
  17.     ComposablePointcut result = null;  
  18.     for (Class<? extends Annotation> retryAnnotationType : retryAnnotationTypes) {  
  19.         Pointcut filter = new AnnotationClassOrMethodPointcut(retryAnnotationType);  
  20.         if (result == null) {  
  21.             result = new ComposablePointcut(filter);  
  22.         }  
  23.         else {  
  24.             result.union(filter);  
  25.         }  
  26.     }  
  27.     return result;  
  28. }

上面代码用到了AnnotationClassOrMethodPointcut，其实它最终还是用到了AnnotationMethodMatcher来根据注解进行切入点的过滤。这里就是@Retryable注解了。

  1. //创建advice对象，即拦截器  
  2. protected Advice buildAdvice() {  
  3.     //下面关注这个对象  
  4.     AnnotationAwareRetryOperationsInterceptor interceptor = new AnnotationAwareRetryOperationsInterceptor();  
  5.     if (retryContextCache != null) {  
  6.         interceptor.setRetryContextCache(retryContextCache);  
  7.     }  
  8.     if (retryListeners != null) {  
  9.         interceptor.setListeners(retryListeners);  
  10.     }  
  11.     if (methodArgumentsKeyGenerator != null) {  
  12.         interceptor.setKeyGenerator(methodArgumentsKeyGenerator);  
  13.     }  
  14.     if (newMethodArgumentsIdentifier != null) {  
  15.         interceptor.setNewItemIdentifier(newMethodArgumentsIdentifier);  
  16.     }  
  17.     if (sleeper != null) {  
  18.         interceptor.setSleeper(sleeper);  
  19.     }  
  20.     return interceptor;  
  21. }

AnnotationAwareRetryOperationsInterceptor

Spring-Retry重试机制实现原理 - 图2可以看出AnnotationAwareRetryOperationsInterceptor是一个MethodInterceptor，在创建AOP代理过程中如果目标方法符合pointcut的规则，它就会加到interceptor列表中，然后做增强，看看invoke方法做了什么增强。

  1. @Override  
  2. public Object invoke(MethodInvocation invocation) throws Throwable {  
  3.     MethodInterceptor delegate = getDelegate(invocation.getThis(), invocation.getMethod());  
  4.     if (delegate != null) {  
  5.         return delegate.invoke(invocation);  
  6.     }  
  7.     else {  
  8.         return invocation.proceed();  
  9.     }  
  10. }

这里用到了委托，主要是需要根据配置委托给具体“有状态”的interceptor还是“无状态”的interceptor。

  1. private MethodInterceptor getDelegate(Object target, Method method) {  
  2.     if (!this.delegates.containsKey(target) || !this.delegates.get(target).containsKey(method)) {  
  3.         synchronized (this.delegates) {  
  4.             if (!this.delegates.containsKey(target)) {  
  5.                 this.delegates.put(target, new HashMap<Method, MethodInterceptor>());  
  6.             }  
  7.             Map<Method, MethodInterceptor> delegatesForTarget = this.delegates.get(target);  
  8.             if (!delegatesForTarget.containsKey(method)) {  
  9.                 Retryable retryable = AnnotationUtils.findAnnotation(method, Retryable.class);  
  10.                 if (retryable == null) {  
  11.                     retryable = AnnotationUtils.findAnnotation(method.getDeclaringClass(), Retryable.class);  
  12.                 }  
  13.                 if (retryable == null) {  
  14.                     retryable = findAnnotationOnTarget(target, method);  
  15.                 }  
  16.                 if (retryable == null) {  
  17.                     return delegatesForTarget.put(method, null);  
  18.                 }  
  19.                 MethodInterceptor delegate;  
  20.                 //支持自定义MethodInterceptor，而且优先级最高  
  21.                 if (StringUtils.hasText(retryable.interceptor())) {  
  22.                     delegate = this.beanFactory.getBean(retryable.interceptor(), MethodInterceptor.class);  
  23.                 }  
  24.                 else if (retryable.stateful()) {  
  25.                     //得到“有状态”的interceptor  
  26.                     delegate = getStatefulInterceptor(target, method, retryable);  
  27.                 }  
  28.                 else {  
  29.                     //得到“无状态”的interceptor  
  30.                     delegate = getStatelessInterceptor(target, method, retryable);  
  31.                 }  
  32.                 delegatesForTarget.put(method, delegate);  
  33.             }  
  34.         }  
  35.     }  
  36.     return this.delegates.get(target).get(method);  
  37. }

getStatefulInterceptor和getStatelessInterceptor都是差不多，先看看比较简单的getStatelessInterceptor。

