hystrix源码分析

public static class Factory {
    // key 是 HystrixCommandKey 的 hashcode
    private static ConcurrentHashMap<String, HystrixCircuitBreaker> circuitBreakersByCommand = new ConcurrentHashMap<String, HystrixCircuitBreaker>();
    public static HystrixCircuitBreaker getInstance(HystrixCommandKey key, HystrixCommandGroupKey group, HystrixCommandProperties properties, HystrixCommandMetrics metrics) {
        // 先从缓存中获取
        HystrixCircuitBreaker previouslyCached = circuitBreakersByCommand.get(key.name());
        if (previouslyCached != null) {
            return previouslyCached;
        }
        // 如果缓存中为空， 就初始化并放入缓存
        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;
        }
    }
    public static HystrixCircuitBreaker getInstance(HystrixCommandKey key) {
        return circuitBreakersByCommand.get(key.name());
    }
    /* package */static void reset() {
        circuitBreakersByCommand.clear();
    }
}

public boolean allowRequest() {
    if (properties.circuitBreakerForceOpen().get()) {
        // 如果断路器强制开启，直接返回 false
        return false;
    }
    if (properties.circuitBreakerForceClosed().get()) {
        // 就算是强制关闭了， 也要调用 isOpen 来记录一些统计数据
        isOpen();
        // 因为是强制关闭， 不用在意 isOpen 的执行结果
        return true;
    }
    // 判断断路器是否关闭或断路器恢复计时器是否到达时间
    return !isOpen() || allowSingleTest();
}

@Override
public boolean isOpen() {
    if (circuitOpen.get()) {
        // 如果是 open 状态， 直接返回 true
        return true;
    }
    // 如果是 cloes 状态， 就需要获取 healthCounts 来判断错误率是否达到需要打开断路器
    HealthCounts health = metrics.getHealthCounts();
    // check if we are past the statisticalWindowVolumeThreshold
    if (health.getTotalRequests() < properties.circuitBreakerRequestVolumeThreshold().get()) {
        // 请求数如果小于判断阈值则返回 false
        return false;
    }
    if (health.getErrorPercentage() < properties.circuitBreakerErrorThresholdPercentage().get()) {
        // 判断错误比例是否小于设定的值， 默认是 50%
        return false;
    } else {
        // 错误比例太高， cas 开启断路器
        if (circuitOpen.compareAndSet(false, true)) {
            // 将当前系统时间设置为短路定时器初始时间
            circuitOpenedOrLastTestedTime.set(System.currentTimeMillis());
            return true;
        } else {
           // 走到这里代表 cas 失败， 就是有其他的线程开启了断路器， 也返回 true
            return true;
        }
    }
}

public boolean allowSingleTest() {
    // 获取断路器定时器的初始时间
    long timeCircuitOpenedOrWasLastTested = circuitOpenedOrLastTestedTime.get();
    // 1) 断路器状态是 open
    // 2) 当前时间与定时器初始时间的差大于 circuitBreakerSleepWindowInMilliseconds(睡眠窗口时间，默认 5s)
    if (circuitOpen.get() && System.currentTimeMillis() > timeCircuitOpenedOrWasLastTested + properties.circuitBreakerSleepWindowInMilliseconds().get()) {
        // cas 将断路定时器初始时间设置为当前时间
        if (circuitOpenedOrLastTestedTime.compareAndSet(timeCircuitOpenedOrWasLastTested, System.currentTimeMillis())) {
            // 如果设置成功则返回 true
            return true;
        }
    }
    // 返回 false 代表有其他线程放行了请求
    return false;
}