Spring Cloud详解（五）Hystrix源码分析

1. @HystrixCommand

首先，我们从@HystrixCommand注解入手

@HystrixCommand(fallbackMethod = "fallBack")
    public String hiService() {
        return restTemplate.getForObject("http://provider/zone?",String.class);
    }

@HystrixCommand注解会为当前方法创建一个HystrixCommand对象，这个类的主要作用是命令模式中对于Command的封装，执行命令有如下两个方法：

//用于异步请求，返回的是Future对象，包含服务执行结束时会返回的结果对象。 
public Future<R> queue() {
        /*
         * The Future returned by Observable.toBlocking().toFuture() does not implement the
         * interruption of the execution thread when the "mayInterrupt" flag of Future.cancel(boolean) is set to true;
         * thus, to comply with the contract of Future, we must wrap around it.
         */
        final Future<R> delegate = toObservable().toBlocking().toFuture();
    
        final Future<R> f = new Future<R>() {
            @Override
            public boolean cancel(boolean mayInterruptIfRunning) {
                if (delegate.isCancelled()) {
                    return false;
                }
                if (HystrixCommand.this.getProperties().executionIsolationThreadInterruptOnFutureCancel().get()) {
                    interruptOnFutureCancel.compareAndSet(false, mayInterruptIfRunning);
        		}

                final boolean res = delegate.cancel(interruptOnFutureCancel.get());

                if (!isExecutionComplete() && interruptOnFutureCancel.get()) {
                    final Thread t = executionThread.get();
                    if (t != null && !t.equals(Thread.currentThread())) {
                        t.interrupt();
                    }
                }

                return res;
			}
            @Override
            public boolean isCancelled() {
                return delegate.isCancelled();
			}
            @Override
            public boolean isDone() {
                return delegate.isDone();
			}
            @Override
            public R get() throws InterruptedException, ExecutionException {
                return delegate.get();
            }
            @Override
            public R get(long timeout, TimeUnit unit) throws InterruptedException, ExecutionException, TimeoutException {
                return delegate.get(timeout, unit);
            }
        	
        };

        /* special handling of error states that throw immediately */
        if (f.isDone()) {
            try {
                f.get();
                return f;
            } catch (Exception e) {
                Throwable t = decomposeException(e);
                if (t instanceof HystrixBadRequestException) {
                    return f;
                } else if (t instanceof HystrixRuntimeException) {
                    HystrixRuntimeException hre = (HystrixRuntimeException) t;
                    switch (hre.getFailureType()) {
					case COMMAND_EXCEPTION:
					case TIMEOUT:
						// we don't throw these types from queue() only from queue().get() as they are execution errors
						return f;
					default:
						// these are errors we throw from queue() as they as rejection type errors
						throw hre;
					}
                } else {
                    throw Exceptions.sneakyThrow(t);
                }
            }
        }

        return f;
    }
//用于同步执行，从依赖服务返回单一对象结果；调用的是queue.get
    public R execute() {
        try {
            return queue().get();
        } catch (Exception e) {
            throw Exceptions.sneakyThrow(decomposeException(e));
        }
    }

除去HystrixCommand，还有HystrixObservableCommand，该类主要实现两个方法：

• observe() 这是一个热观察
• toObservable() 这是一个冷观察

实际上，excute() 的实现是queue()，而queue中，我们可以看到有如下：

//使用了冷观察；并且转化成为Future异步对象。
final Future<R> delegate = toObservable().toBlocking().toFuture();

因此，我们可以知道，Hstrix命令的运行有如下两个重要部分：

• 事件源处理（冷、热观察模式）
• 同、异步处理（同步等待返回结果，异步利用Future对象，可以异步处理其它工作，并且最后获取future中需要的结果返回即可）

以下说一下Hystrix的命令执行，用图说明：

2. HystrixCircuitBreaker断路器

在Hystrix中，断路器概念核心，以下是它的定义：

public interface HystrixCircuitBreaker {

    //每个Hystrix命令请求，在执行之前，都会调用这个方法，判断当前是否可以执行。
    public boolean allowRequest();
	//判断开闭状态
    public boolean isOpen();
	//当断路器处于半开状态，且命令运行成功，则用于关闭断路器
    void markSuccess();
    }
    

