前言
Spring實現了一套重試機制，功能簡單實用。Spring Retry是從Spring Batch 2.2.0版本中獨立出來，變成了Spring Retry模塊。，已經廣泛應用於Spring Batch,Spring Integration, Spring for Apache Hadoop等Spring項目。
使用Spring Retry
(一)Maven依賴
<!-- 重試機制 -->
<dependency>
<groupId>org.springframework.retry</groupId>
<artifactId>spring-retry</artifactId>
<version>1.2.4.RELEASE</version>
</dependency>
<dependency>
<groupId>org.aspectj</groupId>
<artifactId>aspectjweaver</artifactId>
<version>1.9.4</version>
</dependency>
（二）配置類添加註解 @EnableRetry
@EnableRetry
@Configuration
public class RetryConfiguration {
}
（三）Service方法編寫使用以下2個註解，@Retryable標記方法重試的相關信息和@Recover標記重試失敗的回調的方法
@Target({ElementType.METHOD, ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface Retryable {
String interceptor() default "";
Class<? extends Throwable>[] value() default {};
Class<? extends Throwable>[] include() default {};
Class<? extends Throwable>[] exclude() default {};
String label() default "";
boolean stateful() default false;
int maxAttempts() default 3;
String maxAttemptsExpression() default "";
Backoff backoff() default @Backoff;
String exceptionExpression() default "";
String[] listeners() default {};
}
@Retryable註解，其中：
value : 指定發生的異常進行重試
include : 同value , 默認"" , 當exclude也爲空時 , 所有異常都重試
exclude : 排除不重試的異常 , 默認"" , 當include也爲空時 , 所有異常都重試
maxAttemps : 嘗試次數 , 默認3 . 注意該值包含第一次正常執行的次數 , 即失敗之後重新嘗試2次 , 一共執行3次
backoff : 重試補償機制 , 默認無 , 屬性配置如下
delay : 延遲指定時間後(單位毫秒)重試
multiplier : 指定延遲的倍數 , 每次執行間隔加倍延遲 . 如delay=5000L,multiplier=2 , 第一次重 試爲5S後 , 第二次爲第一次的10S後 , 第三次爲第二次的20S後@Recover註解，其中：
當重試次數達到限定時 , 會執行@Recover註解的補償方法 , 只有在入參與發生異常匹配時纔會執行該補償方法，即發生的異常類型需要和@Recover註解的參數一致，@Retryable註解的方法不能有返回值，不然@Recover註解的方法無效
在SpringBoot中使用
@SpringBootApplication
@EnableRetry//啓用重試機制
public class RetryApplication {
public static void main(String[] args) {
SpringApplication.run(RetryApplication.class, args);
}
}
/** 出現指定異常時(RuntimeException) , 再重試3次 , 每次延遲5s , 之後每次延遲翻倍*/
@Retryable(include = {RuntimeException.class}, maxAttempts = 4, backoff = @Backoff(delay = 2000L, multiplier = 2))
public void work2() {
System.out.println("執行方法 : " + LocalDateTime.now());
throw new RuntimeException();//模擬異常
}
/** 當重試次數達到限定時 , 會執行@Recover註解的補償方法 , 只有在入參與發生異常匹配時纔會執行該補償方法 */
@Recover
public void recover(RuntimeException e) {
System.out.println("執行補償方法 : " + LocalDateTime.now());
}
運行結果：
執行方法：2019-12-31T20:17:48.566
執行方法：2019-12-31T20:17:49.567
執行方法：2019-12-31T20:17:50.568
執行補償方法：2019-12-31T20:17:50.568
常見問題
- maxAttemps 參數爲嘗試次數 , 而非重試次數 . 即=3時 , 會重試2次 , =4時會重試3次 , 因爲它包括了首次正常執行的計數
- retry利用了Spring AOP的原理 , 所以底層採用了代理的技術 , 所有同一個方法內調用會使代理失效（導入的要是代理對象而非this對象）
- 有關非冪等的操作(如新增,修改等) , 不要使用重試 , 會影響數據一致性
Spring Retry的重試策略和退避策略
通過源碼可以看出，Spring Retry的重試策略主要在policy包中，常見的包括：
- SimpleRetryPolicy
默認最多重試3次 - TimeoutRetryPolicy
默認在1秒內失敗都會重試 - ExpressionRetryPolicy
符合表達式就會重試 - CircuitBreakerRetryPolicy
增加了熔斷的機制，如果不在熔斷狀態，則允許重試 - CompositeRetryPolicy
可以組合多個重試策略 - NeverRetryPolicy
從不重試（也是一種重試策略哈） - AlwaysRetryPolicy
總是重試
退避策略主要在backoff包裏，退避是指怎麼去做下一次的重試，在這裏其實就是等待多長時間，主要包括：
- FixedBackOffPolicy
默認固定延遲1秒後執行下一次重試 - ExponentialBackOffPolicy
指數遞增延遲執行重試，默認初始0.1秒，係數是2，那麼下次延遲0.2秒，再下次就是延遲0.4秒，如此類推，最大30秒。 - ExponentialRandomBackOffPolicy
在上面那個策略上增加隨機性 - UniformRandomBackOffPolicy
這個跟上面的區別就是，上面的延遲會不停遞增，這個只會在固定的區間隨機 - StatelessBackOffPolicy
這個說明是無狀態的，所謂無狀態就是對上次的退避無感知，從它下面的子類也能看出來
Spring Retry的原理
代碼如何實現重試的，分析@EnableRetry
@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@EnableAspectJAutoProxy(
proxyTargetClass = false
)
@Import({RetryConfiguration.class})
@Documented
public @interface EnableRetry {
boolean proxyTargetClass() default false;
}
我們可以看到
@EnableAspectJAutoProxy(proxyTargetClass = false)就是打開Spring AOP功能，@Import註解註冊RetryConfiguration這個bean，RetryConfigutation繼承AbstractPointcutAdvisor，它有一個pointcut和一個advice。我們知道，在IOC過程中會根據PointcutAdvisor類來對Bean進行Pointcut的過濾，然後生成對應的AOP代理類，用advice來加強處理。
@Configuration
