feign 快速入门与实践

服务注册中心

Feign可以配合eureka等服务注册中心同时使用。eureka来作为服务注册中心，为Feign提供关于服务端信息的获取，比如说IP地址。关于eureka的具体使用可以参考关于eureka的快速入门介绍。

服务提供者

Spring Cloud Feign是声明式RESTful请求客户端，所以它不会侵入服务提供者程序的实现。也就是说，服务提供者只需要提供Web Service的API接口，至于具体实现既可以是Spring Controler也可以是Jersey。我们只需要确保该服务提供者被注册到服务注册中心上。

@RestController
@RequestMapping("/feign-service")
public class FeignServiceController {

    private static final Logger logger = LoggerFactory.getLogger(FeignServiceController.class);

    private static String DEFAULT_SERVICE_ID = "application";
    private static String DEFAULT_HOST = "localhost";
    private static int DEFAULT_PORT = 8080;

    @RequestMapping(value = "/instance/{serviceId}", method = RequestMethod.GET)
    public Instance getInstanceByServiceId(@PathVariable("serviceId") String serviceId){
        logger.info("Get Instance by serviceId {}", serviceId);
        return new Instance(serviceId, DEFAULT_HOST, DEFAULT_PORT);
    }

    @RequestMapping(value = "/instance/{serviceId}", method = RequestMethod.DELETE)
    public String deleteInstanceByServiceId(@PathVariable("serviceId") String serviceId){

        logger.info("Delete Instance by serviceId {}", serviceId);
        return "Instance whose serviceId is " + serviceId + " is deleted";

    }

    @RequestMapping(value = "/instance", method = RequestMethod.POST)
    public String createInstance(@RequestBody Instance instance){

        logger.info("Create Instance whose serviceId is {}", instance.getServiceId());
        return "Instance whose serviceId is" + instance.getServiceId() + " is created";
    }

    @RequestMapping(value = "/instance/{serviceId}", method = RequestMethod.PUT)
    public String updateInstanceByServiceId(@RequestBody Instance instance, @PathVariable("serviceId") String serviceId){
        logger.info("Update Instance whose serviceId is {}", serviceId);
        return "Instance whose serviceId is " + serviceId + " is updated";
    }
}

上述代码中通过@RestController和@RequestMapping声明了四个网络API接口，分别是对Instance资源的增删改查操作。

除了实现网络API接口之外，还需要将该service注册到eureka上。如下列代码所示，需要在application.yml文件中设置服务注册中心的相关信息和代表该应用的名称。

eureka:
  instance:
    instance-id: ${spring.application.name}:${vcap.application.instance_id:${spring.application.instance_id:${random.value}}}
  client:
    service-url:
      default-zone: http://localhost:8761/eureka/
spring:
  application:
    name: feign-service
server:
  port: 0

服务消费者

Feign是声明式RESTful客户端，所以构建Feign项目的关键在于构建服务消费者。通过下面六步可以创建一个Spring Cloud Feign的服务消费者。

第一步: 创建普通的Spring Boot工程

首先创建一个普通的Spring Boot工程，取名为chapter-feign-client。

第二步：添加依赖

需要在pom文件中添加eureka和feign相关的依赖。其中spring-cloud-starter-eureka是eureka的starter依赖包，spring-cloud-starter-feign是feign的starter依赖包。

<dependencies>
    <dependency>
        <groupId>org.springframework.cloud</groupId>
        <artifactId>spring-cloud-starter-eureka</artifactId>
    </dependency>
    <dependency>
        <groupId>org.springframework.cloud</groupId>
        <artifactId>spring-cloud-starter-feign</artifactId>
    </dependency>
</dependencies>

添加注解

然后在工程的入口类上添加@EnableFeignClients注解表示开启Spring Cloud Feign的支持功能，代码如下所示。

@SpringBootApplication
@EnableFeignClients()
public class ChapterFeignClientApplication {
	public static void main(String[] args) {
		SpringApplication.run(ChapterFeignClientApplication.class, args);
	}
}

@EnableFeignClients就像是一个开关，如果你使用了该注解，那么Feign相关的组件和处理机制才会生效，否则不会生效。@EnableFeignClients还可以对Feign相关组件进行自定义配置，它的方法和原理会在本章的源码分析章节在做具体的讲解。

第四步：声明服务

接下来我们定义一个FeignServiceClient接口，通过@FeignClient注解来指定服务名进而绑定服务。这一类被@FeignClient修饰的接口类一般被称为FeignClient。我们可以通过@RequestMapping来修饰相应的方法来定义调用函数。

@FeignClient("feign-service")
@RequestMapping("/feign-service")
public interface FeignServiceClient {

    @RequestMapping(value = "/instance/{serviceId}", method = RequestMethod.GET)
    public Instance getInstanceByServiceId(@PathVariable("serviceId") String serviceId);

    @RequestMapping(value = "/instance/{serviceId}", method = RequestMethod.DELETE)
    public String deleteInstanceByServiceId(@PathVariable("serviceId") String serviceId);

    @RequestMapping(value = "/instance", method = RequestMethod.POST)
    public String createInstance(@RequestBody Instance instance);

    @RequestMapping(value = "/instance/{serviceId}", method = RequestMethod.PUT)
    public String updateInstanceByServiceId(@RequestBody Instance instance, @PathVariable("serviceId") String serviceId);
}

如上面代码片段所显示的，如果你调用FeignServiceClient对象的getInstanceByServiceId函数，那么Feign就会向feign-service服务的/feign-service/instance/{serviceId}接口发送网络请求。

第五步：Controller中调用服务

创建一个Controller来调用上边的服务，通过@Autowired来自动装载FeignServiceClient示例。代码如下：

@RestController
@RequestMapping("/feign-client")
public class FeignClientController {

    @Autowired
    FeignServiceClient feignServiceClient;

    @RequestMapping(value = "/instance/{serviceId}", method = RequestMethod.GET)
    public Instance getInstanceByServiceId(@PathVariable("serviceId") String serviceId){
        return feignServiceClient.getInstanceByServiceId(serviceId);
    }

    @RequestMapping(value = "/instance/{serviceId}", method = RequestMethod.DELETE)
    public String deleteInstanceByServiceId(@PathVariable("serviceId") String serviceId){
        return feignServiceClient.deleteInstanceByServiceId(serviceId);
    }

    @RequestMapping(value = "/instance", method = RequestMethod.POST)
    public String createInstance(@RequestBody Instance instance){
        return feignServiceClient.createInstance(instance);
    }

    @RequestMapping(value = "/instance/{serviceId}", method = RequestMethod.PUT)
    public String updateInstanceByServiceId(@RequestBody Instance instance, @PathVariable("serviceId") String serviceId){
        return feignServiceClient.updateInstanceByServiceId(instance, serviceId);
    }
}

第六步:属性配置

最后，application.yml中需要配置eureka服务注册中心的相关配置，具体配置如下所示：

eureka:
  instance:
    instance-id: ${spring.application.name}:${vcap.application.instance_id:${spring.application.instance_id:${random.value}}}
  client:
    service-url:
      default-zone: http://localhost:8761/eureka/

spring:
  application:
    name: feign-client
server:
  port: 8770

如上的代码，实现了 feign 进行远程调用的过程。相信通过搭建Feign的项目，已经对Feign的相关使用原理有了一定的了解，相信这个过程将对于理解Feign相关的工作原理大有裨益。