hystrix - Fallback是怎么调用的提到了allowRequest办法，咱们看看这里是做了哪些。

public boolean allowRequest() {    // 配置了强制熔断，间接返回false    if (properties.circuitBreakerForceOpen().get()) {        // properties have asked us to force the circuit open so we will allow NO requests        return false;    }    // 强制敞开    if (properties.circuitBreakerForceClosed().get()) {        // we still want to allow isOpen() to perform it's calculations so we simulate normal behavior        isOpen();        // properties have asked us to ignore errors so we will ignore the results of isOpen and just allow all traffic through        return true;    }    return !isOpen() || allowSingleTest();}public boolean isOpen() {    // 获取熔断器的状态，true就是开    if (circuitOpen.get()) {        // 如果是开的，还是返回ture，留给半开的时候用        return true;    }    // 如果是关的，获取HealthCounts信息    HealthCounts health = metrics.getHealthCounts();    // 以后的申请数如果小于设置的阈值，就间接返回false，阐明没开    // 比方咱们设置了20，此时才19，也是没有开的    // 这个值是样品数量的值，只有足够数量的样品，才比拟精确    if (health.getTotalRequests() < properties.circuitBreakerRequestVolumeThreshold().get()) {        // we are not past the minimum volume threshold for the statisticalWindow so we'll return false immediately and not calculate anything        return false;    }    // 谬误比例小于设定的值，比方谬误的比例超过50%，就开启熔断    if (health.getErrorPercentage() < properties.circuitBreakerErrorThresholdPercentage().get()) {        return false;    } else {        // 这边设置工夫，用于半开状态        if (circuitOpen.compareAndSet(false, true)) {            // if the previousValue was false then we want to set the currentTime            circuitOpenedOrLastTestedTime.set(System.currentTimeMillis());            return true;        } else {            // How could previousValue be true? If another thread was going through this code at the same time a race-condition could have            // caused another thread to set it to true already even though we were in the process of doing the same            // In this case, we know the circuit is open, so let the other thread set the currentTime and report back that the circuit is open            return true;        }    }}

流程图如下：

半开状态

下面还有一个办法，allowSingleTest。这个是用于解决半开的时候。
咱们下面晓得，当熔断的时候，会保留一个值，circuitOpenedOrLastTestedTime。这个值加上设置的休眠工夫和以后工夫做比拟，看看能不能允许尝试拜访一次。

public boolean allowSingleTest() {    long timeCircuitOpenedOrWasLastTested = circuitOpenedOrLastTestedTime.get();    // 熔断器开的时候，并且以后工夫>熔断工夫+休眠工夫    // 也就是说休眠工夫设置为5，那5秒后给他一个机会看看能不能拜访，也就是半开状态    // 而后从新设置circuitOpenedOrLastTestedTime    if (circuitOpen.get() && System.currentTimeMillis() > timeCircuitOpenedOrWasLastTested + properties.circuitBreakerSleepWindowInMilliseconds().get()) {        if (circuitOpenedOrLastTestedTime.compareAndSet(timeCircuitOpenedOrWasLastTested, System.currentTimeMillis())) {            return true;        }    }    return false;}

HystrixCircuitBreaker创立

allowRequest是HystrixCircuitBreaker的办法，咱们略微回顾一下command的创立，AbstractCommand中，会调用initCircuitBreaker办法。他会通过HystrixCircuitBreaker的Factory来创立的

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();    }}

在工厂中，他其实会通过缓存来获取的，也就是说每个CommandKey都有对应的HystrixCircuitBreaker。如果缓存没有，则创立一个放入缓存。

public static HystrixCircuitBreaker getInstance(HystrixCommandKey key, HystrixCommandGroupKey group, HystrixCommandProperties properties, HystrixCommandMetrics metrics) {    // 从circuitBreakersByCommand缓存里取，key是CommandKey    HystrixCircuitBreaker previouslyCached = circuitBreakersByCommand.get(key.name());    if (previouslyCached != null) {        return previouslyCached;    }    // 缓存没有，创立一个放入缓存中    HystrixCircuitBreaker cbForCommand = circuitBreakersByCommand.putIfAbsent(key.name(), new HystrixCircuitBreakerImpl(key, group, properties, metrics));    if (cbForCommand == null) {        // this means the putIfAbsent step just created a new one so let's retrieve and return it        return circuitBreakersByCommand.get(key.name());    } else {        // this means a race occurred and while attempting to 'put' another one got there before        // and we instead retrieved it and will now return it        return cbForCommand;    }}