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。本文将讲述如何使用Spring Retry及其实现原理。

背景

重试,其实我们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。

如果我们要做重试,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,我们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:

@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,可以满足你对重试的需求 @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。

然后每个注解里面的参数,都是很简单的,大家看一下就知道是什么意思,怎么用了,我就不多讲了。

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。(下文原理部分会深入分析实现)

  • 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(也是一种重试策略哈)
  • AlwaysRetryPolicy 总是重试

….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试,在这里其实就是等待多长时间。(下文原理部分会深入分析实现)

  • 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

原理部分我想分开两部分来讲,一是重试机制的切入点,即它是如何使得你的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

@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;  

}  

我们可以看到@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

我们看看这个RetryConfiguration是个什么东西

它是一个AbstractPointcutAdvisor,它有一个pointcut和一个advice。我们知道,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:

@PostConstruct  
 public void init() {  
  Set<Class<? extends Annotation>> retryableAnnotationTypes = new LinkedHashSet<Class<? extends Annotation>>(1);  
  retryableAnnotationTypes.add(Retryable.class);  
        //创建pointcut  
  this.pointcut = buildPointcut(retryableAnnotationTypes);  
        //创建advice  
  this.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。

//创建advice对象,即拦截器  
   protected Advice buildAdvice() {  
    //下面关注这个对象  
 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

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,我们看看invoke方法做了什么增强。

@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();  
  }  
 }  

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。

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;  
     //支持自定义MethodInterceptor,而且优先级最高  
     if (StringUtils.hasText(retryable.interceptor())) {  
      delegate = this.beanFactory.getBean(retryable.interceptor(), MethodInterceptor.class);  
     }  
     else if (retryable.stateful()) {  
                     //得到“有状态”的interceptor  
      delegate = getStatefulInterceptor(target, method, retryable);  
     }  
     else {  
                     //得到“无状态”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,我们先看看比较简单的getStatelessInterceptor。

private MethodInterceptor getStatelessInterceptor(Object target, Method method, Retryable retryable) {  
  //生成一个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();  
 }  

具体生成retryPolicy和backoffPolicy的规则,我们等下再回头来看。RetryInterceptorBuilder其实就是为了生成RetryOperationsInterceptor。RetryOperationsInterceptor也是一个MethodInterceptor,我们来看看它的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。  
  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。  
  return this.retryOperations.execute(retryCallback);  

 }  

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。不过,我看这个RetryTemplate并不是很“模板”,因为它没有很多可以扩展的地方。

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”。下面我们继续看看重试的逻辑做了什么。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;  

  //新建一个RetryContext来保存本轮重试的上下文  
  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 {  

   //如果有注册RetryListener,则会调用它的open方法,给调用者一个通知。  
   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);  
    }  
   }  

   //判断能否重试,就是调用RetryPolicy的canRetry方法来判断。  
   //这个循环会直到原方法不抛出异常,或不需要再重试  
   while (canRetry(retryPolicy, context) && !context.isExhaustedOnly()) {  

    try {  
     if (this.logger.isDebugEnabled()) {  
      this.logger.debug("Retry: count=" + context.getRetryCount());  
     }  
     //清除上次记录的异常  
     lastException = null;  
     //doWithRetry方法,一般来说就是原方法  
     return retryCallback.doWithRetry(context);  
    }  
    catch (Throwable e) {  
     //原方法抛出了异常  
     lastException = e;  

     try {  
      //记录异常信息  
      registerThrowable(retryPolicy, state, context, e);  
     }  
     catch (Exception ex) {  
      throw new TerminatedRetryException("Could not register throwable",  
        ex);  
     }  
     finally {  
      //调用RetryListener的onError方法  
      doOnErrorInterceptors(retryCallback, context, e);  
     }  
     //再次判断能否重试  
     if (canRetry(retryPolicy, context) && !context.isExhaustedOnly()) {  
      try {  
       //如果可以重试则走退避策略  
       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.<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;  
   //重试结束后如果有兜底Recovery方法则执行,否则抛异常  
   return handleRetryExhausted(recoveryCallback, context, state);  

  }  
  catch (Throwable e) {  
   throw RetryTemplate.<E>wrapIfNecessary(e);  
  }  
  finally {  
   //处理一些关闭逻辑  
   close(retryPolicy, context, state, lastException == null || exhausted);  
   //调用RetryListener的close方法  
   doCloseInterceptors(retryCallback, context, lastException);  
   RetrySynchronizationManager.clear();  
  }  

 }  

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,我们漏掉了RetryPolicy的canRetry方法和BackOffPolicy的backOff方法,以及这两个Policy是怎么来的。我们回头看看getStatelessInterceptor方法中的getRetryPolicygetRetryPolicy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~,代码不难,这里简单做一下总结好了。就是通过@Retryable注解中的参数,来判断具体使用文章开头说到的哪个重试策略,是SimpleRetryPolicy还是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~,一样的味道。就是通过@Backoff注解中的参数,来判断具体使用文章开头说到的哪个退避策略,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。我们看看SimpleRetryPolicy的

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

同样,我们看看FixedBackOffPolicy的退避方法。

protected void doBackOff() throws BackOffInterruptedException {  
  try {  
   //就是sleep固定的时间  
   sleeper.sleep(backOffPeriod);  
  }  
  catch (InterruptedException e) {  
   throw new BackOffInterruptedException("Thread interrupted while sleeping", e);  
  }  
 }  

至此,重试的主要原理以及逻辑大概就是这样了。

RetryContext

我觉得有必要说说RetryContext,先看看它的继承关系。

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。我刚开始直觉是对每一个需要重试的方法都会new一个策略,这样重试策略之间才不会产生冲突,但是一想就知道这样就可能多出了很多策略对象出来,增加了使用者的负担,这不是一个好的设计。

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。

参考资料

来源:https://blog.csdn.net/bishe_teacher/article/details/110687291

作者:bishe_teacher