Spring中的重試功能！嗯，有點(diǎn)東西
來源：https://albenw.github.io/posts/69a9647f/
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蟮慕嵌葋砜矗覀儜?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 {
@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:
@PostConstruct
public void init() {
Set<Class<? extends Annotation>> retryableAnnotationTypes = new LinkedHashSet<Class<? extends Annotation>>(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<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)。
@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interceptor
delegate = getStatefulInterceptor(target, method, retryable);
}
else {
//得到“無狀態(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 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
@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