public class RetryConfiguration extends AbstractPointcutAdvisor implements IntroductionAdvisor, BeanFactoryAware {
private Advice advice;
private Pointcut pointcut;
@Autowired(
required = false
)
private RetryContextCache retryContextCache;
@Autowired(
required = false
)
private List<RetryListener> retryListeners;
@Autowired(
required = false
)
private MethodArgumentsKeyGenerator methodArgumentsKeyGenerator;
@Autowired(
required = false
)
private NewMethodArgumentsIdentifier newMethodArgumentsIdentifier;
@Autowired(
required = false
)
private Sleeper sleeper;
private BeanFactory beanFactory;
public RetryConfiguration() {
}
@PostConstruct
public void init() {
Set<Class<? extends Annotation>> retryableAnnotationTypes = new LinkedHashSet(1);
retryableAnnotationTypes.add(Retryable.class);
this.pointcut = this.buildPointcut(retryableAnnotationTypes);
this.advice = this.buildAdvice();
if (this.advice instanceof BeanFactoryAware) {
((BeanFactoryAware)this.advice).setBeanFactory(this.beanFactory);
}
}
public void setBeanFactory(BeanFactory beanFactory) {
this.beanFactory = beanFactory;
}
public ClassFilter getClassFilter() {
return this.pointcut.getClassFilter();
}
public Class<?>[] getInterfaces() {
return new Class[]{org.springframework.retry.interceptor.Retryable.class};
}
public void validateInterfaces() throws IllegalArgumentException {
}
public Advice getAdvice() {
return this.advice;
}
public Pointcut getPointcut() {
return this.pointcut;
}
//創建advice對象，即攔截器
protected Advice buildAdvice() {
AnnotationAwareRetryOperationsInterceptor interceptor = new AnnotationAwareRetryOperationsInterceptor();
if (this.retryContextCache != null) {
interceptor.setRetryContextCache(this.retryContextCache);
}
if (this.retryListeners != null) {
interceptor.setListeners(this.retryListeners);
}
if (this.methodArgumentsKeyGenerator != null) {
interceptor.setKeyGenerator(this.methodArgumentsKeyGenerator);
}
if (this.newMethodArgumentsIdentifier != null) {
interceptor.setNewItemIdentifier(this.newMethodArgumentsIdentifier);
}
if (this.sleeper != null) {
interceptor.setSleeper(this.sleeper);
}
return interceptor;
}
protected Pointcut buildPointcut(Set<Class<? extends Annotation>> retryAnnotationTypes) {
ComposablePointcut result = null;
Iterator var3 = retryAnnotationTypes.iterator();
while(var3.hasNext()) {
Class<? extends Annotation> retryAnnotationType = (Class)var3.next();
Pointcut filter = new RetryConfiguration.AnnotationClassOrMethodPointcut(retryAnnotationType);
if (result == null) {
result = new ComposablePointcut(filter);
} else {
result.union(filter);
}
}
return result;
}
private static class AnnotationMethodsResolver {
private Class<? extends Annotation> annotationType;
public AnnotationMethodsResolver(Class<? extends Annotation> annotationType) {
this.annotationType = annotationType;
}
public boolean hasAnnotatedMethods(Class<?> clazz) {
final AtomicBoolean found = new AtomicBoolean(false);
ReflectionUtils.doWithMethods(clazz, new MethodCallback() {
public void doWith(Method method) throws IllegalArgumentException, IllegalAccessException {
if (!found.get()) {
Annotation annotation = AnnotationUtils.findAnnotation(method, AnnotationMethodsResolver.this.annotationType);
if (annotation != null) {
found.set(true);
}
}
}
});
return found.get();
}
}
private final class AnnotationClassOrMethodFilter extends AnnotationClassFilter {
private final RetryConfiguration.AnnotationMethodsResolver methodResolver;
AnnotationClassOrMethodFilter(Class<? extends Annotation> annotationType) {
super(annotationType, true);
this.methodResolver = new RetryConfiguration.AnnotationMethodsResolver(annotationType);
}
public boolean matches(Class<?> clazz) {
return super.matches(clazz) || this.methodResolver.hasAnnotatedMethods(clazz);
}
}
private final class AnnotationClassOrMethodPointcut extends StaticMethodMatcherPointcut {
