前言：
1.介绍Hystrix
在一个分布式系统里，许多依赖不可避免的会调用失败，比如超时、异常等，如何能够保证在一个依赖出问题的情况下，不会导致整体服务失败，这个就是Hystrix需要做的事情。Hystrix提供了熔断、隔离、Fallback、cache、监控等功能，能够在一个、或多个依赖同时出现问题时保证系统依然可用。
2.为什么使用Hystrix
在分布式系统架构中多个系统之间通常是通过远程RPC调用进行通信，也就是 A 系统调用 B 系统服务，B 系统调用 C 系统的服务。当尾部应用 C 发生故障而系统 B 没有服务降级时候可能会导致 B，甚至系统 A 瘫痪，这种现象被称为雪崩现象。所以在系统设计时候要使用一定的降级策略，来保证当服务提供方服务不可用时候，服务调用方可以切换到降级后的策略进行执行。
实战：
1.新建服务消费者：
（1）添加依赖
4.0.0 com.xm.cloud cl_hello_consumer_hy 0.0.1-SNAPSHOT jar cl_hello_consumer_hy This is a Web about springcloud org.springframework.boot spring-boot-starter-parent 2.0.6.RELEASE UTF-8 UTF-8 1.8 Finchley.SR2 org.springframework.boot spring-boot-starter-web org.springframework.cloud spring-cloud-starter-netflix-eureka-client org.springframework.cloud spring-cloud-starter-netflix-hystrix org.springframework.cloud spring-cloud-starter-openfeign org.springframework.boot spring-boot-starter-test test org.springframework.boot spring-boot-starter-actuator org.springframework.cloud spring-cloud-dependencies ${spring-cloud.version} pom import org.springframework.boot spring-boot-maven-plugin
（2）修改配置
eureka.client.service-url.defaultZone=http://127.0.0.1:7001/eureka/eureka.client.register-with-eureka=falsefeign.hystrix.enabled=true
（3）开启注解
package com.xm.cloud;import org.springframework.boot.SpringApplication;import org.springframework.boot.autoconfigure.SpringBootApplication;import org.springframework.cloud.client.circuitbreaker.EnableCircuitBreaker;import org.springframework.cloud.client.discovery.EnableDiscoveryClient;import org.springframework.cloud.openfeign.EnableFeignClients;@EnableDiscoveryClient@EnableFeignClients@SpringBootApplicationpublic class ClHelloConsumerHyApplication { public static void main(String[] args) { SpringApplication.run(ClHelloConsumerHyApplication.class, args); }} （4）配置熔断回调工厂
package com.xm.cloud.fallback;import org.springframework.stereotype.Component;import com.xm.cloud.service.HelloService;import feign.hystrix.FallbackFactory;@Componentpublic class HelloServiceFallbackFactory implements FallbackFactory{ @Override public HelloService create(Throwable cause) { return new HelloService() { @Override public String sayHello() { return "HelloService 异常！"; } }; }}
（5）新建Service
package com.xm.cloud.service;import org.springframework.cloud.openfeign.FeignClient;import org.springframework.web.bind.annotation.GetMapping;import com.xm.cloud.fallback.HelloServiceFallbackFactory;@FeignClient(value="CL-HELLO-PRODUCER",fallbackFactory=HelloServiceFallbackFactory.class)public interface HelloService { @GetMapping("/hello") public String sayHello();} （6）新建Controller
package com.xm.cloud.controller;import java.util.ArrayList;import java.util.List;import org.springframework.beans.factory.annotation.Autowired;import org.springframework.web.bind.annotation.GetMapping;import org.springframework.web.bind.annotation.RestController;import com.xm.cloud.service.HelloService;@RestControllerpublic class HelloController { @Autowired private HelloService helloService; @GetMapping("/hello") public List sayHello() { List list = new ArrayList (); for(int i=0;i<10;i++) { list.add(helloService.sayHello()); } return list; }} 2.新建服务提供者：
（1）添加依赖
4.0.0 com.xm.cloud cl_hello_producer_hy 0.0.1-SNAPSHOT jar cl_hello_producer_hy This is a Web about springcloud org.springframework.boot spring-boot-starter-parent 2.0.6.RELEASE UTF-8 UTF-8 1.8 Finchley.SR2 org.springframework.boot spring-boot-starter-web org.springframework.cloud spring-cloud-starter-netflix-eureka-client org.springframework.cloud spring-cloud-starter-netflix-hystrix org.springframework.boot spring-boot-starter-test test org.springframework.cloud spring-cloud-dependencies ${spring-cloud.version} pom import org.springframework.boot spring-boot-maven-plugin
（2）修改配置
server.port=8001spring.application.name=cl-hello-producereureka.client.service-url.defaultZone=http://127.0.0.1:7001/eureka/
（3）开启注解
package com.xm.cloud;import org.springframework.boot.SpringApplication;import org.springframework.boot.autoconfigure.SpringBootApplication;import org.springframework.cloud.client.circuitbreaker.EnableCircuitBreaker;import org.springframework.cloud.client.discovery.EnableDiscoveryClient;@EnableDiscoveryClient@EnableCircuitBreaker@SpringBootApplicationpublic class ClHelloProducerHyApplication { public static void main(String[] args) { SpringApplication.run(ClHelloProducerHyApplication.class, args); }} （4）新建Controller
package com.xm.cloud.controller;import org.springframework.web.bind.annotation.GetMapping;import org.springframework.web.bind.annotation.RestController;import com.netflix.hystrix.contrib.javanica.annotation.HystrixCommand;@RestControllerpublic class HelloController { @GetMapping("/hello") @HystrixCommand public String sayHello() { if(Math.random()>0.5) { throw new RuntimeException(); } else { return "Hello spring cloud!"; } }} 3.测试
运行：localhost:8080/hello
| 0 | "HelloService 异常！" |
|---|---|
| 1 | "Hello spring cloud!" |
| 2 | "HelloService 异常！" |
| 3 | "Hello spring cloud!" |
| 4 | "Hello spring cloud!" |
| 5 | "Hello spring cloud!" |
| 6 | "Hello spring cloud!" |
| 7 | "Hello spring cloud!" |
| 8 | "HelloService 异常！" |
| 9 | "Hello spring cloud!" |