executeCommandAndObserve方法处理onerror异常。

return execution.doOnNext(markEmits)                .doOnCompleted(markOnCompleted)                .onErrorResumeNext(handleFallback)                .doOnEach(setRequestContext);

　　handleFallback方法处理执行过程中的各种异常

final Func1
     
      > handleFallback = new Func1
      
       >() {            @Override            public Observable
       
         call(Throwable t) {                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);                }            }        };
       
      
     

　　 handleThreadPoolRejectionViaFallback、handleTimeoutViaFallback、handleBadRequestByEmittingError、handleFailureViaFallback最终都会调用getFallbackOrThrowException来处理各种异常。

　　getFallbackOrThrowException方法执行fallback方法并返回结果，如果执行过程中异常，返回异常信息。

private Observable
     
       getFallbackOrThrowException(final AbstractCommand
      
        _cmd, final HystrixEventType eventType, final FailureType failureType, final String message, final Exception originalException) {        ...                Observable
       
         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))                            .doOnNext(markFallbackEmit)                            .doOnCompleted(markFallbackCompleted)                            .onErrorResumeNext(handleFallbackError)                            .doOnTerminate(singleSemaphoreRelease)                            .doOnUnsubscribe(singleSemaphoreRelease);                } else {                   return handleFallbackRejectionByEmittingError();                }            } else {                return handleFallbackDisabledByEmittingError(originalException, failureType, message);            }        }    }
       
      
     

　　handleFallbackError方法，返回异常

final Func1
     
      > handleFallbackError = new Func1
      
       >() {                    @Override                    public Observable
       
         call(Throwable t) {                        Exception e = originalException;                        Exception fe = getExceptionFromThrowable(t);                        if (fe instanceof UnsupportedOperationException) {                            long latency = System.currentTimeMillis() - executionResult.getStartTimestamp();                            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);                            /* executionHook for all errors */                            e = wrapWithOnErrorHook(failureType, e);                            return Observable.error(new HystrixRuntimeException(failureType, _cmd.getClass(), getLogMessagePrefix() + " " + message + " and no fallback available.", e, fe));                        } else {                            long latency = System.currentTimeMillis() - executionResult.getStartTimestamp();                            logger.debug("HystrixCommand execution " + failureType.name() + " and fallback failed.", fe);                            eventNotifier.markEvent(HystrixEventType.FALLBACK_FAILURE, commandKey);                            executionResult = executionResult.addEvent((int) latency, HystrixEventType.FALLBACK_FAILURE);                            /* executionHook for all errors */                            e = wrapWithOnErrorHook(failureType, e);                            return Observable.error(new HystrixRuntimeException(failureType, _cmd.getClass(), getLogMessagePrefix() + " " + message + " and fallback failed.", e, fe));                        }                    }                };