private final MethodMatcher methodResolver;
AnnotationClassOrMethodPointcut(Class<? extends Annotation> annotationType) {
this.methodResolver = new AnnotationMethodMatcher(annotationType);
this.setClassFilter(RetryConfiguration.this.new AnnotationClassOrMethodFilter(annotationType));
}
public boolean matches(Method method, Class<?> targetClass) {
return this.getClassFilter().matches(targetClass) || this.methodResolver.matches(method, targetClass);
}
public boolean equals(Object other) {
if (this == other) {
return true;
} else if (!(other instanceof RetryConfiguration.AnnotationClassOrMethodPointcut)) {
return false;
} else {
RetryConfiguration.AnnotationClassOrMethodPointcut otherAdvisor = (RetryConfiguration.AnnotationClassOrMethodPointcut)other;
return ObjectUtils.nullSafeEquals(this.methodResolver, otherAdvisor.methodResolver);
}
}
}
}
在初始化方法中，創建pointcut和advice的實例，其中AnnotationAwareRetryOperationsInterceptor是一個MethodInterceptor，在創建AOP代理過程中如果目標方法符合pointcut的規則，它就會加到interceptor列表中，然後做增強，我們看看invoke方法做了什麼增強。
public Object invoke(MethodInvocation invocation) throws Throwable {
MethodInterceptor delegate = this.getDelegate(invocation.getThis(), invocation.getMethod());
return delegate != null ? delegate.invoke(invocation) : invocation.proceed();
}
這裏用到了委託，主要是需要根據配置委託給具體“有狀態”的interceptor還是“無狀態”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，而且優先級最高
if (StringUtils.hasText(retryable.interceptor())) {
delegate = this.beanFactory.getBean(retryable.interceptor(), MethodInterceptor.class);
}
else if (retryable.stateful()) {
//得到“有狀態”的interceptor
delegate = getStatefulInterceptor(target, method, retryable);
}
else {
//得到“無狀態”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的規則，我們等下再來看。 RetryInterceptorBuilder其實就是爲了生成 RetryOperationsInterceptor。RetryOperationsInterceptor也是一個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RetryCallback，其實看它的doWithRetry方法，調用了invocation的proceed()方法，是不是有點眼熟，這就是AOP的攔截鏈調用，如果沒有攔截鏈，那就是對原來方法的調用。
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);
}
//最終還是進入到retryOperations的execute方法，這個retryOperations就是在之前的builder set進來的RetryTemplate。
return this.retryOperations.execute(retryCallback);
}
無論是RetryOperationsInterceptor還是StatufulRetryOperationsInterceptor，最終的攔截處理邏輯還是調用RetryTemplate的execute方法，RetryTemplate作爲一個模板類，裏面包含了重試統一邏輯。
重試邏輯及策略實現
上面介紹了Spring Retry利用了AOP代理使重試機制對業務代碼進行“入侵”。繼續看看重試的邏輯做了什麼：
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 {
//如果有註冊RetryListener，則會調用它的open方法，給調用者一個通知。
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，就是調用RetryPolicy的canRetry方法來判斷。
//這個循環會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;
//重試結束後如果有兜底Recovery方法則執行，否則拋異常
return handleRetryExhausted(recoveryCallback, context, state);
}
catch (Throwable e) {
throw RetryTemplate.<E>wrapIfNecessary(e);
}
finally {
//處理一些關閉邏輯
close(retryPolicy, context, state, lastException == null || exhausted);
//調用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，在上面，我們漏掉了RetryPolicy的canRetry方法和BackOffPolicy的backOff方法，以及這兩個Policy是怎麼來的。 回頭看看getStatelessInterceptor方法的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);
}
}
總結：就是通過@Retryable註解中的參數，來判斷具體使用文章開頭說到的哪個重試策略，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;
}
通過@Backoff註解中的參數，來判斷具體使用文章開頭說到的哪個退避策略，是FixedBackOffPolicy還是UniformRandomBackOffPolicy等。那麼每個RetryPolicy都會重寫canRetry方法，然後在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
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
每一個策略都有對應的Context。在Spring Retry裏，其實每一個策略都是單例來的。我剛開始直覺是對每一個需要重試的方法都會new一個策略，這樣重試策略之間纔不會產生衝突，但是一想就知道這樣就可能多出了很多策略對象出來，增加了使用者的負擔，這不是一個好的設計。Spring Retry採用了一個更加輕量級的做法，就是針對每一個需要重試的方法只new一個上下文Context對象，然後在重試時，把這個Context傳到策略裏，策略再根據這個Context做重試，而且Spring Retry還對這個Context做了cache。這樣就相當於對重試的上下文做了優化。
總結
Spring Retry通過AOP機制來實現對業務代碼的重試”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。