Hystrix源码解析流程（一）：真正调用前的准备工作

SpringCloud版本：
<dependency>
   <groupId>org.springframework.cloud</groupId>
   <artifactId>spring-cloud-dependencies</artifactId>
   <version>Edgware.SR3</version>
   <type>pom</type>
   <scope>import</scope>
</dependency>

从Spring.factories中
org.springframework.cloud.client.circuitbreaker.EnableCircuitBreaker=\
org.springframework.cloud.netflix.hystrix.HystrixCircuitBreakerConfiguration
@Configuration
public class HystrixCircuitBreakerConfiguration {
   @Bean
   public HystrixCommandAspect hystrixCommandAspect() {
      //初始化HystrixCommandAspect
      return new HystrixCommandAspect();
   }
   @Bean
   public HystrixShutdownHook hystrixShutdownHook() {
      return new HystrixShutdownHook();
   }
   @Bean
   public HasFeatures hystrixFeature() {
      return HasFeatures.namedFeatures(new NamedFeature("Hystrix", HystrixCommandAspect.class));
   }
}

@Aspect
public class HystrixCommandAspect {
   //AOP切点（带有HystrixCommand注解的方法）
    @Pointcut("@annotation(com.netflix.hystrix.contrib.javanica.annotation.HystrixCommand)")
    public void hystrixCommandAnnotationPointcut() {
    }
 //AOP切点（带有HystrixCollapser注解的方法）
    @Pointcut("@annotation(com.netflix.hystrix.contrib.javanica.annotation.HystrixCollapser)")
    public void hystrixCollapserAnnotationPointcut() {
    }
    @Around("hystrixCommandAnnotationPointcut() || hystrixCollapserAnnotationPointcut()")
    public Object methodsAnnotatedWithHystrixCommand(final ProceedingJoinPoint joinPoint) throws Throwable {
        Method method = getMethodFromTarget(joinPoint);
        //如果方法中同时有HystrixCollapser和HystrixCommand将会报错
        if (method.isAnnotationPresent(HystrixCommand.class) && method.isAnnotationPresent(HystrixCollapser.class))                       {throw new IllegalStateException("");}
        //根据不同的注解（HystrixCommand和HystrixCollapser）获取不同的MetaHolderFactory 
        MetaHolderFactory metaHolderFactory = META_HOLDER_FACTORY_MAP.get(HystrixPointcutType.of(method));
        // 创建metaHolder，其中包含了各种必要信息，如注解的方法包含的各种信息以及fallback方法的各种信息，下面有源码
        MetaHolder metaHolder = metaHolderFactory.create(joinPoint);//详见注解一
        //通过HystrixCommandFactory获取执行对象
        HystrixInvokable invokable = HystrixCommandFactory.getInstance().create(metaHolder);//详见注解二
        //获取执行模式
        ExecutionType executionType = metaHolder.isCollapserAnnotationPresent() ?
                metaHolder.getCollapserExecutionType() : metaHolder.getExecutionType();
        Object result;
        try {
            if (!metaHolder.isObservable()) {
                //开始执行具体的逻辑（）
                result = CommandExecutor.execute(invokable, executionType, metaHolder);//详见下一篇请求过程分析
            } else {
                result = executeObservable(invokable, executionType, metaHolder);
            }
        } catch (HystrixBadRequestException e) {
            throw e.getCause() != null ? e.getCause() : e;
        } catch (HystrixRuntimeException e) {
            throw hystrixRuntimeExceptionToThrowable(metaHolder, e);
        }
        return result;
    }

注解一

metaHolderFactory.create(joinPoint)（HytrixCommandAspect）public MetaHolder create(final ProceedingJoinPoint joinPoint) {
    Method method = getMethodFromTarget(joinPoint);
    Object obj = joinPoint.getTarget();
    Object[] args = joinPoint.getArgs();
    Object proxy = joinPoint.getThis();
    return create(proxy, method, obj, args, joinPoint);
}
public abstract MetaHolder create(Object proxy, Method method, Object obj, Object[] args, final ProceedingJoinPoint joinPoint);
//CommandMetaHolderFactory对create方法进行了实现
private static class CommandMetaHolderFactory extends MetaHolderFactory {
    @Override
    public MetaHolder create(Object proxy, Method method, Object obj, Object[] args, final ProceedingJoinPoint joinPoint) {
        HystrixCommand hystrixCommand = method.getAnnotation(HystrixCommand.class);
  //获取ExecutionType，包含种类: ASYNCHRONOUS（同步 ）、OBSERVABLE（Rxjava）、SYNCHRONOUS（异步 ）（默认）;
        ExecutionType executionType = ExecutionType.getExecutionType(method.getReturnType());
        MetaHolder.Builder builder = metaHolderBuilder(proxy, method, obj, args, joinPoint);
        if (isCompileWeaving()) {
            builder.ajcMethod(getAjcMethodFromTarget(joinPoint));
        }
        return builder.defaultCommandKey(method.getName())
                        .hystrixCommand(hystrixCommand)
                        //ObservableExecutionMode:EAGER（Observable 会在被创建后立刻执行）、LAZY（则会产生一个 Observable 被 subscribe 后执行）
                        .observableExecutionMode(hystrixCommand.observableExecutionMode())
                        .executionType(executionType)
                        //执行方式：rxjava执行还是其他方式执行
                        .observable(ExecutionType.OBSERVABLE == executionType)
                        .build();
    }
}
metaHolderBuilder(proxy, method, obj, args, joinPoint);调用父类的方法
（HystrixCommandAspect）MetaHolder.Builder metaHolderBuilder(Object proxy, Method method, Object obj, Object[] args, final ProceedingJoinPoint joinPoint) {
    MetaHolder.Builder builder = MetaHolder.builder()
            .args(args).method(method).obj(obj).proxyObj(proxy)
            .joinPoint(joinPoint);
    //设置服务降级的方法
    setFallbackMethod(builder, obj.getClass(), method);
    //设置defaultGroupKey、defaultThreadPoolKey
    builder = setDefaultProperties(builder, obj.getClass(), joinPoint);
    return builder;
}

