Spring Cloud Feign 之Fallback

转自：https://www.jianshu.com/p/d84768f7996d

环境信息: java 1.8、Spring boot 1.5.10.RELEASE、spring cloud-Edgware.SR3、maven 3.3+

在网络请求时，可能会出现异常请求，如果还想再异常情况下使系统可用，那么就需要容错处理，比如:网络请求超时时给用户提示“稍后重试”或使用本地快照数据等等。

Spring Cloud Feign就是通过Fallback实现的，有两种方式：

1、@FeignClient.fallback = UserFeignFallback.class指定一个实现Feign接口的实现类。

2、@FeignClient.fallbackFactory = UserFeignFactory.class指定一个实现FallbackFactory<T>工厂接口类

因为Fallback是通过Hystrix实现的， 所以需要开启Hystrix，spring boot application.properties文件配置feign.hystrix.enabled=true，这样就开启了Fallback

Fallback-实现Feign接口

UserFeignFallback回调实现，由spring创建使用@Component(其他的注册也可以)注解

HystrixTargeter.targetWithFallback方法实现了@FeignClient.fallback处理逻辑，通过源码可以知道UserFeignFallback回调类是从Spring容器中获取的，所以UserFeignFallback由spring创建。

UserFeign配置:

package com.example.feign;

import org.springframework.cloud.netflix.feign.FeignClient;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.PathVariable;
import org.springframework.web.bind.annotation.PostMapping;

import java.util.List;

@FeignClient(name = "user",url = "${user.url}",fallback = UserFeignFallback.class
        /*fallbackFactory = UserFeignFactory.class*/)
public interface UserFeign {

    @PostMapping
    void save(User user);

    @GetMapping("/{id}")
    User getUserByID(@PathVariable("id") String id);

    @GetMapping
    List<User> findAll();
}

UserFeignFallback类：

package com.example.feign;

import org.springframework.stereotype.Component;

import java.util.List;

@Component
public class UserFeignFallback implements UserFeign {

    @Override
    public void save(User user) {

    }

    @Override
    public User getUserByID(String id) {
        User user = new User();
        user.setId("100");
        user.setName("fallback 回调用户");
        return user;
    }

    @Override
    public List<User> findAll() {
        return null;
    }
}

为了模拟回调失败服务提供方，抛出500错误。

 @GetMapping("/{id}")
    public User getUserByID(@PathVariable("id") String id) {

//        return userMap.get(id);
        throw new RuntimeException("服务端测试异常！");
    }

运行单元测试UserFeignTest.getUserByID控制台输出结果:

2018-08-18 11:47:59.800  INFO 8660 --- [ hystrix-user-1] com.example.feign.UserFeign              : [UserFeign#getUserByID] ---> GET http://localhost:8080/user/1 HTTP/1.1
2018-08-18 11:47:59.800  INFO 8660 --- [ hystrix-user-1] com.example.feign.UserFeign              : [UserFeign#getUserByID] ---> END HTTP (0-byte body)
2018-08-18 11:47:59.828  INFO 8660 --- [ hystrix-user-1] com.example.feign.UserFeign              : [UserFeign#getUserByID] <--- HTTP/1.1 500 (27ms)
2018-08-18 11:47:59.828  INFO 8660 --- [ hystrix-user-1] com.example.feign.UserFeign              : [UserFeign#getUserByID] connection: close
2018-08-18 11:47:59.828  INFO 8660 --- [ hystrix-user-1] com.example.feign.UserFeign              : [UserFeign#getUserByID] content-type: application/json;charset=UTF-8
2018-08-18 11:47:59.828  INFO 8660 --- [ hystrix-user-1] com.example.feign.UserFeign              : [UserFeign#getUserByID] date: Sat, 18 Aug 2018 03:47:59 GMT
2018-08-18 11:47:59.828  INFO 8660 --- [ hystrix-user-1] com.example.feign.UserFeign              : [UserFeign#getUserByID] transfer-encoding: chunked
2018-08-18 11:47:59.828  INFO 8660 --- [ hystrix