hystrix总结之异常降级

private Observable<R> applyHystrixSemantics(final AbstractCommand<R> _cmd) {
        if (circuitBreaker.allowRequest()) {
...
            if (executionSemaphore.tryAcquire()) {
                try {
...
                    return executeCommandAndObserve(_cmd)//执行run方法，如果异常，执行fallback方法
                            .doOnError(markExceptionThrown)
                            .doOnTerminate(singleSemaphoreRelease)
                            .doOnUnsubscribe(singleSemaphoreRelease);
                } catch (RuntimeException e) {
                    return Observable.error(e);
                }
            } else {
                return handleSemaphoreRejectionViaFallback();//执行fallback方法，并返回响应Observable
            }
        } else {
            return handleShortCircuitViaFallback();//执行fallback方法，并返回响应Observable
        }
    }

 private Observable<R> executeCommandAndObserve(final AbstractCommand<R> _cmd) {
...
        Observable<R> execution;
        if (properties.executionTimeoutEnabled().get()) {
            execution = executeCommandWithSpecifiedIsolation(_cmd)//执行run方法。
                    .lift(new HystrixObservableTimeoutOperator<R>(_cmd));
        } else {
            execution = executeCommandWithSpecifiedIsolation(_cmd);
        }
        return execution.doOnNext(markEmits)
                .doOnCompleted(markOnCompleted)
                .onErrorResumeNext(handleFallback)//如果执行异常，则执行fallback方法。
                .doOnEach(setRequestContext);
    }