其實只是Hystrix初始化部分，我們從源碼的角度分析一下@EnableCircuitBreaker以及@HystrixCommand注解的初始化過程。

從@EnableCircuitBreaker入手

我們是通過在啟動類添加@EnableCircuitBreaker注解啟用Hystrix的，所以，源碼解析也要從這個注解入手。

該注解已經被標了@Deprecated了，不過，擋不住…

Spring關于@Enablexxxx注解的套路：通過@Import注解引入了EnableCircuitBreakerImportSelector.class：

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

跟蹤EnableCircuitBreakerImportSelector，繼承自SpringFactoryImportSelector，注意SpringFactoryImportSelector類的泛型類型為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);}}

看一下類圖：

屬性annotationClass通過GenericTypeResolver.resolveTypeArgument方法獲取當前對象的泛型參數的具體類型，現在我們知道是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注解我們前面做過詳細分析了，實現了DeferredImportSelector接口表示延遲加載全限定名稱為selectImports方法返回的類。看selectImports方法：

@Overridepublic String[] selectImports(AnnotationMetadata metadata) {//Enable參數沒有打開的話就不加載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機制調用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調用SpringFactoriesLoader.loadFactoryNames方法，該方法的目的是通過SPI機制讀取spring.factories文件中的org.springframework.cloud.client.circuitbreaker相關配置，返回。

返回的信息是類全限定名，從selectImports方法作用可知，這些類會加載到Spring Ioc容器中。

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

所以該配置下的org.springframework.cloud.netflix.hystrix.HystrixCircuitBreakerConfiguration會被調用并加載到Spring IoC容器中。

HystrixCircuitBreakerConfiguration

跟蹤配置類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();}

注入了一個叫HystrixCommandAspect 的bean，從名字上看，應該是關于HystrixCommand的切面，用到了AOP。其實也容易理解，加了@HystrixCommand注解的方法的執行邏輯發生了變化：方法增強了執行時長是否超時、執行是否成功（是否返回異常）、超時或異常的情況下調用fallback…方法功能的增強正是AOP的強項。

繼續跟蹤HystrixCommandAspect 類。

HystrixCommandAspect

打開代碼，首先是非常熟悉的@Aspect注解：

@Aspect
public class HystrixCommandAspect {

表明當前類是AOP的切面。

繼續看代碼：

@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 {

添加了針對注解HystrixCommand的切點，以及針對該切點的環繞增強方法methodsAnnotatedWithHystrixCommand。

最終的熔斷、限流、服務降級功能，都是在這個methodsAnnotatedWithHystrixCommand方法里實現的，繼續向下研究這個方法的代碼邏輯，需要RxJava背景知識做支撐。

有空再聊:) ~!