hystrix源码之AbstractCommand

AbstractCommand

 　　HystrixCommand和HystrixObservableCommand的父类。每个command对应一个HystrixCommandKey、HystrixThreadPoolKey和HystrixCommandGroupKey。内部有HystrixCircuitBreaker、HystrixThreadPool、HystrixCommandProperties、HystrixCommandMetrics、HystrixCommandExecutionHook、HystrixConcurrencyStrategy、HystrixRequestCache、HystrixRequestLog、HystrixEventNotifier组件来实现各个功能。

构造函数

　　创建时，首先初始化内部的各个组件对象。

protected AbstractCommand(HystrixCommandGroupKey group, HystrixCommandKey key, HystrixThreadPoolKey threadPoolKey, HystrixCircuitBreaker circuitBreaker, HystrixThreadPool threadPool,
            HystrixCommandProperties.Setter commandPropertiesDefaults, HystrixThreadPoolProperties.Setter threadPoolPropertiesDefaults,
            HystrixCommandMetrics metrics, TryableSemaphore fallbackSemaphore, TryableSemaphore executionSemaphore,
            HystrixPropertiesStrategy propertiesStrategy, HystrixCommandExecutionHook executionHook) {
        this.commandGroup = initGroupKey(group);
        this.commandKey = initCommandKey(key, getClass());
        this.properties = initCommandProperties(this.commandKey, propertiesStrategy, commandPropertiesDefaults);
        this.threadPoolKey = initThreadPoolKey(threadPoolKey, this.commandGroup, this.properties.executionIsolationThreadPoolKeyOverride().get());
        this.metrics = initMetrics(metrics, this.commandGroup, this.threadPoolKey, this.commandKey, this.properties);
        this.circuitBreaker = initCircuitBreaker(this.properties.circuitBreakerEnabled().get(), circuitBreaker, this.commandGroup, this.commandKey, this.properties, this.metrics);
        this.threadPool = initThreadPool(threadPool, this.threadPoolKey, threadPoolPropertiesDefaults);
        //Strategies from plugins
        this.eventNotifier = HystrixPlugins.getInstance().getEventNotifier();
        this.concurrencyStrategy = HystrixPlugins.getInstance().getConcurrencyStrategy();
　　　　  //开始commandpublisher
        HystrixMetricsPublisherFactory.createOrRetrievePublisherForCommand(this.commandKey, this.commandGroup, this.metrics, this.circuitBreaker, this.properties);
        this.executionHook = initExecutionHook(executionHook);
        this.requestCache = HystrixRequestCache.getInstance(this.commandKey, this.concurrencyStrategy);
        this.currentRequestLog = initRequestLog(this.properties.requestLogEnabled().get(), this.concurrencyStrategy);
        /* 设置fallback的并发量 */
        this.fallbackSemaphoreOverride = fallbackSemaphore;
        /* 设置excution的并发量 */
        this.executionSemaphoreOverride = executionSemaphore;
    }

　　通过HystrixPropertiesFactory获得指定commandKey的HystrixCommandProperties。

private static HystrixCommandProperties initCommandProperties(HystrixCommandKey commandKey, HystrixPropertiesStrategy propertiesStrategy, HystrixCommandProperties.Setter commandPropertiesDefaults) {
        if (propertiesStrategy == null) {
            return HystrixPropertiesFactory.getCommandProperties(commandKey, commandPropertiesDefaults);
        } else {
            // used for unit testing
            return propertiesStrategy.getCommandProperties(commandKey, commandPropertiesDefaults);
        }
    }

　　通过HystrixCommandMetrics获得指定commandKey的HystrixCommandMetrics。

private static HystrixCommandMetrics initMetrics(HystrixCommandMetrics fromConstructor, HystrixCommandGroupKey groupKey,
                                                     HystrixThreadPoolKey threadPoolKey, HystrixCommandKey commandKey,
                                                     HystrixCommandProperties properties) {
        if (fromConstructor == null) {
            return HystrixCommandMetrics.getInstance(commandKey, groupKey, threadPoolKey, properties);
        } else {
            return fromConstructor;
        }
    }

