Hystrix 旨在通过节制那些会见长途系统、处事和第三方库的节点，劳务派遣管理系统，从而对延迟和妨碍提供更强大的容错本领。Hystrix具备拥有回退机制和断路器成果的线程和信号断绝，请求缓存和请求打包，昆山软件开发，以及监控和设置等成果。

Dubbo是Alibaba开源的，今朝海内最风行的java rpc框架。

本文先容在spring应用里，怎么把Dubbo和Hystrix团结起来利用。

Spring Boot应用

Demo地点

生成dubbo集成spring boot的应用

对付不熟悉dubbo 集成spring boot应用的同学，可以在这里直接生成dubbo + spring boot的工程： http://start.dubbo.io/

hystrix">设置spring-cloud-starter-netflix-hystrix

spring boot官方提供了对hystrix的集成，直接在pom.xml里插手依赖：

        <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：

@SpringBootApplication
@EnableHystrix
public class ProviderApplication {

设置Provider端

在Dubbo的Provider上增加@HystrixCommand设置，这样子挪用就会颠末Hystrix署理。

@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") })
    @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.");
    }
}

设置Consumer端

对付Consumer端，则可以增加一层method挪用，并在method上设置@HystrixCommand。当挪用堕落时，会走到fallbackMethod = "reliable"的挪用里。

    @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";
    }

通过上面的设置，很简朴地就完成了Spring Boot里Dubbo + Hystrix的集成。

传统Spring Annotation应用

Demo地点

传统spring annotation应用的设置其实也很简朴，和spring boot应用差异的是：

1. 显式设置Spring AOP支持：@EnableAspectJAutoProxy
2. 显式通过@Configuration设置HystrixCommandAspect Bean。

    @Configuration
    @EnableDubbo(scanBasePackages = "com.alibaba.dubbo.samples.annotation.action")
    @PropertySource("classpath:/spring/dubbo-consumer.properties")
    @ComponentScan(value = {"com.alibaba.dubbo.samples.annotation.action"})
    @EnableAspectJAutoProxy
    static public class ConsumerConfiguration {

        @Bean
        public HystrixCommandAspect hystrixCommandAspect() {
            return new HystrixCommandAspect();
        }
    }

Hystrix集成Spring AOP道理

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。下面简朴阐明下实现。

@Aspect
public class HystrixCommandAspect {
    @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 {
        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;
    }

1. HystrixCommandAspect里界说了两个注解的AspectJ Pointcut：@HystrixCommand, @HystrixCollapser。所有带这两个注解的spring bean城市颠末AOP处理惩罚
2. 在@Around AOP处理惩罚函数里，可以看到Hystrix会建设出HystrixInvokable，再通过CommandExecutor来执行

spring-cloud-starter-netflix-hystrix的代码阐明

1. @EnableHystrix 引入了@EnableCircuitBreaker，@EnableCircuitBreaker引入了EnableCircuitBreakerImportSelector

   @EnableCircuitBreaker
   public @interface EnableHystrix {
   }
   
   @Import(EnableCircuitBreakerImportSelector.class)
   public @interface EnableCircuitBreaker {
   }

2. EnableCircuitBreakerImportSelector担任了SpringFactoryImportSelector<EnableCircuitBreaker>，使spring加载META-INF/spring.factories里的EnableCircuitBreaker声明的设置在META-INF/spring.factories里可以找到下面的设置，也就是引入了HystrixCircuitBreakerConfiguration。

   org.springframework.cloud.client.circuitbreaker.EnableCircuitBreaker=\
   org.springframework.cloud.netflix.hystrix.HystrixCircuitBreakerConfiguration

3. 在HystrixCircuitBreakerConfiguration里可以发明建设了HystrixCommandAspect

   @Configuration
   public class HystrixCircuitBreakerConfiguration {
   
       @Bean
       public HystrixCommandAspect hystrixCommandAspect() {
           return new HystrixCommandAspect();
       }

可见spring-cloud-starter-netflix-hystrix实际上也是建设了HystrixCommandAspect来集成Hystrix。