Greenwich.SR2版本的Spring Cloud Feign实例

　　前面我们了解了Spring Cloud Ribbon和Hystrix，在使用上它们基本上会成队出现，那么是不是可以把它们组合起来使用？而且我们发现，在服务消费方a-beautiful-client里通过REST调用服务提供方时，会有很多RestTemplate的代码，这些重复代码能否简化掉呢？答案是肯定的，Spring Cloud为我们提供了Feign，就是整合了Ribbon和Hystrix，并且提供了一种声明式的Web服务客户端定义方式，让我们只需要定义好REST接口即可，服务消费方无需再写实现了。

　　我们这次新增一个a-feign-client吧，把它跟a-beautiful-client（参见Greenwich.SR2版本的Spring Cloud Eureka实例）来做一番比较。三板斧祭出：

　　1、pom里我们去掉了ribbon和hystrix，只需引入openfeign即可：

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <groupId>com.example</groupId>
    <artifactId>feign</artifactId>
    <version>0.0.1-SNAPSHOT</version>
    <packaging>jar</packaging>

    <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>2.1.6.RELEASE</version>
        <relativePath/> <!-- lookup parent from repository -->
    </parent>

    <properties>
        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
        <java.version>1.8</java.version>
    </properties>

    <dependencies>
        <dependency>
            <groupId>org.springframework.cloud</groupId>
            <artifactId>spring-cloud-starter-netflix-eureka-client</artifactId>
        </dependency>
        <dependency>
            <groupId>org.springframework.cloud</groupId>
            <artifactId>spring-cloud-starter-openfeign</artifactId>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-test</artifactId>
            <scope>test</scope>
        </dependency>
        <dependency>
            <groupId>org.springframework.cloud</groupId>
            <artifactId>spring-cloud-starter-netflix-eureka-server</artifactId>
            <scope>test</scope>
        </dependency>

    </dependencies>

    <dependencyManagement>
        <dependencies>
            <dependency>
                <groupId>org.springframework.cloud</groupId>
                <artifactId>spring-cloud-dependencies</artifactId>
                <version>Greenwich.SR2</version>
                <type>pom</type>
                <scope>import</scope>
            </dependency>
        </dependencies>
    </dependencyManagement>

    <build>
        <plugins>
            <plugin>
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-maven-plugin</artifactId>
            </plugin>
        </plugins>
    </build>

</project>

　　2、application：

#本机端口
server.port=8764

#本机服务名
spring.application.name=a-feign-client

#服务提供方实例地址
app.service.url=http://A-BOOTIFUL-CLIENT/

#注册中心地址
eureka.client.service-url.defaultZone=http://localhost:8888/eureka/

#开启熔断
feign.hystrix.enabled=true

#负载均衡配置
a-bootiful-client.ribbon.NFLoadBalancerRuleClassName=com.netflix.loadbalancer.RandomRule

　　主类我们用@EnableFeignClients代替了@EnableCircuitBreaker和@LoadBalance：

package hello;

import hello.service.ConsumerService;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.cloud.client.ServiceInstance;
import org.springframework.cloud.client.discovery.DiscoveryClient;
import org.springframework.cloud.client.discovery.EnableDiscoveryClient;
import org.springframework.cloud.openfeign.EnableFeignClients;
import org.springframework.web.bind.annotation.*;

import java.util.List;

@EnableFeignClients
@EnableDiscoveryClient
@SpringBootApplication
public class FeignApplication {

    public static void main(String[] args) {
        SpringApplication.run(FeignApplication.class, args);
    }
}

@RestController
class ServiceInstanceRestController {

    @Autowired
    private ConsumerService consumerService;

    @Autowired
    private DiscoveryClient discoveryClient;

    @RequestMapping("/consumer/sayHi")
    public String sayHi(@RequestParam(value = "name") String name, @RequestParam(value = "accessToken", required = false) String accessToken) {
        return name + " say hi: " + consumerService.hello(name);
    }

    @RequestMapping("/service-instances/{applicationName}")
    public List<ServiceInstance> serviceInstancesByApplicationName(
            @PathVariable String applicationName) {
        return this.discoveryClient.getInstances(applicationName);
    }
}

　　3、我们改写一下接口，再新增一个熔断的服务降级类：

package hello.service;

import hello.service.impl.BackUPCallHi;
import org.springframework.cloud.openfeign.FeignClient;
import org.springframework.stereotype.Component;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RequestParam;

@Component
@FeignClient(name = "a-bootiful-client", fallback = BackUPCallHi.class)
public interface ConsumerService {
    @RequestMapping("/hello")
    String hello(@RequestParam(value = "name") String name);
}

package hello.service.impl;

import hello.service.ConsumerService;
import org.springframework.stereotype.Component;

@Component
public class BackUPCallHi implements ConsumerService {

    @Override
    public String hello(String name) {
        return "I'm feign hystrix.";
    }
}

　　搞定，可以看到原来a-beautiful-client能干的事情，a-feign-client也做到了：

　　负载均衡：

　　熔断：

　　上面我们看到url后面多了一个accessToken的参数，这个其实是给网关鉴权用的，详见Greenwich.SR2版本的Spring Cloud Zuul实例。