　　通过HystrixCircuitBreaker.Factory获得指定commandKey的HystrixCircuitBreaker。

private static HystrixCircuitBreaker initCircuitBreaker(boolean enabled, HystrixCircuitBreaker fromConstructor,
                                                            HystrixCommandGroupKey groupKey, HystrixCommandKey commandKey,
                                                            HystrixCommandProperties properties, HystrixCommandMetrics metrics) {
        if (enabled) {
            if (fromConstructor == null) {
                // get the default implementation of HystrixCircuitBreaker
                return HystrixCircuitBreaker.Factory.getInstance(commandKey, groupKey, properties, metrics);
            } else {
                return fromConstructor;
            }
        } else {
            return new NoOpCircuitBreaker();
        }
    }

　　通过HystrixThreadPool.Factory获得指定threadPoolKey的HystrixThreadPool。

private static HystrixThreadPool initThreadPool(HystrixThreadPool fromConstructor, HystrixThreadPoolKey threadPoolKey, HystrixThreadPoolProperties.Setter threadPoolPropertiesDefaults) {
        if (fromConstructor == null) {
            // get the default implementation of HystrixThreadPool
            return HystrixThreadPool.Factory.getInstance(threadPoolKey, threadPoolPropertiesDefaults);
        } else {
            return fromConstructor;
        }
    }

　　通过HystrixPlugins获得自定义插架HystrixCommandExecutionHook。

private static HystrixCommandExecutionHook initExecutionHook(HystrixCommandExecutionHook fromConstructor) {
        if (fromConstructor == null) {
            return new ExecutionHookDeprecationWrapper(HystrixPlugins.getInstance().getCommandExecutionHook());
        } else {
            // used for unit testing
            if (fromConstructor instanceof ExecutionHookDeprecationWrapper) {
                return fromConstructor;
            } else {
                return new ExecutionHookDeprecationWrapper(fromConstructor);
            }
        }
    }

　　通过HystrixPlugins获得自定义插架HystrixEventNotifier、HystrixConcurrencyStrategy。

this.eventNotifier = HystrixPlugins.getInstance().getEventNotifier();
this.concurrencyStrategy = HystrixPlugins.getInstance().getConcurrencyStrategy();

　　通过HystrixRequestCache获得指定commandKey的HystrixRequestCache。

this.requestCache = HystrixRequestCache.getInstance(this.commandKey, this.concurrencyStrategy);

　　通过HystrixRequestLog获得当前的HystrixRequestLog。

private static HystrixRequestLog initRequestLog(boolean enabled, HystrixConcurrencyStrategy concurrencyStrategy) {
        if (enabled) {
            /* store reference to request log regardless of which thread later hits it */
            return HystrixRequestLog.getCurrentRequest(concurrencyStrategy);
        } else {
            return null;
        }
    }

 observe方法

 　　使用了ReplaySubject缓存了toObservable的消息，使得执行后再监听也可以收到所有消息。

public Observable<R> observe() {
        // us a ReplaySubject to buffer the eagerly subscribed-to Observable
        ReplaySubject<R> subject = ReplaySubject.create();
        // eagerly kick off subscription
        final Subscription sourceSubscription = toObservable().subscribe(subject);
        // return the subject that can be subscribed to later while the execution has already started
        return subject.doOnUnsubscribe(new Action0() {
            @Override
            public void call() {
                sourceSubscription.unsubscribe();
            }
        });
    }

 toObservable方法

public Observable<R> toObservable() {
　　　　　...
return Observable.defer(new Func0<Observable<R>>() {
            @Override
            public Observable<R> call() {
                 /* 每个command对象，只能被执行一次 */
                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 make a new error type for this
                    throw new HystrixRuntimeException(FailureType.BAD_REQUEST_EXCEPTION, _cmd.getClass(), getLogMessagePrefix() + " command executed multiple times - this is not permitted.", ex, null);
                }

                commandStartTimestamp = System.currentTimeMillis();
　　　　　　　　   //如果开启了记录请求，则向HystrixRequestLog添加执行命令
                if (properties.requestLogEnabled().get()) {
                    // log this command execution regardless of what happened
                    if (currentRequestLog != null) {
                        currentRequestLog.addExecutedCommand(_cmd);
                    }
                }
　　　　　　　　　//是否开启缓存，如果开启缓存并且实现了getCacheKey方法，则首先从HystrixRequestCache获取结果数据
                final boolean requestCacheEnabled = isRequestCachingEnabled();
                final String cacheKey = getCacheKey();
                /* try from cache first */
                if (requestCacheEnabled) {
                    HystrixCommandResponseFromCache<R> fromCache = (HystrixCommandResponseFromCache<R>) requestCache.get(cacheKey);
                    if (fromCache != null) {
                        isResponseFromCache = true;
                        return handleRequestCacheHitAndEmitValues(fromCache, _cmd);
                    }
                }
　　　　　　　　　//执行命令返回结果
                Observable<R> hystrixObservable =
                        Observable.defer(applyHystrixSemantics)
                                .map(wrapWithAllOnNextHooks);

