SpringCloud入门实战之三：服务消费者（Feign）
上一篇文章，讲述了如何通过RestTemplate+Ribbon去消费服务，这篇文章主要讲述如何通过Feign去消费服务。
Feign是一个声明式的伪Http客户端，它使得写Http客户端变得更简单。使用Feign，只需要创建一个接口并注解。它具有可插拔的注解特 性，可使用Feign 注解和JAX-RS注解。Feign支持可插拔的编码器和解码器。Feign默认集成了Ribbon，并和Eureka结合，默认实现了负载均衡的效果。
简而言之：
- Feign 采用的是基于接口的注解
- Feign 整合了ribbon，具有负载均衡的能力
- 整合了Hystrix，具有熔断的能力
继续用上一节的工程， 必须先启动eureka-server 端口为8761; 之后启动service-hi 两次，端口分别为8762 、8773.
创建一个feign的服务
使用STS开发工具创建一个名称为service-feign的工程，这是第4个独立的工程
创建服务消费者服务工程service-feign自动生成的pom.xml,这一篇blog范例中勾选了Web选项，
自动会加入spring-boot-starter-web依赖，所以就不需要手动加入这web依赖，pom.xml文件的内容如下：
4.0.0
com.contoso
service-feign
0.0.1-SNAPSHOT
jar
service-feign
Demo project for Spring Boot
org.springframework.boot
spring-boot-starter-parent
2.0.3.RELEASE
UTF-8
UTF-8
1.8
Finchley.RELEASE
org.springframework.boot
spring-boot-starter-web
org.springframework.cloud
spring-cloud-starter-netflix-eureka-client
org.springframework.cloud
spring-cloud-starter-openfeign
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
在工程的配置文件application.yml文件，指定程序名为service-feign，端口号为8765，服务注册地址为http://localhost:8761/eureka/ ，代码如下：
eureka:
client:
serviceUrl:
defaultZone: http://localhost:8761/eureka/
server:
port: 8765
spring:
application:
name: service-feign在程序的启动类ServiceFeignApplication ，加上@EnableFeignClients注解开启Feign的功能：
package com.contoso;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.cloud.client.discovery.EnableDiscoveryClient;
import org.springframework.cloud.netflix.eureka.EnableEurekaClient;
import org.springframework.cloud.openfeign.EnableFeignClients;
@SpringBootApplication
@EnableEurekaClient
@EnableDiscoveryClient
@EnableFeignClients
public class ServiceFeignApplication {
public static void main(String[] args) {
SpringApplication.run( ServiceFeignApplication.class, args );
}
}定义一个feign接口，通过@FeignClient（“服务名”），来指定调用哪个服务。比如在代码中调用了service-hi服务的“/hi”接口，代码如下：
package com.contoso;
import org.springframework.cloud.openfeign.FeignClient;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RequestMethod;
import org.springframework.web.bind.annotation.RequestParam;
@FeignClient(value = "service-hi")
public interface SchedualServiceHi {
@RequestMapping(value = "/hi",method = RequestMethod.GET)
String sayHiFromClientOne(@RequestParam(value = "name") String name);
}定义一个HiController控制器，对外暴露一个”/hi”的API接口，通过上面定义的Feign客户端SchedualServiceHi 来消费服务。代码如下：
package com.contoso;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RequestParam;
import org.springframework.web.bind.annotation.RestController;
@RestController
public class HiController {
//编译器报错，无视。 因为这个Bean是在程序启动的时候注入的，编译器感知不到，所以报错。
@Autowired
SchedualServiceHi schedualServiceHi;
@GetMapping(value = "/hi")
public String sayHi(@RequestParam String name) {
return schedualServiceHi.sayHiFromClientOne( name );
}
}编译报错如下：
org.springframework.beans.factory.BeanCreationNotAllowedException: Error creating bean with name 'eurekaAutoServiceRegistration': Singleton bean creation not allowed while singletons of this factory are in destruction (Do not request a bean from a BeanFactory in a destroy method implementation!)
