其实只是Hystrix初始化部分，我们从源码的角度分析一下@EnableCircuitBreaker以及@HystrixCommand注解的初始化过程。

从@EnableCircuitBreaker入手

我们是通过在启动类添加@EnableCircuitBreaker注解启用Hystrix的，所以，源码解析也要从这个注解入手。

该注解已经被标了@Deprecated了，不过，挡不住…

Spring关于@Enablexxxx注解的套路：通过@Import注解引入了EnableCircuitBreakerImportSelector.class：

@Deprecated
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@Import(EnableCircuitBreakerImportSelector.class)
public @interface EnableCircuitBreaker {}

跟踪EnableCircuitBreakerImportSelector，继承自SpringFactoryImportSelector，注意SpringFactoryImportSelector类的泛型类型为EnableCircuitBreaker：

@Order(Ordered.LOWEST_PRECEDENCE - 100)
public class EnableCircuitBreakerImportSelector extends SpringFactoryImportSelector<EnableCircuitBreaker> {@Overrideprotected boolean isEnabled() {return getEnvironment().getProperty("spring.cloud.circuit.breaker.enabled", Boolean.class, Boolean.TRUE);}}

看一下类图：

属性annotationClass通过GenericTypeResolver.resolveTypeArgument方法获取当前对象的泛型参数的具体类型，现在我们知道是EnableCircuitBreaker（在org.springframework.cloud.client.circuitbreaker包下）。

public abstract class SpringFactoryImportSelector<T>implements DeferredImportSelector, BeanClassLoaderAware, EnvironmentAware {private final Log log = LogFactory.getLog(SpringFactoryImportSelector.class);private ClassLoader beanClassLoader;private Class<T> annotationClass;private Environment environment;@SuppressWarnings("unchecked")protected SpringFactoryImportSelector() {this.annotationClass = (Class<T>) GenericTypeResolver.resolveTypeArgument(this.getClass(),SpringFactoryImportSelector.class);}

@Import注解我们前面做过详细分析了，实现了DeferredImportSelector接口表示延迟加载全限定名称为selectImports方法返回的类。看selectImports方法：

@Overridepublic String[] selectImports(AnnotationMetadata metadata) {//Enable参数没有打开的话就不加载if (!isEnabled()) {return new String[0];}AnnotationAttributes attributes = AnnotationAttributes.fromMap(metadata.getAnnotationAttributes(this.annotationClass.getName(), true));Assert.notNull(attributes, "No " + getSimpleName() + " attributes found. Is " + metadata.getClassName()+ " annotated with @" + getSimpleName() + "?");// Find all possible auto configuration classes, filtering duplicates//SPI机制调用spring.factories文件下的EnableCircuitBreakerList<String> factories = new ArrayList<>(new LinkedHashSet<>(SpringFactoriesLoader.loadFactoryNames(this.annotationClass, this.beanClassLoader)));if (factories.isEmpty() && !hasDefaultFactory()) {throw new IllegalStateException("Annotation @" + getSimpleName()+ " found, but there are no implementations. Did you forget to include a starter?");}if (factories.size() > 1) {// there should only ever be one DiscoveryClient, but there might be more than// one factorythis.log.warn("More than one implementation " + "of @" + getSimpleName()+ " (now relying on @Conditionals to pick one): " + factories);}return factories.toArray(new String[factories.size()]);}

selectImports方法的主要功能就是调用SpringFactoriesLoader.loadFactoryNames方法，该方法的目的是通过SPI机制读取spring.factories文件中的org.springframework.cloud.client.circuitbreaker相关配置，返回。

返回的信息是类全限定名，从selectImports方法作用可知，这些类会加载到Spring Ioc容器中。

org.springframework.cloud.client.circuitbreaker在spring-cloud-netflix-hystrix-2.2.10.RELEASE包下：

所以该配置下的org.springframework.cloud.netflix.hystrix.HystrixCircuitBreakerConfiguration会被调用并加载到Spring IoC容器中。

HystrixCircuitBreakerConfiguration

跟踪配置类HystrixCircuitBreakerConfiguration：

/*** @author Spencer Gibb* @author Christian Dupuis* @author Venil Noronha*/
@Configuration(proxyBeanMethods = false)
public class HystrixCircuitBreakerConfiguration {@Beanpublic HystrixCommandAspect hystrixCommandAspect() {return new HystrixCommandAspect();}@Beanpublic HystrixShutdownHook hystrixShutdownHook() {return new HystrixShutdownHook();}

注入了一个叫HystrixCommandAspect 的bean，从名字上看，应该是关于HystrixCommand的切面，用到了AOP。其实也容易理解，加了@HystrixCommand注解的方法的执行逻辑发生了变化：方法增强了执行时长是否超时、执行是否成功（是否返回异常）、超时或异常的情况下调用fallback…方法功能的增强正是AOP的强项。

继续跟踪HystrixCommandAspect 类。

HystrixCommandAspect

打开代码，首先是非常熟悉的@Aspect注解：

@Aspect
public class HystrixCommandAspect {

表明当前类是AOP的切面。

继续看代码：

@Pointcut("@annotation(com.netflix.hystrix.contrib.javanica.annotation.HystrixCommand)")public void hystrixCommandAnnotationPointcut() {}@Pointcut("@annotation(com.netflix.hystrix.contrib.javanica.annotation.HystrixCollapser)")public void hystrixCollapserAnnotationPointcut() {}@Around("hystrixCommandAnnotationPointcut() || hystrixCollapserAnnotationPointcut()")public Object methodsAnnotatedWithHystrixCommand(final ProceedingJoinPoint joinPoint) throws Throwable {

添加了针对注解HystrixCommand的切点，以及针对该切点的环绕增强方法methodsAnnotatedWithHystrixCommand。

最终的熔断、限流、服务降级功能，都是在这个methodsAnnotatedWithHystrixCommand方法里实现的，继续向下研究这个方法的代码逻辑，需要RxJava背景知识做支撑。

有空再聊:) ~!