                Observable<R> afterCache;

　　　　　　　　　　　　　　
                // 是否开启缓存，如果开启缓存并且实现了getCacheKey方法，将结果放到HystrixRequestCache
                if (requestCacheEnabled && cacheKey != null) {
                    // wrap it for caching
                    HystrixCachedObservable<R> toCache = HystrixCachedObservable.from(hystrixObservable, _cmd);
                    HystrixCommandResponseFromCache<R> fromCache = (HystrixCommandResponseFromCache<R>) requestCache.putIfAbsent(cacheKey, toCache);
                    if (fromCache != null) {
                        // another thread beat us so we'll use the cached value instead
                        toCache.unsubscribe();
                        isResponseFromCache = true;
                        return handleRequestCacheHitAndEmitValues(fromCache, _cmd);
                    } else {
                        // we just created an ObservableCommand so we cast and return it
                        afterCache = toCache.toObservable();
                    }
                } else {
                    afterCache = hystrixObservable;
                }

                return afterCache
                        .doOnTerminate(terminateCommandCleanup)     // perform cleanup once (either on normal terminal state (this line), or unsubscribe (next line))
                        .doOnUnsubscribe(unsubscribeCommandCleanup) // perform cleanup once
                        .doOnCompleted(fireOnCompletedHook);
            }
        });
    }

 　　在获取缓存结束后，需要更新command状态，记录执行时间，输出metrics，发送消息。

private Observable<R> handleRequestCacheHitAndEmitValues(final HystrixCommandResponseFromCache<R> fromCache, final AbstractCommand<R> _cmd) {
        try {
            executionHook.onCacheHit(this);
        } catch (Throwable hookEx) {
            logger.warn("Error calling HystrixCommandExecutionHook.onCacheHit", hookEx);
        }

        return fromCache.toObservableWithStateCopiedInto(this)
                .doOnTerminate(new Action0() {
                    @Override
                    public void call() {
                        if (commandState.compareAndSet(CommandState.OBSERVABLE_CHAIN_CREATED, CommandState.TERMINAL)) {
                            cleanUpAfterResponseFromCache(false); //user code never ran
                        } else if (commandState.compareAndSet(CommandState.USER_CODE_EXECUTED, CommandState.TERMINAL)) {
                            cleanUpAfterResponseFromCache(true); //user code did run
                        }
                    }
                })
                .doOnUnsubscribe(new Action0() {
                    @Override
                    public void call() {
                        if (commandState.compareAndSet(CommandState.OBSERVABLE_CHAIN_CREATED, CommandState.UNSUBSCRIBED)) {
                            cleanUpAfterResponseFromCache(false); //user code never ran
                        } else if (commandState.compareAndSet(CommandState.USER_CODE_EXECUTED, CommandState.UNSUBSCRIBED)) {
                            cleanUpAfterResponseFromCache(true); //user code did run
                        }
                    }
                });
    }

    private void cleanUpAfterResponseFromCache(boolean commandExecutionStarted) {
        Reference<TimerListener> tl = timeoutTimer.get();
        if (tl != null) {
            tl.clear();
        }

        final long latency = System.currentTimeMillis() - commandStartTimestamp;
        executionResult = executionResult
                .addEvent(-1, HystrixEventType.RESPONSE_FROM_CACHE)
                .markUserThreadCompletion(latency)
                .setNotExecutedInThread();
        ExecutionResult cacheOnlyForMetrics = ExecutionResult.from(HystrixEventType.RESPONSE_FROM_CACHE)
                .markUserThreadCompletion(latency);
        metrics.markCommandDone(cacheOnlyForMetrics, commandKey, threadPoolKey, commandExecutionStarted);
        eventNotifier.markEvent(HystrixEventType.RESPONSE_FROM_CACHE, commandKey);
    }