  1. private MethodInterceptor getStatelessInterceptor(Object target, Method method, Retryable retryable) {  
  2.     //生成一个RetryTemplate  
  3.     RetryTemplate template = createTemplate(retryable.listeners());  
  4.     //生成retryPolicy  
  5.     template.setRetryPolicy(getRetryPolicy(retryable));  
  6.     //生成backoffPolicy  
  7.     template.setBackOffPolicy(getBackoffPolicy(retryable.backoff()));  
  8.     return RetryInterceptorBuilder.stateless()  
  9.         .retryOperations(template)  
  10.         .label(retryable.label())  
  11.         .recoverer(getRecoverer(target, method))  
  12.         .build();  
  13. }

具体生成retryPolicy和backoffPolicy的规则，等下再回头来看。RetryInterceptorBuilder其实就是为了生成RetryOperationsInterceptor。RetryOperationsInterceptor也是一个MethodInterceptor，来看看它的invoke方法。

  1. public Object invoke(final MethodInvocation invocation) throws Throwable {  
  3.     String name;  
  4.     if (StringUtils.hasText(label)) {  
  5.         name = label;  
  6.     } else {  
  7.         name = invocation.getMethod().toGenericString();  
  8.     }  
  9.     final String label = name;  
  11.     //定义了一个RetryCallback，其实看它的doWithRetry方法，调用了invocation的proceed()方法，是不是有点眼熟，这就是AOP的拦截链调用，如果没有拦截链，那就是对原来方法的调用。  
  12.     RetryCallback<Object, Throwable> retryCallback = new RetryCallback<Object, Throwable>() {  
  14.         public Object doWithRetry(RetryContext context) throws Exception {  
  16.             context.setAttribute(RetryContext.NAME, label);  
  18.             /*  
  19.              * If we don't copy the invocation carefully it won't keep a reference to  
  20.              * the other interceptors in the chain. We don't have a choice here but to  
  21.              * specialise to ReflectiveMethodInvocation (but how often would another  
  22.              * implementation come along?).  
  23.              */  
  24.             if (invocation instanceof ProxyMethodInvocation) {  
  25.                 try {  
  26.                     return ((ProxyMethodInvocation) invocation).invocableClone().proceed();  
  27.                 }  
  28.                 catch (Exception e) {  
  29.                     throw e;  
  30.                 }  
  31.                 catch (Error e) {  
  32.                     throw e;  
  33.                 }  
  34.                 catch (Throwable e) {  
  35.                     throw new IllegalStateException(e);  
  36.                 }  
  37.             }  
  38.             else {  
  39.                 throw new IllegalStateException(  
  40.                     "MethodInvocation of the wrong type detected - this should not happen with Spring AOP, " +  
  41.                     "so please raise an issue if you see this exception");  
  42.             }  
  43.         }  
  45.     };  
  47.     if (recoverer != null) {  
  48.         ItemRecovererCallback recoveryCallback = new ItemRecovererCallback(  
  49.             invocation.getArguments(), recoverer);  
  50.         return this.retryOperations.execute(retryCallback, recoveryCallback);  
  51.     }  
  52.     //最终还是进入到retryOperations的execute方法，这个retryOperations就是在之前的builder set进来的RetryTemplate。  
  53.     return this.retryOperations.execute(retryCallback);  
  55. }

无论是RetryOperationsInterceptor还是StatefulRetryOperationsInterceptor，最终的拦截处理逻辑还是调用到RetryTemplate的execute方法，从名字也看出来，RetryTemplate作为一个模板类，里面包含了重试统一逻辑。

重试逻辑及策略实现

上面介绍了Spring Retry利用了AOP代理使重试机制对业务代码进行“入侵”。下面继续看看重试的逻辑做了什么。RetryTemplate的doExecute方法。

