在上篇文章Hystrix工作流程解析中，我们整体介绍了Hystrix的工作流程，知道了Hystrix会在下面四种情况下发生降级：
- 熔断器打开
- 线程池/信号量跑满
- 调用超时
- 调用失败
本篇文章则介绍一下在发生降级时Hystrix的处理细节，下面的方法异常的处理逻辑：
代码语言：javascript复制final Func1<Throwable, Observable<R>> handleFallback = new Func1<Throwable, Observable<R>>() {
@Override
public Observable<R> call(Throwable t) {
circuitBreaker.markNonSuccess();
Exception e = getExceptionFromThrowable(t);
executionResult = executionResult.setExecutionException(e);
if (e instanceof RejectedExecutionException) {
return handleThreadPoolRejectionViaFallback(e);
} else if (t instanceof HystrixTimeoutException) {
return handleTimeoutViaFallback();
} else if (t instanceof HystrixBadRequestException) {
return handleBadRequestByEmittingError(e);
} else {
/*
* Treat HystrixBadRequestException from ExecutionHook like a plain HystrixBadRequestException.
*/
if (e instanceof HystrixBadRequestException) {
eventNotifier.markEvent(HystrixEventType.BAD_REQUEST, commandKey);
return Observable.error(e);
}
return handleFailureViaFallback(e);
}
}
};
线程池跑满
代码语言：javascript复制private Observable<R> handleThreadPoolRejectionViaFallback(Exception underlying) {
eventNotifier.markEvent(HystrixEventType.THREAD_POOL_REJECTED, commandKey);
threadPool.markThreadRejection();
// use a fallback instead (or throw exception if not implemented)
return getFallbackOrThrowException(this, HystrixEventType.THREAD_POOL_REJECTED, FailureType.REJECTED_THREAD_EXECUTION, "could not be queued for execution", underlying);
}- 第一行发布了一个线程池拒绝的事件
- 第二行记录了线程池拒绝的次数
- 获取Fallback方法
获取Fallback方法
代码语言：javascript复制 final HystrixRequestContext requestContext = HystrixRequestContext.getContextForCurrentThread();
long latency = System.currentTimeMillis() - executionResult.getStartTimestamp();
// record the executionResult
// do this before executing fallback so it can be queried from within getFallback (see See https://github.com/Netflix/Hystrix/pull/144)
executionResult = executionResult.addEvent((int) latency, eventType);
if (isUnrecoverable(originalException)) {
logger.error("Unrecoverable Error for HystrixCommand so will throw HystrixRuntimeException and not apply fallback. ", originalException);
/* executionHook for all errors */
Exception e = wrapWithOnErrorHook(failureType, originalException);
return Observable.error(new HystrixRuntimeException(failureType, this.getClass(), getLogMessagePrefix() " " message " and encountered unrecoverable error.", e, null));
} else {
if (isRecoverableError(originalException)) {
logger.warn("Recovered from java.lang.Error by serving Hystrix fallback", originalException);
}
if (properties.fallbackEnabled().get()) {
/* fallback behavior is permitted so attempt */
final Action1<Notification<? super R>> setRequestContext = new Action1<Notification<? super R>>() {
@Override
public void call(Notification<? super R> rNotification) {
setRequestContextIfNeeded(requestContext);
}
};
final Action1<R> markFallbackEmit = new Action1<R>() {
@Override
public void call(R r) {
if (shouldOutputOnNextEvents()) {
executionResult = executionResult.addEvent(HystrixEventType.FALLBACK_EMIT);
eventNotifier.markEvent(HystrixEventType.FALLBACK_EMIT, commandKey);
}
}
};
final Action0 markFallbackCompleted = new Action0() {
@Override
public void call() {
long latency = System.currentTimeMillis() - executionResult.getStartTimestamp();
eventNotifier.markEvent(HystrixEventType.FALLBACK_SUCCESS, commandKey);
executionResult = executionResult.addEvent((int) latency, HystrixEventType.FALLBACK_SUCCESS);