　　在如果不是通过缓存在结束后，也需要更新command状态，记录执行时间，输出metrics，发送消息。

final Action0 terminateCommandCleanup = new Action0() {

            @Override
            public void call() {
                if (_cmd.commandState.compareAndSet(CommandState.OBSERVABLE_CHAIN_CREATED, CommandState.TERMINAL)) {
                    handleCommandEnd(false); //user code never ran
                } else if (_cmd.commandState.compareAndSet(CommandState.USER_CODE_EXECUTED, CommandState.TERMINAL)) {
                    handleCommandEnd(true); //user code did run
                }
            }
        };
        final Action0 unsubscribeCommandCleanup = new Action0() {
            @Override
            public void call() {
                circuitBreaker.markNonSuccess();
                if (_cmd.commandState.compareAndSet(CommandState.OBSERVABLE_CHAIN_CREATED, CommandState.UNSUBSCRIBED)) {
                    if (!_cmd.executionResult.containsTerminalEvent()) {
                        _cmd.eventNotifier.markEvent(HystrixEventType.CANCELLED, _cmd.commandKey);
                        try {
                            executionHook.onUnsubscribe(_cmd);
                        } catch (Throwable hookEx) {
                            logger.warn("Error calling HystrixCommandExecutionHook.onUnsubscribe", hookEx);
                        }
                        _cmd.executionResultAtTimeOfCancellation = _cmd.executionResult
                                .addEvent((int) (System.currentTimeMillis() - _cmd.commandStartTimestamp), HystrixEventType.CANCELLED);
                    }
                    handleCommandEnd(false); //user code never ran
                } else if (_cmd.commandState.compareAndSet(CommandState.USER_CODE_EXECUTED, CommandState.UNSUBSCRIBED)) {
                    if (!_cmd.executionResult.containsTerminalEvent()) {
                        _cmd.eventNotifier.markEvent(HystrixEventType.CANCELLED, _cmd.commandKey);
                        try {
                            executionHook.onUnsubscribe(_cmd);
                        } catch (Throwable hookEx) {
                            logger.warn("Error calling HystrixCommandExecutionHook.onUnsubscribe", hookEx);
                        }
                        _cmd.executionResultAtTimeOfCancellation = _cmd.executionResult
                                .addEvent((int) (System.currentTimeMillis() - _cmd.commandStartTimestamp), HystrixEventType.CANCELLED);
                    }
                    handleCommandEnd(true); //user code did run
                }
            }
        };
        final Action0 fireOnCompletedHook = new Action0() {
            @Override
            public void call() {
                try {
                    executionHook.onSuccess(_cmd);
                } catch (Throwable hookEx) {
                    logger.warn("Error calling HystrixCommandExecutionHook.onSuccess", hookEx);
                }
            }
        };
        private void handleCommandEnd(boolean commandExecutionStarted) {
            Reference<TimerListener> tl = timeoutTimer.get();
            if (tl != null) {
                tl.clear();
            }

            long userThreadLatency = System.currentTimeMillis() - commandStartTimestamp;
            executionResult = executionResult.markUserThreadCompletion((int) userThreadLatency);
            if (executionResultAtTimeOfCancellation == null) {
                metrics.markCommandDone(executionResult, commandKey, threadPoolKey, commandExecutionStarted);
            } else {
                metrics.markCommandDone(executionResultAtTimeOfCancellation, commandKey, threadPoolKey, commandExecutionStarted);
            }

            if (endCurrentThreadExecutingCommand != null) {
                endCurrentThreadExecutingCommand.call();
            }
        }

 　　如果没有获取到缓存，则需要执行命令获得结果。命令执行流程如下：

private Observable<R> applyHystrixSemantics(final AbstractCommand<R> _cmd) {// executionHook调用
        executionHook.onStart(_cmd);
        /* determine if we're allowed to execute */
        if (circuitBreaker.attemptExecution()) { //使用熔断器判断是否熔断
            final TryableSemaphore executionSemaphore = getExecutionSemaphore();
            final AtomicBoolean semaphoreHasBeenReleased = new AtomicBoolean(false);
            final Action0 singleSemaphoreRelease = new Action0() {
                @Override
                public void call() {
                    if (semaphoreHasBeenReleased.compareAndSet(false, true)) {
                        executionSemaphore.release();
                    }
                }
            };

            final Action1<Throwable> markExceptionThrown = new Action1<Throwable>() {
                @Override
                public void call(Throwable t) {
                    eventNotifier.markEvent(HystrixEventType.EXCEPTION_THROWN, commandKey);
                }
            };

