# spring cloud feign

introduction

netflix feign是一個類似retrofit進行http呼叫框架，Feign makes writing java http clients easier 使得編寫http client程式碼更加簡單

netflix feign

直接給出一段簡單的案例

package com.lkl.netflix.feign;

import feign.*;
import feign.codec.ErrorDecoder;
import feign.codec.Decoder;
import feign.gson.GsonDecoder;

import java.io.IOException;
import java.util.List
 
;

/**
 * Created by liaokailin on 16/5/9.
 */
public class GitHubExample {

    interface GitHub {  // 1
        @RequestLine("GET /repos/{owner}/{repo}/contributors")
        List<Contributor> contributors(@Param("owner") String owner, @Param("repo") String repo);
    }


    public static void main(String... args) {
        Decoder decoder = new
 
 GsonDecoder();  // 2
        GitHub github = Feign.builder()
                .decoder(decoder) // 3
                .errorDecoder(new GitHubErrorDecoder(decoder))  // 4
                .logger(new Logger.ErrorLogger())  // 5
                .logLevel(Logger.Level.BASIC)  // 5
                .target(GitHub.class, "https://api.github.com"
 
);  // 6

        System.out.println("Let's fetch and print a list of the contributors to this library.");
        List<Contributor> contributors = github.contributors("netflix", "feign");  // 7
        for (Contributor contributor : contributors) {
            System.out.println(contributor.login + " (" + contributor.contributions + ")");
        }

        System.out.println("Now, let's cause an error.");
        try {
            github.contributors("netflix", "some-unknown-project");  // 8
        } catch (GitHubClientError e) {
            System.out.println(e.getMessage());
        }
    }

    static class Contributor {
        String login;
        int contributions;
    }

    static class GitHubClientError extends RuntimeException {  // 4
        private String message; // parsed from json

        @Override
        public String getMessage() {
            return message;
        }
    }

    static class GitHubErrorDecoder implements ErrorDecoder {   // 4

        final Decoder decoder;
        final ErrorDecoder defaultDecoder = new ErrorDecoder.Default();

        GitHubErrorDecoder(Decoder decoder) {
            this.decoder = decoder;
        }

        @Override
        public Exception decode(String methodKey, Response response) {
            try {
                return (Exception) decoder.decode(response, GitHubClientError.class);
            } catch (IOException fallbackToDefault) {
                return defaultDecoder.decode(methodKey, response);
            }
        }
    }


}

1.首先定義介面GitHub,使用註解RequestLine 表明contributors()方法為一個get請求，請求相對為/repos/{owner}/{repo}/contributors
2.Decoder decoder = new GsonDecoder(); 建立一個GsonDecoder解碼器，表明通過Gson解析返回資料
3.decoder()方法設定解碼器
4.errorDecoder()指定發生異常時的解碼器，需要實現ErrorDecoder介面，覆寫decode方法，通過指定的Decoder解析錯誤資訊，這裡還是使用GsonDecoder
5.logger 相關的表示配置日誌系你係
6.target() 方法指定訪問url以及返回的型別
7.通過建立的github物件呼叫contributors獲取結果
8.模擬異常情況

note: feign使用起來很簡單，其原理和retrofit及其類似，通過介面定義訪問訪問，用jdk的動態代理建立介面的實現類，在類中解析方法上的註解資訊用以識別使用者配置的http請求資訊，然後執行請求；
feign可以通過ReflectiveFeign下的newInstance()方法看到

return (T) Proxy
        .newProxyInstance(target.type().getClassLoader(), new Class<?>[]{target.type()}, handler);

spring cloud feign

spring cloud feign通過註解的封裝使用起來更加簡單

package com.lkl.springcloud.feign;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.EnableAutoConfiguration;
import org.springframework.cloud.netflix.feign.EnableFeignClients;
import org.springframework.cloud.netflix.feign.FeignClient;
import org.springframework.context.annotation.Configuration;
import org.springframework.http.ResponseEntity;
import org.springframework.web.bind.annotation.PathVariable;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RequestMethod;
import org.springframework.web.bind.annotation.RestController;

import java.util.List;

/**
 * Created by liaokailin on 16/5/11.
 */
@Configuration
@EnableAutoConfiguration
@EnableFeignClients
@RestController
public class Application {

    @Autowired
    GitHub gitHub;
    public static void main(String[] args) {
        SpringApplication.run(Application.class, args);
    }

    @FeignClient(url = "https://api.github.com")
    interface GitHub {
        @RequestMapping(value = "/repos/{owner}/{repo}/contributors", method = RequestMethod.GET)
      ResponseEntity<List<Contributor>> contributors(@PathVariable("owner") String owner, @PathVariable("repo") String repo);
    }

    @RequestMapping("/")
    public void test(){
        ResponseEntity< List<Contributor>>  responseEntity  =   gitHub.contributors("netflix", "feign");
        List<Contributor> contributors = responseEntity.getBody();
         for (Contributor contributor : contributors) {
         System.out.println(contributor.login + " (" + contributor.contributions + ")");
         }
    }


    static class Contributor {
        String login;
        int contributions;
    }
}

註解EnableFeignClients表明需要掃描使用FeignClient註解的介面，在程式碼中定義了@FeignClient(url = "https://api.github.com") 表明該介面為一個feign介面
通過url指定訪問路徑RequestMapping表明相對路徑

在程式碼中注入beanGitHub，直接呼叫其中申明的方法即可。

重點說明下FeignClient註解

該註解表示申明建立一個rest client bean,可以直接通過Autowired注入使用，如果ribbon在工程中啟用，則會使用load balance進行後端請求呼叫，可以為FeignClient指定value表明需要訪問的serviceId

feign + ribbon + eureka

package com.lkl.springcloud.feign;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.cloud.client.discovery.EnableDiscoveryClient;
import org.springframework.cloud.netflix.feign.EnableFeignClients;
import org.springframework.cloud.netflix.feign.FeignClient;
import org.springframework.cloud.netflix.ribbon.RibbonClient;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;

import static org.springframework.web.bind.annotation.RequestMethod.GET;

/**
 * Created by liaokailin on 16/5/9.
 */
@SpringBootApplication
@EnableDiscoveryClient
@RestController
@RibbonClient("hello")
@EnableFeignClients
public class Application {

    @Autowired
    HelloClient client;

    @RequestMapping("/")
    public String hello() {
        return client.hello();
    }

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

    @FeignClient("simple")
    interface HelloClient {
        @RequestMapping(value = "/", method = GET)
        String hello();
    }

}

使用@FeignClient("simple")指定要訪問的service id 由於hello()對應的mapping為@RequestMapping(value = "/", method = GET) 那麼該方法實際呼叫url為
http://simple/，因此在執行

@RequestMapping("/")
    public String hello() {
        return client.hello();
    }

呼叫的為SimpleApplication.java中

@RequestMapping("/")
    public String hello() {
        return "Hello";
    }

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