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

Spring Cloud Feign 之Fallback

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-user-1] com.example.feign.UserFeign : [UserFeign#getUserByID] 2018-08-18 11:47:59.829 INFO 8660 --- [ hystrix-user-1] com.example.feign.UserFeign : [UserFeign#getUserByID] {"timestamp":1534564079825,"status":500,"error":"Internal Server Error","exception":"java.lang.RuntimeException","message":"服务端测试异常！","path":"/user/1"} 2018-08-18 11:47:59.829 INFO 8660 --- [ hystrix-user-1] com.example.feign.UserFeign : [UserFeign#getUserByID] <--- END HTTP (167-byte body) User{id='100', name='fallback 回调用户'} 

服务提供方抛出的500错误代码，但是客户端程序还可以正常运行输出了UserFeignFallback.getUserByID方法返回的结果。

FallbackFactory<T>工厂

上面的实现方式简单，但是获取不到HTTP请求错误状态码和信息 ，这时就可以使用工厂模式来实现Fallback

同样工厂实现类也要交由spring管理，同时结合UserFeignFallback使用，这里需要注意的create方法返回值类型一定要实现Feign接口，否则会报错。

UserFeignFactory只做了打印异常处理：

package com.example.feign; import feign.hystrix.FallbackFactory; import org.springframework.stereotype.Component; @Component public class UserFeignFactory implements FallbackFactory<UserFeign> { private final UserFeignFallback userFeignFallback; public UserFeignFactory(UserFeignFallback userFeignFallback) { this.userFeignFallback = userFeignFallback; } @Override public UserFeign create(Throwable cause) { //打印下异常 cause.printStackTrace(); return userFeignFallback; } } 

UserFeign:

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(); } 

运行单元测试UserFeignTest.getUserByID可以看到控制台打印的异常feign.FeignException更多信息省略。

ErrorDecoder接口处理请求错误信息，默认实现ErrorDecoder.Default抛出FeignException异常

FeignException.status 方法返回HTTP状态码，FallbackFactory.create默认情况下可以强制转换成FeignException异常这样就可以获取到HTTP状态码了。

自定义ErrorDecoder

第一种:application.properties

全局配置,通过application.properties配置文件

feign.client.default-config=my-config
feign.client.config.my-config.error-decoder=com.example.feign.MyErrorDecoder

错误解码实现类MyErrorDecoder

package com.example.feign; import feign.Response; import feign.codec.ErrorDecoder; public class MyErrorDecoder implements ErrorDecoder { @Override public Exception decode(String methodKey, Response response) { return new MyFeignException(methodKey,response); } } 

自定义异常MyFeignException

package com.example.feign; import feign.Response; public class MyFeignException extends RuntimeException { private final String methodKey; private Response response; MyFeignException(String methodKey, Response response) { this.methodKey = methodKey; this.response = response; } public Response getResponse() { return response; } public String getMethodKey() { return methodKey; } } 

第二种:@EnableFeignClients

全局配置,@EnableFeignClients.defaultConfiguration注解

package com.example; import com.example.feign.FeignClientsConfig; import org.springframework.boot.SpringApplication; import org.springframework.boot.autoconfigure.SpringBootApplication; import org.springframework.cloud.netflix.feign.EnableFeignClients; /** * 启动类 * * @author: sunshaoping * @date: Create by in 上午10:47 2018/8/7 */ @EnableFeignClients( defaultConfiguration = FeignClientsConfig.class ) @SpringBootApplication public class FeignApplication { public static void main(String[] args) { SpringApplication.run(FeignApplication.class, args); } } 

第三种:@FeignClient

作用范围是Feign接口,优先级要高于上面两种，@FeignClient.configuration 注解

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*/ decode404 = true, fallbackFactory = UserFeignFactory.class, configuration = FeignClientsConfig.class ) public interface UserFeign { @PostMapping void save(User user); @GetMapping("/{id}") User getUserByID(@PathVariable("id") String id); @GetMapping List<User> findAll(); } 

总结

本章节讲了如下内容

Spring Cloud Feign HTTP请求异常Fallback容错机制，它是基于Hystrix实现的，所以要通过配置参数feign.hystrix.enabled=true开启该功能，及其两种实现方式。

Fallback工厂方式引出了ErrorDecoder错误解码自定义处理，有三种方式，可根据实际请求选择，举一反三其他自定义配置也可以通过这种方式实现如：Decoder、Encoder、Logger(第二、三章有介绍)。

如果开启的Hystrix就不要用feign的超时配置了,单位是毫秒

feign.client.config.defalut.connect-timeout=10000

defalut是默认配置名称，可以使用feign.client.default-config替换自定义名称

feign.client.default-config=my-config
feign.client.config.my-config.connect-timeout=10000

请使用如下属性配置超时时间，单位毫秒

hystrix.command.default.execution.isolation.thread.timeoutInMilliseconds=20000

样例地址 spring-cloud-feign 分支 Spring-Cloud-Feign-之fallback

作者：孙平平
链接：https://www.jianshu.com/p/d84768f7996d
来源：简书
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