            if (executionSemaphore.tryAcquire()) {//执行并发信号量
                try {
                    /* used to track userThreadExecutionTime */
                    executionResult = executionResult.setInvocationStartTime(System.currentTimeMillis());
                    return executeCommandAndObserve(_cmd) //执行命令
                            .doOnError(markExceptionThrown)
                            .doOnTerminate(singleSemaphoreRelease)
                            .doOnUnsubscribe(singleSemaphoreRelease);
                } catch (RuntimeException e) {
                    return Observable.error(e);
                }
            } else {
                return handleSemaphoreRejectionViaFallback();//超过并发信号量fallback
            }
        } else {
            return handleShortCircuitViaFallback();//执行熔断fallback
        }
    }

 executeCommandAndObserve方法

　调用命令方法执行，并对执行的各种事件进行响应。

Observable<R> execution;
        if (properties.executionTimeoutEnabled().get()) {
            execution = executeCommandWithSpecifiedIsolation(_cmd)
                    .lift(new HystrixObservableTimeoutOperator<R>(_cmd));
        } else {
            execution = executeCommandWithSpecifiedIsolation(_cmd);
        }

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

　　命令返回数据时

final Action1<R> markEmits = new Action1<R>() {
            @Override
            public void call(R r) {
                if (shouldOutputOnNextEvents()) {
                    executionResult = executionResult.addEvent(HystrixEventType.EMIT);
                    eventNotifier.markEvent(HystrixEventType.EMIT, commandKey);//发送emit事件
                }
                if (commandIsScalar()) {
                    long latency = System.currentTimeMillis() - executionResult.getStartTimestamp();
                    eventNotifier.markEvent(HystrixEventType.SUCCESS, commandKey);//发送SUCCESS事件
                    executionResult = executionResult.addEvent((int) latency, HystrixEventType.SUCCESS);
                    eventNotifier.markCommandExecution(getCommandKey(), properties.executionIsolationStrategy().get(), (int) latency, executionResult.getOrderedList());
                    circuitBreaker.markSuccess();
                }
            }
        };

　　　命令完成时

final Action0 markOnCompleted = new Action0() {
            @Override
            public void call() {
                if (!commandIsScalar()) {
                    long latency = System.currentTimeMillis() - executionResult.getStartTimestamp();
                    eventNotifier.markEvent(HystrixEventType.SUCCESS, commandKey);
                    executionResult = executionResult.addEvent((int) latency, HystrixEventType.SUCCESS);
                    eventNotifier.markCommandExecution(getCommandKey(), properties.executionIsolationStrategy().get(), (int) latency, executionResult.getOrderedList());
                    circuitBreaker.markSuccess();
                }
            }
        };

　　命令出现异常时

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);
                }
            }
        };

handleThreadPoolRejectionViaFallback、handleTimeoutViaFallback、handleBadRequestByEmittingError、handleFailureViaFallback地层都调用了getFallbackOrThrowException方法。

private Observable<R> handleThreadPoolRejectionViaFallback(Exception underlying) {
        eventNotifier.markEvent(HystrixEventType.THREAD_POOL_REJECTED, commandKey);
        threadPool.markThreadRejection();
        return getFallbackOrThrowException(this, HystrixEventType.THREAD_POOL_REJECTED, FailureType.REJECTED_THREAD_EXECUTION, "could not be queued for execution", underlying);
    }
    private Observable<R> handleTimeoutViaFallback() {
        return getFallbackOrThrowException(this, HystrixEventType.TIMEOUT, FailureType.TIMEOUT, "timed-out", new TimeoutException());
    }
    private Observable<R> handleBadRequestByEmittingError(Exception underlying) {
        Exception toEmit = underlying;
        try {
            long executionLatency = System.currentTimeMillis() - executionResult.getStartTimestamp();
            eventNotifier.markEvent(HystrixEventType.BAD_REQUEST, commandKey);
            executionResult = executionResult.addEvent((int) executionLatency, HystrixEventType.BAD_REQUEST);
            Exception decorated = executionHook.onError(this, FailureType.BAD_REQUEST_EXCEPTION, underlying);
            if (decorated instanceof HystrixBadRequestException) {
                toEmit = decorated;
            } else {
                logger.warn("ExecutionHook.onError returned an exception that was not an instance of HystrixBadRequestException so will be ignored.", decorated);
            }
        } catch (Exception hookEx) {
            logger.warn("Error calling HystrixCommandExecutionHook.onError", hookEx);
        }
        return Observable.error(toEmit);
    }

    private Observable<R> handleFailureViaFallback(Exception underlying) {// report failure
        eventNotifier.markEvent(HystrixEventType.FAILURE, commandKey);
        // record the exception
        executionResult = executionResult.setException(underlying);
        return getFallbackOrThrowException(this, HystrixEventType.FAILURE, FailureType.COMMAND_EXCEPTION, "failed", underlying);
    }

 getFallbackOrThrowException

　　核心就是调用用户定义的fallback方法，并在各自情况下调用eventNotifier事件和返回executionResult。

...
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();
                }
...

