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Spring实现了一套重试机制#xff0c;功能简单实用。Spring Retry是从Spring Batch独立出来的一个功能#xff0c;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retries1timeout500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我们应该把重试的代码独立出来。
使用介绍 先举个例子
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。 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
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;}
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:
PostConstructpublic void init() {SetClass? extends Annotation retryableAnnotationTypes new LinkedHashSetClass? extends Annotation(1);retryableAnnotationTypes.add(Retryable.class);//创建pointcutthis.pointcut buildPointcut(retryableAnnotationTypes);//创建advicethis.advice buildAdvice();if (this.advice instanceof BeanFactoryAware) {((BeanFactoryAware) this.advice).setBeanFactory(beanFactory);}}protected Pointcut buildPointcut(SetClass? 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
继承关系 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。
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();}}
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 HashMapMethod, MethodInterceptor());}MapMethod, 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interceptordelegate getStatefulInterceptor(target, method, retryable);}else {//得到“无状态”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
先看看比较简单的getStatelessInterceptor。
getStatelessInterceptor。
private MethodInterceptor getStatelessInterceptor(Object target, Method method, Retryable retryable) {//生成一个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。
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。RetryCallbackObject, Throwable retryCallback new RetryCallbackObject, Throwable() {public Object doWithRetry(RetryContext context) throws Exception {context.setAttribute(RetryContext.NAME, label);/** If we dont copy the invocation carefully it wont keep a reference to* the other interceptors in the chain. We dont 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(RetryCallbackT, E retryCallback,RecoveryCallbackT 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 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.EwrapIfNecessary(e);}}/** A stateful attempt that can retry may rethrow the exception before now,* but if we get this far in a stateful retry theres 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.EwrapIfNecessary(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
getRetryPolicy和getRetryPolicy方法。
private RetryPolicy getRetryPolicy(Annotation retryable) {MapString, 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;}MapClass? extends Throwable, Boolean policyMap new HashMapClass? 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);}}
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;}
一样的味道。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
Overridepublic 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。