断路器的实现有多个，采用内部类实现：

//都不实现
static class NoOpCircuitBreaker implements HystrixCircuitBreaker {
        @Override
        public boolean allowRequest() {
            return true;
        }
        @Override
        public boolean isOpen() {
            return false;
        }
        @Override
        public void markSuccess() {
        }
    }
  //默认实现
  static class HystrixCircuitBreakerImpl implements HystrixCircuitBreaker {
        private final HystrixCommandProperties properties;
        private final HystrixCommandMetrics metrics;
        /* track whether this circuit is open/closed at any given point in time (default to false==closed) */
        private AtomicBoolean circuitOpen = new AtomicBoolean(false);
        /* when the circuit was marked open or was last allowed to try a 'singleTest' */
        private AtomicLong circuitOpenedOrLastTestedTime = new AtomicLong();
    //还有具体几个方法的实现。
  }

对于断路器来说，断路器会根据本身的状态，即断路器是否开启，来决定这个HystrixCommand命令是否执行，还是调用fallBack进行降级，断路器开启和关闭的度量标准如下，由度量对象metrics获取healthCount进行判断，这个时间窗口默认为10S：

• QPS在预设的阈值之内，则返回false，默认为20。
• 如果错误请求百分比，在预设阈值之下，则返回false。

源码如下：

 @Override
        public boolean isOpen() {
            if (circuitOpen.get()) {
                return true;
            }
            HealthCounts health = metrics.getHealthCounts();
            // check if we are past the statisticalWindowVolumeThreshold
            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;
            }
            if (health.getErrorPercentage() < properties.circuitBreakerErrorThresholdPercentage().get()) {
                return false;
            } else {
                // our failure rate is too high, trip the circuit
                if (circuitOpen.compareAndSet(false, true)) {
                    // if the previousValue was false then we want to set the currentTime
                    circuitOpenedOrLastTestedTime.set(System.currentTimeMillis());
                    return true;
                } else {
                    return true;
                }
            }
        }

判断是否允许：

 @Override
        public boolean allowRequest() {
          //这个地方要注意，我们看到了，强制开启的优先级大于强制关闭。
            if (properties.circuitBreakerForceOpen().get()) {
                return false;
            }
            if (properties.circuitBreakerForceClosed().get()) {
                isOpen();
                return true;
            }
          	//这里通过两个方法的结合使用，实现关闭、开启的切换
            return !isOpen() || allowSingleTest();
        }
//这个方法的功能是尝试允许命令请求；
 public boolean allowSingleTest() {
            long timeCircuitOpenedOrWasLastTested = circuitOpenedOrLastTestedTime.get();
            // 1) if the circuit is open
            // 2) and it's been longer than 'sleepWindow' since we opened the circuit
            if (circuitOpen.get() && System.currentTimeMillis() > timeCircuitOpenedOrWasLastTested + properties.circuitBreakerSleepWindowInMilliseconds().get()) {
                // We push the 'circuitOpenedTime' ahead by 'sleepWindow' since we have allowed one request to try.
                // If it succeeds the circuit will be closed, otherwise another singleTest will be allowed at the end of the 'sleepWindow'.
                if (circuitOpenedOrLastTestedTime.compareAndSet(timeCircuitOpenedOrWasLastTested, System.currentTimeMillis())) {
                  //尝试允许请求，则使得断路器处于半开状态。
                    return true;
                }
            }
            return false;
        }

以上是允许请求的实现：

• 通过allowSingleTest，尝试允许请求访问。
• 通过设置休眠时间，在休眠时间过去之后，则将断路器处于“半开状态”，这种半开状态，实际是就是允许命令请求的执行，如果执行又失败，或者超时，则又将断路器置于全开状态；并且在休眠时间之后，尝试允许，继续处于半开状态。

关闭断路器的实现如下：

   public void markSuccess() {
            if (circuitOpen.get()) {
              //关闭断路器
                if (circuitOpen.compareAndSet(true, false)) {
                    //win the thread race to reset metrics
                    //Unsubscribe from the current stream to reset the health counts stream.  This only affects the health counts view,
                    //and all other metric consumers are unaffected by the reset
                  //重置统计信息
                    metrics.resetStream();
                }
            }
        }