timeout实现

private static class HystrixObservableTimeoutOperator<R> implements Operator<R, R> {

        final AbstractCommand<R> originalCommand;

        public HystrixObservableTimeoutOperator(final AbstractCommand<R> originalCommand) {
            this.originalCommand = originalCommand;
        }

        @Override
        public Subscriber<? super R> call(final Subscriber<? super R> child) {
            final CompositeSubscription s = new CompositeSubscription();
            // if the child unsubscribes we unsubscribe our parent as well
            child.add(s);

            //capture the HystrixRequestContext upfront so that we can use it in the timeout thread later
            final HystrixRequestContext hystrixRequestContext = HystrixRequestContext.getContextForCurrentThread();

            TimerListener listener = new TimerListener() {

                @Override
                public void tick() {
                    // if we can go from NOT_EXECUTED to TIMED_OUT then we do the timeout codepath
                    // otherwise it means we lost a race and the run() execution completed or did not start
                    if (originalCommand.isCommandTimedOut.compareAndSet(TimedOutStatus.NOT_EXECUTED, TimedOutStatus.TIMED_OUT)) {
                        // report timeout failure
                        originalCommand.eventNotifier.markEvent(HystrixEventType.TIMEOUT, originalCommand.commandKey);

                        // shut down the original request
                        s.unsubscribe();

                        final HystrixContextRunnable timeoutRunnable = new HystrixContextRunnable(originalCommand.concurrencyStrategy, hystrixRequestContext, new Runnable() {

                            @Override
                            public void run() {
                                child.onError(new HystrixTimeoutException());
                            }
                        });


                        timeoutRunnable.run();
                        //if it did not start, then we need to mark a command start for concurrency metrics, and then issue the timeout
                    }
                }

                @Override
                public int getIntervalTimeInMilliseconds() {
                    return originalCommand.properties.executionTimeoutInMilliseconds().get();
                }
            };

            final Reference<TimerListener> tl = HystrixTimer.getInstance().addTimerListener(listener);

            // set externally so execute/queue can see this
            originalCommand.timeoutTimer.set(tl);

            /**
             * If this subscriber receives values it means the parent succeeded/completed
             */
            Subscriber<R> parent = new Subscriber<R>() {

                @Override
                public void onCompleted() {
                    if (isNotTimedOut()) {
                        // stop timer and pass notification through
                        tl.clear();
                        child.onCompleted();
                    }
                }

                @Override
                public void onError(Throwable e) {
                    if (isNotTimedOut()) {
                        // stop timer and pass notification through
                        tl.clear();
                        child.onError(e);
                    }
                }

                @Override
                public void onNext(R v) {
                    if (isNotTimedOut()) {
                        child.onNext(v);
                    }
                }

                private boolean isNotTimedOut() {
                    // if already marked COMPLETED (by onNext) or succeeds in setting to COMPLETED
                    return originalCommand.isCommandTimedOut.get() == TimedOutStatus.COMPLETED ||
                            originalCommand.isCommandTimedOut.compareAndSet(TimedOutStatus.NOT_EXECUTED, TimedOutStatus.COMPLETED);
                }

            };

