org.springframework.retry.support.RetryTemplate
Spring新建了一个类RetryTemplate来封装增强的逻辑，可以看看这个类中最里面的逻辑doExecute()方法，里面有个while循环就知道是重试的核心。这段逻辑还没有涉及到Spring框架，还只是一个独立的重试设计。
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;
// Allow the retry policy to initialise itself...
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 {
// Give clients a chance to enhance the context...
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);
}
}
/*
* We allow the whole loop to be skipped if the policy or context already
* forbid the first try. This is used in the case of external retry to allow a
* recovery in handleRetryExhausted without the callback processing (which
* would throw an exception).
*/
while (canRetry(retryPolicy, context) && !context.isExhaustedOnly()) {
try {
if (this.logger.isDebugEnabled()) {
this.logger.debug("Retry: count=" + context.getRetryCount());
}
// Reset the last exception, so if we are successful
// the close interceptors will not think we failed...
lastException = null;
T result = retryCallback.doWithRetry(context);
doOnSuccessInterceptors(retryCallback, context, result);
return result;
}
catch (Throwable e) {
lastException = e;
try {
registerThrowable(retryPolicy, state, context, e);
}
catch (Exception ex) {
throw new TerminatedRetryException("Could not register throwable", ex);
}
finally {
doOnErrorInterceptors(retryCallback, context, e);
}
if (canRetry(retryPolicy, context) && !context.isExhaustedOnly()) {
try {
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;
return handleRetryExhausted(recoveryCallback, context, state);
}
catch (Throwable e) {
throw RetryTemplate.<E>wrapIfNecessary(e);
}
finally {
close(retryPolicy, context, state, lastException == null || exhausted);
doCloseInterceptors(retryCallback, context, lastException);
RetrySynchronizationManager.clear();
}
}这个具体逻辑先放下，需要继续往上找到谁在调用这个重试逻辑
org.springframework.retry.interceptor.RetryOperationsInterceptor
到了这里就涉及到Spring框架了，其中MethodInterceptor是Spring引入aop联盟的接口，用来封装方法增强的逻辑。MethodInterceptor只提供了一个方法invoke()，实现这个方法的人只需要加上自己的增强逻辑后再调用参数的Methodinvocation，执行被增强的逻辑即可。
Object invoke(@Nonnull MethodInvocation invocation) throws Throwable;public class RetryOperationsInterceptor implements MethodInterceptor {
private RetryOperations retryOperations = new RetryTemplate();
private MethodInvocationRecoverer<?> recoverer;
private String label;
public void setLabel(String label) {
this.label = label;
}
public void setRetryOperations(RetryOperations retryTemplate) {
Assert.notNull(retryTemplate, "'retryOperations' cannot be null.");
this.retryOperations = retryTemplate;
}
public void setRecoverer(MethodInvocationRecoverer<?> recoverer) {
this.recoverer = recoverer;
}
@Override
public Object invoke(final MethodInvocation invocation) throws Throwable {
String name;
if (StringUtils.hasText(this.label)) {
name = this.label;
}
else {
name = invocation.getMethod().toGenericString();
}
final String label = name;
RetryCallback<Object, Throwable> retryCallback = new MethodInvocationRetryCallback<Object, Throwable>(
invocation, label) {
@Override
public Object doWithRetry(RetryContext context) throws Exception {
context.setAttribute(RetryContext.NAME, this.label);
context.setAttribute("ARGS", new Args(invocation.getArguments()));
/*
* 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 (this.invocation instanceof ProxyMethodInvocation) {
context.setAttribute("___proxy___", ((ProxyMethodInvocation) this.invocation).getProxy());
try {
return ((ProxyMethodInvocation) this.invocation).invocableClone().proceed();
}
catch (Exception | 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 (this.recoverer != null) {
ItemRecovererCallback recoveryCallback = new ItemRecovererCallback(invocation.getArguments(),
this.recoverer);
try {
Object recovered = this.retryOperations.execute(retryCallback, recoveryCallback);
return recovered;
}
finally {
RetryContext context = RetrySynchronizationManager.getContext();
if (context != null) {
context.removeAttribute("__proxy__");
}
}
}
return this.retryOperations.execute(retryCallback);
}
/**
* @author Dave Syer
*
*/
private static final class ItemRecovererCallback implements RecoveryCallback<Object> {
private final Object[] args;
private final MethodInvocationRecoverer<?> recoverer;
/**
* @param args the item that failed.
*/
private ItemRecovererCallback(Object[] args, MethodInvocationRecoverer<?> recoverer) {
this.args = Arrays.asList(args).toArray();
this.recoverer = recoverer;
}
@Override
public Object recover(RetryContext context) {
return this.recoverer.recover(this.args, context.getLastThrowable());
}
}
}接下来需要继续往上再追一层，看看是谁调用RetryOperationsInterceptor的invoke()方法，
org.springframework.retry.annotation.AnnotationAwareRetryOperationsInterceptor
这个类将自身融入到了Spring框架中，因为他实现了org.springframework.aop.IntroductionInterceptor接口，而IntroductionInterceptor有继承了MethodInterceptor，也就是说AnnotationAwareRetryOperationsInterceptor其实是个MethodInterceptor，而它却要调用另外一个MethodInterceptor--------RetryOperationsInterceptor，这种手法很常见，就是静态代理了一层，用于扩展出2种类型。而AnnotationAwareRetryOperationsInterceptor是会在Spring启动时被发现，然后进行动态代理生成。这样就自动增强了。