背景
Hystrix 旨在通过控制那些访问远程系统、服务和第三方库的节点，从而对延迟和故障提供更强大的容错能力。Hystrix具备拥有回退机制和断路器功能的线程和信号隔离，请求缓存和请求打包，以及监控和配置等功能。
Dubbo是Alibaba开源的，目前国内最流行的java rpc框架。
本文介绍在spring应用里，怎么把Dubbo和Hystrix结合起来使用。
Spring Boot应用
生成dubbo集成spring boot的应用
配置spring-cloud-starter-netflix-hystrix
spring boot官方提供了对hystrix的集成，直接在pom.xml里加入依赖：
<dependency>
<groupId>org.springframework.cloud</groupId>
<artifactId>spring-cloud-starter-netflix-hystrix</artifactId>
<version>1.4.4.RELEASE</version>
</dependency>
然后在Application类上增加@EnableHystrix来启用hystrix starter：br/>@SpringBootApplication
@EnableHystrix
public class ProviderApplication {br/>配置Provider端
在Dubbo的Provider上增加@HystrixCommand配置，这样子调用就会经过Hystrix代理。
@Service(version = "1.0.0")
public class HelloServiceImpl implements HelloService {
@HystrixCommand(commandProperties = {
@HystrixProperty(name = "circuitBreaker.requestVolumeThreshold", value = "10"),
@HystrixProperty(name = "execution.isolation.thread.timeoutInMilliseconds", value = "2000") })br/>@Override
public String sayHello(String name) {
// System.out.println("async provider received: " + name);
// return "annotation: hello, " + name;
throw new RuntimeException("Exception to show hystrix enabled.");
}
}
配置Consumer端
对于Consumer端，则可以增加一层method调用，并在method上配置@HystrixCommand。当调用出错时，会走到fallbackMethod = "reliable"的调用里。
@Reference(version = "1.0.0")
private HelloService demoService;

@HystrixCommand(fallbackMethod = "reliable")
public String doSayHello(String name) {
return demoService.sayHello(name);
}
public String reliable(String name) {
return "hystrix fallback value";
}
通过上面的配置，很简单地就完成了Spring Boot里Dubbo + Hystrix的集成。
传统Spring Annotation应用
传统spring annotation应用的配置其实也很简单，和spring boot应用不同的是：
显式配置Spring AOP支持：@EnableAspectJAutoProxy
显式通过@Configuration配置HystrixCommandAspect Bean。br/>@Configuration
@EnableDubbo(scanBasePackages = "com.alibaba.dubbo.samples.annotation.action")br/>@PropertySource("classpath:/spring/dubbo-consumer.properties")
@ComponentScan(value = {"com.alibaba.dubbo.samples.annotation.action"})br/>@EnableAspectJAutoProxy
static public class ConsumerConfiguration {

@Bean
public HystrixCommandAspect hystrixCommandAspect() {
return new HystrixCommandAspect();
}
}
Hystrix集成Spring AOP原理
在上面的例子里可以看到，Hystrix对Spring的集成是通过Spring AOP来实现的。下面简单分析下实现。br/>@Aspect
public class HystrixCommandAspect {br/>@Pointcut("@annotation(com.netflix.hystrix.contrib.javanica.annotation.HystrixCommand)")
public void hystrixCommandAnnotationPointcut() {br/>}
@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);
Validate.notNull(method, "failed to get method from joinPoint: %s", joinPoint);
if (method.isAnnotationPresent(HystrixCommand.class) && method.isAnnotationPresent(HystrixCollapser.class)) {
throw new IllegalStateException("method cannot be annotated with HystrixCommand and HystrixCollapser " +
"annotations at the same time");
}
MetaHolderFactory metaHolderFactory = META_HOLDER_FACTORY_MAP.get(HystrixPointcutType.of(method));
MetaHolder metaHolder = metaHolderFactory.create(joinPoint);
HystrixInvokable invokable = HystrixCommandFactory.getInstance().create(metaHolder);
ExecutionType executionType = metaHolder.isCollapserAnnotationPresent() ?
metaHolder.getCollapserExecutionType() : metaHolder.getExecutionType();

Object result;
try {
if (!metaHolder.isObservable()) {
result = CommandExecutor.execute(invokable, executionType, metaHolder);
} 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;
}
HystrixCommandAspect里定义了两个注解的AspectJ Pointcut：@HystrixCommand, @HystrixCollapser。所有带这两个注解的spring bean都会经过AOP处理
在@Around AOP处理函数里，可以看到Hystrix会创建出HystrixInvokable，再通过CommandExecutor来执行
spring-cloud-starter-netflix-hystrix的代码分析
@EnableHystrix 引入了@EnableCircuitBreaker，@EnableCircuitBreaker引入了EnableCircuitBreakerImportSelector1br/>@EnableCircuitBreaker
public @interface EnableHystrix {
}

@Import(EnableCircuitBreakerImportSelector.class)
public @interface EnableCircuitBreaker {br/>}
EnableCircuitBreakerImportSelector继承了SpringFactoryImportSelector<EnableCircuitBreaker>，使spring加载META-INF/spring.factories里的EnableCircuitBreaker声明的配置在META-INF/spring.factories里可以找到下面的配置，也就是引入了HystrixCircuitBreakerConfiguration。1
org.springframework.cloud.client.circuitbreaker.EnableCircuitBreaker=\
org.springframework.cloud.netflix.hystrix.HystrixCircuitBreakerConfiguration
在HystrixCircuitBreakerConfiguration里可以发现创建了HystrixCommandAspect1
@Configuration
public class HystrixCircuitBreakerConfiguration {

@Bean
public HystrixCommandAspect hystrixCommandAspect() {
return new HystrixCommandAspect();
}
可见spring-cloud-starter-netflix-hystrix实际上也是创建了HystrixCommandAspect来集成Hystrix。
另外spring-cloud-starter-netflix-hystrix里还有metrics, health, dashboard等集成。
总结
对于dubbo provider的@Service是一个spring bean，直接在上面配置@HystrixCommand即可
对于dubbo consumer的@Reference，可以通过加一层简单的spring method包装，配置@HystrixCommand即可
Hystrix本身提供HystrixCommandAspect来集成Spring AOP，配置了@HystrixCommand和@HystrixCollapser的spring method都会被Hystrix处理