}
};
final Func1<Throwable, Observable<R>> handleFallbackError = new Func1<Throwable, Observable<R>>() {
@Override
public Observable<R> call(Throwable t) {
/* executionHook for all errors */
Exception e = wrapWithOnErrorHook(failureType, originalException);
Exception fe = getExceptionFromThrowable(t);
long latency = System.currentTimeMillis() - executionResult.getStartTimestamp();
Exception toEmit;
if (fe instanceof UnsupportedOperationException) {
logger.debug("No fallback for HystrixCommand. ", fe); // debug only since we're throwing the exception and someone higher will do something with it
eventNotifier.markEvent(HystrixEventType.FALLBACK_MISSING, commandKey);
executionResult = executionResult.addEvent((int) latency, HystrixEventType.FALLBACK_MISSING);
toEmit = new HystrixRuntimeException(failureType, _cmd.getClass(), getLogMessagePrefix() " " message " and no fallback available.", e, fe);
} else {
logger.debug("HystrixCommand execution " failureType.name() " and fallback failed.", fe);
eventNotifier.markEvent(HystrixEventType.FALLBACK_FAILURE, commandKey);
executionResult = executionResult.addEvent((int) latency, HystrixEventType.FALLBACK_FAILURE);
toEmit = new HystrixRuntimeException(failureType, _cmd.getClass(), getLogMessagePrefix() " " message " and fallback failed.", e, fe);
}
// NOTE: we're suppressing fallback exception here
if (shouldNotBeWrapped(originalException)) {
return Observable.error(e);
}
return Observable.error(toEmit);
}
};
final TryableSemaphore fallbackSemaphore = getFallbackSemaphore();
final AtomicBoolean semaphoreHasBeenReleased = new AtomicBoolean(false);
final Action0 singleSemaphoreRelease = new Action0() {
@Override
public void call() {
if (semaphoreHasBeenReleased.compareAndSet(false, true)) {
fallbackSemaphore.release();
}
}
};
Observable<R> fallbackExecutionChain;
// acquire a permit
if (fallbackSemaphore.tryAcquire()) {
try {
if (isFallbackUserDefined()) {
executionHook.onFallbackStart(this);
fallbackExecutionChain = getFallbackObservable();
} else {
//same logic as above without the hook invocation
fallbackExecutionChain = getFallbackObservable();
}
} catch (Throwable ex) {
//If hook or user-fallback throws, then use that as the result of the fallback lookup
fallbackExecutionChain = Observable.error(ex);
}
return fallbackExecutionChain
.doOnEach(setRequestContext)
.lift(new FallbackHookApplication(_cmd))
.lift(new DeprecatedOnFallbackHookApplication(_cmd))
.doOnNext(markFallbackEmit)
.doOnCompleted(markFallbackCompleted)
.onErrorResumeNext(handleFallbackError)
.doOnTerminate(singleSemaphoreRelease)
.doOnUnsubscribe(singleSemaphoreRelease);
} else {
return handleFallbackRejectionByEmittingError();
}
} else {
return handleFallbackDisabledByEmittingError(originalException, failureType, message);
}
}
}方法比较长，主要做了以下事情：
- 直接看
isUnrecoverable方法，判断异常是否为不可恢复异常，如果不可恢复则直接返回失败 - 如果是可恢复异常则打印日志
- 判断是否开启执行回退方法，如果开启进入步骤4
- 创建开始和完成需要发送的两个事件：
FALLBACK_EMIT、FALLBACK_SUCCESS - 创建调用回退方法出现异常时的处理逻辑：
handleFallbackError，而这种场景发生的异常只有两种情况：UnsupportedOperationException异常：未实现getFallback抽象方法- 其他异常
- 创建释放信号量的Action：
singleSemaphoreRelease - 获取信号量，如果成功执行回退逻辑，也就是调用用户实现的
getFallback方法
final protected Observable<R> getFallbackObservable() {
return Observable.defer(new Func0<Observable<R>>() {
@Override
public Observable<R> call() {
try {
return Observable.just(getFallback());
} catch (Throwable ex) {
return Observable.error(ex);
}
}
});
}