注解二

HystrixInvokable invokable = HystrixCommandFactory.getInstance().create(metaHolder);

//根据注解和executionType来判断采用什么样的HystrixInvokable 
public HystrixInvokable create(MetaHolder metaHolder) {
    HystrixInvokable executable;
    if (metaHolder.isCollapserAnnotationPresent()) {
        executable = new CommandCollapser(metaHolder);
    } else if (metaHolder.isObservable()) {
        executable = new GenericObservableCommand(HystrixCommandBuilderFactory.getInstance().create(metaHolder));
    } else {
        executable = new GenericCommand(HystrixCommandBuilderFactory.getInstance().create(metaHolder));
    }
    return executable;
}

HystrixCommandBuilderFactory.getInstance().create(metaHolder)

public HystrixCommandBuilder create(MetaHolder metaHolder) {
    return create(metaHolder, Collections.<HystrixCollapser.CollapsedRequest<Object, Object>>emptyList());
}

public <ResponseType> HystrixCommandBuilder create(MetaHolder metaHolder, Collection<HystrixCollapser.CollapsedRequest<ResponseType, Object>> collapsedRequests) {
    validateMetaHolder(metaHolder);

    return HystrixCommandBuilder.builder()
            //builder中包含groupKey、threadPoolKey、commandKey等注解信息
            .setterBuilder(createGenericSetterBuilder(metaHolder))
            //构建commandAction，将具体调用的方法封装为commandAction，例：@HystrixCommand，包含commandAction和fallbackAction；commandAction为实际的请求逻辑。fallbackAction为服务降级之后的调用方法。
            .commandActions(createCommandActions(metaHolder))
            //builder中包含groupKey、threadPoolKey、commandKey等注解信息
            .collapsedRequests(collapsedRequests)
            //如果存在@CacheResult，对结果缓存的基础信息构建
            .cacheResultInvocationContext(createCacheResultInvocationContext(metaHolder))
            //如果存在@CacheRemove，对删除缓存的基础信息构建
            .cacheRemoveInvocationContext(createCacheRemoveInvocationContext(metaHolder))
            .ignoreExceptions(metaHolder.getCommandIgnoreExceptions())
            .executionType(metaHolder.getExecutionType())
            .build();
}

GenericCommand构造方法的执行;

public GenericCommand(HystrixCommandBuilder builder) {
    super(builder);
}
(AbstractHystrixCommand)protected AbstractHystrixCommand(HystrixCommandBuilder builder) {
    super(builder.getSetterBuilder().build());
    this.commandActions = builder.getCommandActions();
    this.collapsedRequests = builder.getCollapsedRequests();
    this.cacheResultInvocationContext = builder.getCacheResultInvocationContext();
    this.cacheRemoveInvocationContext = builder.getCacheRemoveInvocationContext();
    this.ignoreExceptions = builder.getIgnoreExceptions();
    this.executionType = builder.getExecutionType();
}
(AbstractCommand)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) {
//初始化group，group主要是用来对不同的command key进行统一管理
    this.commandGroup = initGroupKey(group);
// 初始化command key，用来标识降级逻辑
    this.commandKey = initCommandKey(key, getClass());
// 初始化自定义的降级策略
    this.properties = initCommandProperties(this.commandKey, propertiesStrategy, commandPropertiesDefaults);
// 初始化线程池key，相同的线程池key将公用线程池
    this.threadPoolKey = initThreadPoolKey(threadPoolKey, this.commandGroup, this.properties.executionIsolationThreadPoolKeyOverride().get());
// 初始化监控器,里面会保存每个命令相关的执行信息
    this.metrics = initMetrics(metrics, this.commandGroup, this.threadPoolKey, this.commandKey, this.properties);
// 初始化断路器，控制着熔断器的开闭。
    this.circuitBreaker = initCircuitBreaker(this.properties.circuitBreakerEnabled().get(), circuitBreaker, this.commandGroup, this.commandKey, this.properties, this.metrics);
//// 初始化线程池
    this.threadPool = initThreadPool(threadPool, this.threadPoolKey, threadPoolPropertiesDefaults);

    //Strategies from plugins
    this.eventNotifier = HystrixPlugins.getInstance().getEventNotifier();
    this.concurrencyStrategy = HystrixPlugins.getInstance().getConcurrencyStrategy();
    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);

    /* fallback semaphore override if applicable */
    this.fallbackSemaphoreOverride = fallbackSemaphore;

    /* execution semaphore override if applicable */
    this.executionSemaphoreOverride = executionSemaphore;
}
initCircuitBreaker(this.properties.circuitBreakerEnabled().get(), circuitBreaker, this.commandGroup, this.commandKey, this.properties, this.metrics);
每一个command对应有一个HystrixCircuitBreaker，保存断路器的HashMap声明在HystrixCircuitBreaker.Factory中，key是Command key。
private static ConcurrentHashMap<String, HystrixCircuitBreaker> circuitBreakersByCommand = new ConcurrentHashMap<String, HystrixCircuitBreaker>();