  1. protected <T, E extends Throwable> T doExecute(RetryCallback<T, E> retryCallback,  
  2.                                                RecoveryCallback<T> recoveryCallback, RetryState state)  
  3.     throws E, ExhaustedRetryException {  
  5.     RetryPolicy retryPolicy = this.retryPolicy;  
  6.     BackOffPolicy backOffPolicy = this.backOffPolicy;  
  8.     //新建一个RetryContext来保存本轮重试的上下文  
  9.     RetryContext context = open(retryPolicy, state);  
  10.     if (this.logger.isTraceEnabled()) {  
  11.         this.logger.trace("RetryContext retrieved: " + context);  
  12.     }  
  14.     // Make sure the context is available globally for clients who need  
  15.     // it...  
  16.     RetrySynchronizationManager.register(context);  
  18.     Throwable lastException = null;  
  20.     boolean exhausted = false;  
  21.     try {  
  23.         //如果有注册RetryListener，则会调用它的open方法，给调用者一个通知。  
  24.         boolean running = doOpenInterceptors(retryCallback, context);  
  26.         if (!running) {  
  27.             throw new TerminatedRetryException(  
  28.                 "Retry terminated abnormally by interceptor before first attempt");  
  29.         }  
  31.         // Get or Start the backoff context...  
  32.         BackOffContext backOffContext = null;  
  33.         Object resource = context.getAttribute("backOffContext");  
  35.         if (resource instanceof BackOffContext) {  
  36.             backOffContext = (BackOffContext) resource;  
  37.         }  
  39.         if (backOffContext == null) {  
  40.             backOffContext = backOffPolicy.start(context);  
  41.             if (backOffContext != null) {  
  42.                 context.setAttribute("backOffContext", backOffContext);  
  43.             }  
  44.         }  
  46.         //判断能否重试，就是调用RetryPolicy的canRetry方法来判断。  
  47.         //这个循环会直到原方法不抛出异常，或不需要再重试  
  48.         while (canRetry(retryPolicy, context) && !context.isExhaustedOnly()) {  
  50.             try {  
  51.                 if (this.logger.isDebugEnabled()) {  
  52.                     this.logger.debug("Retry: count=" + context.getRetryCount());  
  53.                 }  
  54.                 //清除上次记录的异常  
  55.                 lastException = null;  
  56.                 //doWithRetry方法，一般来说就是原方法  
  57.                 return retryCallback.doWithRetry(context);  
  58.             }  
  59.             catch (Throwable e) {  
  60.                 //原方法抛出了异常  
  61.                 lastException = e;  
  63.                 try {  
  64.                     //记录异常信息  
  65.                     registerThrowable(retryPolicy, state, context, e);  
  66.                 }  
  67.                 catch (Exception ex) {  
  68.                     throw new TerminatedRetryException("Could not register throwable",  
  69.                                                        ex);  
  70.                 }  
  71.                 finally {  
  72.                     //调用RetryListener的onError方法  
  73.                     doOnErrorInterceptors(retryCallback, context, e);  
  74.                 }  
  75.                 //再次判断能否重试  
  76.                 if (canRetry(retryPolicy, context) && !context.isExhaustedOnly()) {  
  77.                     try {  
  78.                         //如果可以重试则走退避策略  
  79.                         backOffPolicy.backOff(backOffContext);  
  80.                     }  
  81.                     catch (BackOffInterruptedException ex) {  
  82.                         lastException = e;  
  83.                         // back off was prevented by another thread - fail the retry  
  84.                         if (this.logger.isDebugEnabled()) {  
  85.                             this.logger  
  86.                                 .debug("Abort retry because interrupted: count="  
  87.                                        + context.getRetryCount());  
  88.                         }  
  89.                         throw ex;  
  90.                     }  
  91.                 }  
  93.                 if (this.logger.isDebugEnabled()) {  
  94.                     this.logger.debug(  
  95.                         "Checking for rethrow: count=" + context.getRetryCount());  
  96.                 }  
  98.                 if (shouldRethrow(retryPolicy, context, state)) {  
  99.                     if (this.logger.isDebugEnabled()) {  
  100.                         this.logger.debug("Rethrow in retry for policy: count="  
  101.                                           + context.getRetryCount());  
  102.                     }  
  103.                     throw RetryTemplate.<E>wrapIfNecessary(e);  
  104.                 }  
  106.             }  
  108.             /*  
  109.              * A stateful attempt that can retry may rethrow the exception before now,  
  110.              * but if we get this far in a stateful retry there's a reason for it,  
  111.              * like a circuit breaker or a rollback classifier.  
  112.              */  
  113.             if (state != null && context.hasAttribute(GLOBAL_STATE)) {  
  114.                 break;  
  115.             }  
  116.         }  
  118.         if (state == null && this.logger.isDebugEnabled()) {  
  119.             this.logger.debug(  
  120.                 "Retry failed last attempt: count=" + context.getRetryCount());  
  121.         }  
  123.         exhausted = true;  
  124.         //重试结束后如果有兜底Recovery方法则执行，否则抛异常  
  125.         return handleRetryExhausted(recoveryCallback, context, state);  
  127.     }  
  128.     catch (Throwable e) {  
  129.         throw RetryTemplate.<E>wrapIfNecessary(e);  
  130.     }  
  131.     finally {  
  132.         //处理一些关闭逻辑  
  133.         close(retryPolicy, context, state, lastException == null || exhausted);  
  134.         //调用RetryListener的close方法  
  135.         doCloseInterceptors(retryCallback, context, lastException);  
  136.         RetrySynchronizationManager.clear();  
  137.     }  
  138. }