            // if s is unsubscribed we want to unsubscribe the parent
            s.add(parent);

            return parent;
        }

    }

executeCommandWithSpecifiedIsolation方法

　　真正执行命令的方法，有两种方式执行，一种是使用线程池执行，一种是使用当前线程执行。

private Observable<R> executeCommandWithSpecifiedIsolation(final AbstractCommand<R> _cmd) {
        if (properties.executionIsolationStrategy().get() == ExecutionIsolationStrategy.THREAD) {
            //使用线程池执行
            return Observable.defer(new Func0<Observable<R>>() {
                @Override
                public Observable<R> call() {
                    executionResult = executionResult.setExecutionOccurred();
                    if (!commandState.compareAndSet(CommandState.OBSERVABLE_CHAIN_CREATED, CommandState.USER_CODE_EXECUTED)) {
                        return Observable.error(new IllegalStateException("execution attempted while in state : " + commandState.get().name()));
                    }

                    metrics.markCommandStart(commandKey, threadPoolKey, ExecutionIsolationStrategy.THREAD);//调用命令开始执行metrics

                    if (isCommandTimedOut.get() == TimedOutStatus.TIMED_OUT) {
                        // the command timed out in the wrapping thread so we will return immediately
                        // and not increment any of the counters below or other such logic
                        return Observable.error(new RuntimeException("timed out before executing run()"));
                    }
                    if (threadState.compareAndSet(ThreadState.NOT_USING_THREAD, ThreadState.STARTED)) {//修改threadState状态
                        //we have not been unsubscribed, so should proceed
                        HystrixCounters.incrementGlobalConcurrentThreads();
                        threadPool.markThreadExecution(); //调用线程池开始方法metrics
                        // store the command that is being run
                        endCurrentThreadExecutingCommand = Hystrix.startCurrentThreadExecutingCommand(getCommandKey());
                        executionResult = executionResult.setExecutedInThread();//设置在线程池中执行
                        try {
                            executionHook.onThreadStart(_cmd);//调用hook的onThread
                            executionHook.onRunStart(_cmd);//调用hook的onRunStart
                            executionHook.onExecutionStart(_cmd);//调用hook的onExecutionStart
                            return getUserExecutionObservable(_cmd);
                        } catch (Throwable ex) {
                            return Observable.error(ex);
                        }
                    } else {
                        //command has already been unsubscribed, so return immediately
                        return Observable.empty();
                    }
                }
            }).doOnTerminate(new Action0() {
                @Override
                public void call() {
                    if (threadState.compareAndSet(ThreadState.STARTED, ThreadState.TERMINAL)) {
                        handleThreadEnd(_cmd);
                    }
                    if (threadState.compareAndSet(ThreadState.NOT_USING_THREAD, ThreadState.TERMINAL)) {
                        //if it was never started and received terminal, then no need to clean up (I don't think this is possible)
                    }
                    //if it was unsubscribed, then other cleanup handled it
                }
            }).doOnUnsubscribe(new Action0() {
                @Override
                public void call() {
                    if (threadState.compareAndSet(ThreadState.STARTED, ThreadState.UNSUBSCRIBED)) {
                        handleThreadEnd(_cmd);
                    }
                    if (threadState.compareAndSet(ThreadState.NOT_USING_THREAD, ThreadState.UNSUBSCRIBED)) {
                        //if it was never started and was cancelled, then no need to clean up
                    }
                    //if it was terminal, then other cleanup handled it
                }
            }).subscribeOn(threadPool.getScheduler(new Func0<Boolean>() {//使用线程池执行
                @Override
                public Boolean call() {
                    return properties.executionIsolationThreadInterruptOnTimeout().get() && _cmd.isCommandTimedOut.get() == TimedOutStatus.TIMED_OUT;
                }
            }));
        } else {
            return Observable.defer(new Func0<Observable<R>>() {
                @Override
                public Observable<R> call() {
                    executionResult = executionResult.setExecutionOccurred();
                    if (!commandState.compareAndSet(CommandState.OBSERVABLE_CHAIN_CREATED, CommandState.USER_CODE_EXECUTED)) {
                        return Observable.error(new IllegalStateException("execution attempted while in state : " + commandState.get().name()));
                    }
                    metrics.markCommandStart(commandKey, threadPoolKey, ExecutionIsolationStrategy.SEMAPHORE);//调用命令开始执行metrics
                    // semaphore isolated
                    // store the command that is being run
                    endCurrentThreadExecutingCommand = Hystrix.startCurrentThreadExecutingCommand(getCommandKey());
                    try {
                        executionHook.onRunStart(_cmd);//调用hook的onRunStart
                        executionHook.onExecutionStart(_cmd);//调用hook的onExecutionStart
                        return getUserExecutionObservable(_cmd);  //the getUserExecutionObservable method already wraps sync exceptions, so this shouldn't throw
                    } catch (Throwable ex) {
                        //If the above hooks throw, then use that as the result of the run method
                        return Observable.error(ex);
                    }
                }
            });
        }
    }