Java Feign 客户端后备 - 无法调用后备

Posted 2023-03-16

【中文标题】Java Feign 客户端后备 - 无法调用后备

【英文标题】：Java Feign client Fallback - can't call fallback

【发布时间】：2020-07-27 05:54:27

【问题描述】：

我正在尝试理解Feign客户端在Java中的工作，并在出现错误时调用Fallback，我遇到了一些困难。 我写了一个最简单的例子来理解它是如何工作的，但似乎我很困惑。 请帮助我了解我的错误在哪里。

我还在 github 上发布了这个示例，以使其更方便： https://github.com/ksereda/feign-fallback-example

client通过Feign调用service-one，收到字符串“Get String from SERVICE-ONE”。

сlient也通过Feign调用service-two，得到字符串“Get String from SERVICE-TWO”。

先决条件： 在 service-two 不可用的情况下，client 必须调用指示对 service-three 的调用的 Fallback 类，而不是“从 SERVICE-TWO 获取字符串”，客户端应该从 service-3 获取“从 SERVICE-THREE 获取字符串”。

Service-two和service-three是一样的，只是返回的字符串不同。

服务一码：

@RestController
@RequestMapping("/")
public class OneController 

    Logger logger = Logger.getLogger(OneController.class.getName());

    @GetMapping("/getStringFromServiceOne")
    public String getString() 
        logger.info("Get String from SERVICE-ONE");
        return "Get String from SERVICE-ONE";
    

服务二代码：

@RestController
@RequestMapping("/")
public class TwoController 

    Logger logger = Logger.getLogger(TwoController.class.getName());

    @GetMapping("/getStringFromServiceTwo")
    public String getString() 
        logger.info("Get String from SERVICE-TWO");
        return "Get String from SERVICE-TWO";
    

服务三码：

@RestController
@RequestMapping("/")
public class ThreeController 

    Logger logger = Logger.getLogger(ThreeController.class.getName());

    @GetMapping("/getStringFromServiceThree")
    public String getString() 
        logger.info("Get String from SERVICE-THREE");
        return "Get String from SERVICE-THREE";
    

客户端代码：

控制器：

@RestController
@RequestMapping("/")
public class ClientController 

   Logger logger = Logger.getLogger(ClientController.class.getName());

    @Autowired
    private ServiceOneFeignClient serviceOneFeignClient;

    @Autowired
    private ServiceTwoFeignClient serviceTwoFeignClient;

    @RequestMapping(path = "/getDataFromServiceOneByFeign")
    public String getDataFromServiceOne() 
        String result = serviceOneFeignClient.getString();
        logger.info("Calling through Feign Client");
        return result;
    

    @RequestMapping(path = "/getDataFromServiceTwoByFeign")
    public String getDataFromServiceTwo() 
        String result = serviceTwoFeignClient.getString();
        logger.info("Calling through Feign Client");
        return result;
    

假装1：

@FeignClient(name = "service-one", url = "http://localhost:8081/")
public interface ServiceOneFeignClient 

    @GetMapping("/getStringFromServiceOne")
    String getString();

和 Feign 2：

@FeignClient(name = "service-two", url = "http://localhost:8082/", fallback = Fallback.class)
public interface ServiceTwoFeignClient 

    @GetMapping("/getStringFromServiceTwo")
    String getString();

和 Feign 3：

@FeignClient(name = "service-three", url = "http://localhost:8083/")
public interface ServiceThreeFeignClient 

    @GetMapping("/getStringFromServiceThree")
    String getString();

主类：

@SpringBootApplication
@EnableFeignClients
@EnableCircuitBreaker
@EnableHystrix
public class ClientApplication 

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

和 application.yml 文件

server:
  port: 8070

feign:
  hystrix:
    enabled: true

我在这里创建了 Fallback 类

@Component
public class Fallback implements ServiceThreeFeignClient 

    @Override
    public String getString() 
        System.out.println("Called MOVIE-SERVICE with Fallback class!");
        return new String("FALLBACK STRING");
    

如果 service-two 不可用，我想在这里调用 service-three。

但是当我启动我的应用程序时，我立即得到一个错误：

Caused by: java.lang.IllegalStateException: Incompatible fallback instance. Fallback/fallbackFactory of type class com.example.client.service.Fallback is not assignable to interface com.example.client.service.ServiceTwoFeignClient for feign client service-two
    at org.springframework.cloud.openfeign.HystrixTargeter.getFromContext(HystrixTargeter.java:86) ~[spring-cloud-openfeign-core-2.1.1.RELEASE.jar:2.1.1.RELEASE]
    at org.springframework.cloud.openfeign.HystrixTargeter.targetWithFallback(HystrixTargeter.java:70) ~[spring-cloud-openfeign-core-2.1.1.RELEASE.jar:2.1.1.RELEASE]
    at org.springframework.cloud.openfeign.HystrixTargeter.target(HystrixTargeter.java:46) ~[spring-cloud-openfeign-core-2.1.1.RELEASE.jar:2.1.1.RELEASE]
    at org.springframework.cloud.openfeign.FeignClientFactoryBean.getTarget(FeignClientFactoryBean.java:284) ~[spring-cloud-openfeign-core-2.1.1.RELEASE.jar:2.1.1.RELEASE]
    at org.springframework.cloud.openfeign.FeignClientFactoryBean.getObject(FeignClientFactoryBean.java:247) ~[spring-cloud-openfeign-core-2.1.1.RELEASE.jar:2.1.1.RELEASE]
    at org.springframework.beans.factory.support.FactoryBeanRegistrySupport.doGetObjectFromFactoryBean(FactoryBeanRegistrySupport.java:171) ~[spring-beans-5.1.7.RELEASE.jar:5.1.7.RELEASE]

我明白他写的是什么：我必须指出

implements ServiceTwoFeignClient

改为

implements ServiceThreeFeignClient

但是如果服务二不可用，我想调用服务三。

我不明白该怎么做。 我会很感激你的帮助。 谢谢

【参考方案1】：

我是在我的 Fallback 课程中决定的：

@Component
public class Fallback implements ServiceTwoFeignClient 

    @Autowired
    private ServiceThreeFeignClient serviceThreeFeignClient;

    @Override
    public String getString() 
        return serviceThreeFeignClient.getString();