at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:208) [spring-beans-5.0.7.RELEASE.jar:5.0.7.RELEASE]
at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:315) ~[spring-beans-5.0.7.RELEASE.jar:5.0.7.RELEASE]
at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:199) ~[spring-beans-5.0.7.RELEASE.jar:5.0.7.RELEASE]
at org.springframework.context.support.AbstractApplicationContext.getBean(AbstractApplicationContext.java:1089) [spring-context-5.0.7.RELEASE.jar:5.0.7.RELEASE]
at org.springframework.context.event.ApplicationListenerMethodAdapter.getTargetBean(ApplicationListenerMethodAdapter.java:288) ~[spring-context-5.0.7.RELEASE.jar:5.0.7.RELEASE]
at org.springframework.context.event.ApplicationListenerMethodAdapter.doInvoke(ApplicationListenerMethodAdapter.java:258) ~[spring-context-5.0.7.RELEASE.jar:5.0.7.RELEASE]
at org.springframework.context.event.ApplicationListenerMethodAdapter.processEvent(ApplicationListenerMethodAdapter.java:180) ~[spring-context-5.0.7.RELEASE.jar:5.0.7.RELEASE]
at org.springframework.context.event.ApplicationListenerMethodAdapter.onApplicationEvent(ApplicationListenerMethodAdapter.java:142) ~[spring-context-5.0.7.RELEASE.jar:5.0.7.RELEASE]
at org.springframework.context.event.SimpleApplicationEventMulticaster.doInvokeListener(SimpleApplicationEventMulticaster.java:172) ~[spring-context-5.0.7.RELEASE.jar:5.0.7.RELEASE]
at org.springframework.context.event.SimpleApplicationEventMulticaster.invokeListener(SimpleApplicationEventMulticaster.java:165) ~[spring-context-5.0.7.RELEASE.jar:5.0.7.RELEASE]
at org.springframework.context.event.SimpleApplicationEventMulticaster.multicastEvent(SimpleApplicationEventMulticaster.java:139) ~[spring-context-5.0.7.RELEASE.jar:5.0.7.RELEASE]
at org.springframework.context.support.AbstractApplicationContext.publishEvent(AbstractApplicationContext.java:400) [spring-context-5.0.7.RELEASE.jar:5.0.7.RELEASE]
at org.springframework.context.support.AbstractApplicationContext.publishEvent(AbstractApplicationContext.java:406) [spring-context-5.0.7.RELEASE.jar:5.0.7.RELEASE]
at org.springframework.context.support.AbstractApplicationContext.publishEvent(AbstractApplicationContext.java:354) [spring-context-5.0.7.RELEASE.jar:5.0.7.RELEASE]
at org.springframework.context.support.AbstractApplicationContext.doClose(AbstractApplicationContext.java:1000) [spring-context-5.0.7.RELEASE.jar:5.0.7.RELEASE]
at org.springframework.context.support.AbstractApplicationContext.close(AbstractApplicationContext.java:967) [spring-context-5.0.7.RELEASE.jar:5.0.7.RELEASE]
at org.springframework.cloud.context.named.NamedContextFactory.destroy(NamedContextFactory.java:76) [spring-cloud-context-2.0.0.RELEASE.jar:2.0.0.RELEASE]
at org.springframework.beans.factory.support.DisposableBeanAdapter.destroy(DisposableBeanAdapter.java:256) [spring-beans-5.0.7.RELEASE.jar:5.0.7.RELEASE]
at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.destroyBean(DefaultSingletonBeanRegistry.java:571) [spring-beans-5.0.7.RELEASE.jar:5.0.7.RELEASE]
at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.destroySingleton(DefaultSingletonBeanRegistry.java:543) [spring-beans-5.0.7.RELEASE.jar:5.0.7.RELEASE]
at org.springframework.beans.factory.support.DefaultListableBeanFactory.destroySingleton(DefaultListableBeanFactory.java:957) [spring-beans-5.0.7.RELEASE.jar:5.0.7.RELEASE]
at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.destroySingletons(DefaultSingletonBeanRegistry.java:504) [spring-beans-5.0.7.RELEASE.jar:5.0.7.RELEASE]
at org.springframework.beans.factory.support.DefaultListableBeanFactory.destroySingletons(DefaultListableBeanFactory.java:964) [spring-beans-5.0.7.RELEASE.jar:5.0.7.RELEASE]
at org.springframework.context.support.AbstractApplicationContext.destroyBeans(AbstractApplicationContext.java:1041) [spring-context-5.0.7.RELEASE.jar:5.0.7.RELEASE]
at org.springframework.context.support.AbstractApplicationContext.doClose(AbstractApplicationContext.java:1017) [spring-context-5.0.7.RELEASE.jar:5.0.7.RELEASE]
at org.springframework.context.support.AbstractApplicationContext$1.run(AbstractApplicationContext.java:937) [spring-context-5.0.7.RELEASE.jar:5.0.7.RELEASE]
[INFO] 启动工程顺序必须为eureka-server,service-hi(两次端口分别为8762和8763)，访问服务消费者（Feign）对外对外提供的接口地址 http://localhost:8765/hi?name=Tom 浏览器交替显示：
Hello Tom ,this message is from port:8763
Hello Tom ,this message is from port:8762
Hello Tom ,this message is from port:8763
Hello Tom ,this message is from port:8762
本篇blog源码spring-cloud-example3.zip