return Observable.defer(new Func0<Observable<R>>() {
            @Override
            public Observable<R> call() {
                 /* 这是一个有状态的对象，因此只能使用一次 */
                if (!commandState.compareAndSet(CommandState.NOT_STARTED, CommandState.OBSERVABLE_CHAIN_CREATED)) {
                    IllegalStateException ex = new IllegalStateException("This instance can only be executed once. Please instantiate a new instance.");
                    // TODO 为此创建新的错误类型
                    throw new HystrixRuntimeException(FailureType.BAD_REQUEST_EXCEPTION, _cmd.getClass(), getLogMessagePrefix() + " command executed multiple times - this is not permitted.", ex, null);
                }
                commandStartTimestamp = System.currentTimeMillis();
                if (properties.requestLogEnabled().get()) {
                    // 记录此命令执行，无论发生什么情况
                    if (currentRequestLog != null) {
                        currentRequestLog.addExecutedCommand(_cmd);
                    }
                }
                final boolean requestCacheEnabled = isRequestCachingEnabled();
                final String cacheKey = getCacheKey();
                /* 首先尝试从缓存 */
                if (requestCacheEnabled) {
                    HystrixCommandResponseFromCache<R> fromCache = (HystrixCommandResponseFromCache<R>) requestCache.get(cacheKey);
                    if (fromCache != null) {
                        isResponseFromCache = true;
                        return handleRequestCacheHitAndEmitValues(fromCache, _cmd);
                    }
                }
                Observable<R> hystrixObservable =
                        Observable.defer(applyHystrixSemantics)
                                .map(wrapWithAllOnNextHooks);
                Observable<R> afterCache;
                // 放入缓存
                if (requestCacheEnabled && cacheKey != null) {
                    // 包装以缓存
                    HystrixCachedObservable<R> toCache = HystrixCachedObservable.from(hystrixObservable, _cmd);
                    HystrixCommandResponseFromCache<R> fromCache = (HystrixCommandResponseFromCache<R>) requestCache.putIfAbsent(cacheKey, toCache);
                    if (fromCache != null) {
                        // 另一个线程击败了我们，因此使用缓存值
                        toCache.unsubscribe();
                        isResponseFromCache = true;
                        return handleRequestCacheHitAndEmitValues(fromCache, _cmd);
                    } else {
                        // 我们刚刚创建了一个ObservableCommand，所以我们进行了强制转换并返回了它
                        afterCache = toCache.toObservable();
                    }
                } else {
                    afterCache = hystrixObservable;
                }
                return afterCache
                        .doOnTerminate(terminateCommandCleanup)     // 进行一次清理(在正常终端状态(此行)或退订(下一行))
                        .doOnUnsubscribe(unsubscribeCommandCleanup) // 进行一次清理
                        .doOnCompleted(fireOnCompletedHook);
            }
        });