主要核心重试逻辑就是上面的代码了，看上去还是挺简单的。在上面，漏掉了RetryPolicy的canRetry方法和BackOffPolicy的backOff方法，以及这两个Policy是怎么来的。回头看看getStatelessInterceptor方法中的getRetryPolicy和getRetryPolicy方法。

  1. private RetryPolicy getRetryPolicy(Annotation retryable) {  
  2.     Map<String, Object> attrs = AnnotationUtils.getAnnotationAttributes(retryable);  
  3.     @SuppressWarnings("unchecked")  
  4.     Class<? extends Throwable>[] includes = (Class<? extends Throwable>[]) attrs.get("value");  
  5.     String exceptionExpression = (String) attrs.get("exceptionExpression");  
  6.     boolean hasExpression = StringUtils.hasText(exceptionExpression);  
  7.     if (includes.length == 0) {  
  8.         @SuppressWarnings("unchecked")  
  9.         Class<? extends Throwable>[] value = (Class<? extends Throwable>[]) attrs.get("include");  
  10.         includes = value;  
  11.     }  
  12.     @SuppressWarnings("unchecked")  
  13.     Class<? extends Throwable>[] excludes = (Class<? extends Throwable>[]) attrs.get("exclude");  
  14.     Integer maxAttempts = (Integer) attrs.get("maxAttempts");  
  15.     String maxAttemptsExpression = (String) attrs.get("maxAttemptsExpression");  
  16.     if (StringUtils.hasText(maxAttemptsExpression)) {  
  17.         maxAttempts = PARSER.parseExpression(resolve(maxAttemptsExpression), PARSER_CONTEXT)  
  18.             .getValue(this.evaluationContext, Integer.class);  
  19.     }  
  20.     if (includes.length == 0 && excludes.length == 0) {  
  21.         SimpleRetryPolicy simple = hasExpression ? new ExpressionRetryPolicy(resolve(exceptionExpression))  
  22.             .withBeanFactory(this.beanFactory)  
  23.             : new SimpleRetryPolicy();  
  24.         simple.setMaxAttempts(maxAttempts);  
  25.         return simple;  
  26.     }  
  27.     Map<Class<? extends Throwable>, Boolean> policyMap = new HashMap<Class<? extends Throwable>, Boolean>();  
  28.     for (Class<? extends Throwable> type : includes) {  
  29.         policyMap.put(type, true);  
  30.     }  
  31.     for (Class<? extends Throwable> type : excludes) {  
  32.         policyMap.put(type, false);  
  33.     }  
  34.     boolean retryNotExcluded = includes.length == 0;  
  35.     if (hasExpression) {  
  36.         return new ExpressionRetryPolicy(maxAttempts, policyMap, true, exceptionExpression, retryNotExcluded)  
  37.             .withBeanFactory(this.beanFactory);  
  38.     }  
  39.     else {  
  40.         return new SimpleRetryPolicy(maxAttempts, policyMap, true, retryNotExcluded);  
  41.     }  
  42. }

这里简单做一下总结。就是通过@Retryable注解中的参数，来判断具体使用哪个重试策略，是SimpleRetryPolicy还是ExpressionRetryPolicy等。

