Spring cloud Hystrix注解初始化源码过程解读
作者：福
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<EnableCircuitBreaker>，注意SpringFactoryImportSelector类的泛型类型为EnableCircuitBreaker：
@Order(Ordered.LOWEST_PRECEDENCE - 100)
public class EnableCircuitBreakerImportSelector extends SpringFactoryImportSelector<EnableCircuitBreaker> {
@Override
protected 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：
@Override
public 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EnableCircuitBreaker
List<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 factory
this.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 {
@Bean
public HystrixCommandAspect hystrixCommandAspect() {
return new HystrixCommandAspect();
}
@Bean
public 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。
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