  1. private BackOffPolicy getBackoffPolicy(Backoff backoff) {  
  2.     long min = backoff.delay() == 0 ? backoff.value() : backoff.delay();  
  3.     if (StringUtils.hasText(backoff.delayExpression())) {  
  4.         min = PARSER.parseExpression(resolve(backoff.delayExpression()), PARSER_CONTEXT)  
  5.             .getValue(this.evaluationContext, Long.class);  
  6.     }  
  7.     long max = backoff.maxDelay();  
  8.     if (StringUtils.hasText(backoff.maxDelayExpression())) {  
  9.         max = PARSER.parseExpression(resolve(backoff.maxDelayExpression()), PARSER_CONTEXT)  
  10.             .getValue(this.evaluationContext, Long.class);  
  11.     }  
  12.     double multiplier = backoff.multiplier();  
  13.     if (StringUtils.hasText(backoff.multiplierExpression())) {  
  14.         multiplier = PARSER.parseExpression(resolve(backoff.multiplierExpression()), PARSER_CONTEXT)  
  15.             .getValue(this.evaluationContext, Double.class);  
  16.     }  
  17.     if (multiplier > 0) {  
  18.         ExponentialBackOffPolicy policy = new ExponentialBackOffPolicy();  
  19.         if (backoff.random()) {  
  20.             policy = new ExponentialRandomBackOffPolicy();  
  21.         }  
  22.         policy.setInitialInterval(min);  
  23.         policy.setMultiplier(multiplier);  
  24.         policy.setMaxInterval(max > min ? max : ExponentialBackOffPolicy.DEFAULT_MAX_INTERVAL);  
  25.         if (this.sleeper != null) {  
  26.             policy.setSleeper(this.sleeper);  
  27.         }  
  28.         return policy;  
  29.     }  
  30.     if (max > min) {  
  31.         UniformRandomBackOffPolicy policy = new UniformRandomBackOffPolicy();  
  32.         policy.setMinBackOffPeriod(min);  
  33.         policy.setMaxBackOffPeriod(max);  
  34.         if (this.sleeper != null) {  
  35.             policy.setSleeper(this.sleeper);  
  36.         }  
  37.         return policy;  
  38.     }  
  39.     FixedBackOffPolicy policy = new FixedBackOffPolicy();  
  40.     policy.setBackOffPeriod(min);  
  41.     if (this.sleeper != null) {  
  42.         policy.setSleeper(this.sleeper);  
  43.     }  
  44.     return policy;  
  45. }

通过@Backoff注解中的参数，来判断具体使用哪个退避策略，是FixedBackOffPolicy还是UniformRandomBackOffPolicy等。那么每个RetryPolicy都会重写canRetry方法，然后在RetryTemplate判断是否需要重试。看看SimpleRetryPolicy的

  1. @Override  
  2. public boolean canRetry(RetryContext context) {  
  3.     Throwable t = context.getLastThrowable();  
  4.     //判断抛出的异常是否符合重试的异常  
  5.     //还有，是否超过了重试的次数  
  6.     return (t == null || retryForException(t)) && context.getRetryCount() < maxAttempts;  
  7. }

同样，看看FixedBackOffPolicy的退避方法。

  1. protected void doBackOff() throws BackOffInterruptedException {  
  2.     try {  
  3.         //就是sleep固定的时间  
  4.         sleeper.sleep(backOffPeriod);  
  5.     }  
  6.     catch (InterruptedException e) {  
  7.         throw new BackOffInterruptedException("Thread interrupted while sleeping", e);  
  8.     }  
  9. }

至此，重试的主要原理以及逻辑大概就是这样了。

RetryContext

先看看RetryContext的继承关系。Spring-Retry重试机制实现原理 - 图3可以看出对每一个策略都有对应的Context。在Spring Retry里，其实每一个策略都是单例来的。Spring Retry采用了一个更加轻量级的做法，就是针对每一个需要重试的方法只new一个上下文Context对象，然后在重试时，把这个Context传到策略里，策略再根据这个Context做重试，而且Spring Retry还对这个Context做了cache。这样就相当于对重试的上下文做了优化。

总结

Spring Retry通过AOP机制来实现对业务代码的重试”入侵“，RetryTemplate中包含了核心的重试逻辑，还提供了丰富的重试策略和退避策略。