Feign客户端异常处理

1. 概述

在这个教程中，我们将展示如何在Feign中处理异常。Feign是微服务开发者强大的工具，它支持**错误解码器(ErrorDecoder)和回退工厂(FallbackFactory)**来处理异常。

2. Maven依赖

首先，我们创建一个基于Spring Boot的项目，通过添加spring-cloud-starter-openfeign依赖。spring-cloud-starter-openfeign内部包含了feign-core依赖：

<dependency>
    <groupId>org.springframework.cloud</groupId>
    <artifactId>spring-cloud-starter-openfeign</artifactId>
    <version>3.1.3</version>
</dependency>

或者，我们可以在pom.xml文件中直接添加feign-core依赖：

<dependency>
    <groupId>io.github.openfeign</groupId>
    <artifactId>feign-core</artifactId>
    <version>11.9.1</version>
</dependency>

3. 使用ErrorDecoder处理异常

我们可以通过配置ErrorDecoder来处理异常，这还允许我们在需要时自定义错误信息。当发生错误时，Feign客户端会隐藏原始消息。为了获取它，我们可以编写自定义的ErrorDecoder。让我们覆盖默认的ErrorDecoder实现：

public class RetreiveMessageErrorDecoder implements ErrorDecoder {
    private final ErrorDecoder errorDecoder = new Default();
    @Override
    public Exception decode(String methodKey, Response response) {
        ExceptionMessage message = null;
        try (InputStream bodyIs = response.body().asInputStream()) {
            ObjectMapper mapper = new ObjectMapper();
            message = mapper.readValue(bodyIs, ExceptionMessage.class);
        } catch (IOException e) {
            return new Exception(e.getMessage());
        }
        switch (response.status()) {
            case 400:
                return new BadRequestException(message.getMessage() != null ? message.getMessage() : "Bad Request");
            case 404:
                return new NotFoundException(message.getMessage() != null ? message.getMessage() : "Not found");
            default:
                return errorDecoder.decode(methodKey, response);
        }
    }
}

在这个编码器中，我们重写了默认行为，以便更好地控制异常。

4. 使用Fallback处理异常

我们也可以通过配置Fallback来处理异常。首先，创建一个客户端并配置Fallback：

@FeignClient(name = "file", url = "http://localhost:8081", 
  configuration = FeignSupportConfig.class, fallback = FileUploadClientWithFallbackImpl.class)
public interface FileUploadClientWithFallBack {
    @PostMapping(value = "/upload-error", consumes = MediaType.MULTIPART_FORM_DATA_VALUE)
    String fileUpload(@RequestPart(value = "file") MultipartFile file);
}

接下来，创建FileUploadClientWithFallbackImpl来根据我们的需求处理异常：

@Component
public class FileUploadClientWithFallbackImpl implements FileUploadClientWithFallBack {
    @Override
    public String fileUpload(MultipartFile file) {
        try {
            throw new NotFoundException("hi, something wrong");
        } catch (Exception ex) {
            if (ex instanceof BadRequestException) {
                return "Bad Request!!!";
            }
            if (ex instanceof NotFoundException) {
                return "Not Found!!!";
            }
            if (ex instanceof Exception) {
                return "Exception!!!";
            }
            return "Successfully Uploaded file!!!";
        }
    }
}

现在，让我们编写一个简单的测试验证Fallback选项：

@Test(expected = NotFoundException.class)
public void whenFileUploadClientFallback_thenFileUploadError() throws IOException {
    ClassLoader classloader = Thread.currentThread().getContextClassLoader();
    File file = new File(classloader.getResource(FILE_NAME).getFile());
    Assert.assertTrue(file.exists());
    FileInputStream input = new FileInputStream(file);
    MultipartFile multipartFile = new MockMultipartFile("file", file.getName(), "text/plain",
      IOUtils.toByteArray(input));
    uploadService.uploadFileWithFallback(multipartFile);
}

5. 使用FallbackFactory处理异常

我们还可以通过配置FallbackFactory来处理异常。同样，先创建一个客户端并配置FallbackFactory：

@FeignClient(name = "file", url = "http://localhost:8081", 
  configuration = FeignSupportConfig.class, fallbackFactory = FileUploadClientFallbackFactory.class)
public interface FileUploadClient {
    @PostMapping(value = "/upload-file", consumes = MediaType.MULTIPART_FORM_DATA_VALUE)
    String fileUpload(@RequestPart(value = "file") MultipartFile file);
}

接着，创建FileUploadClientFallbackFactory来根据需要处理异常：

@Component
public class FileUploadClientFallbackFactory implements FallbackFactory<FileUploadClient> {
    @Override
    public FileUploadClient create(Throwable cause) {
        return new FileUploadClient() {
            @Override
            public String fileUpload(MultipartFile file) {
                if (cause instanceof BadRequestException) {
                    return "Bad Request!!!";
                }
                if (cause instanceof NotFoundException) {
                    return "Not Found!!!";
                }
                if (cause instanceof Exception) {
                    return "Exception!!!";
                }
                return "Successfully Uploaded file!!!";
            }
        };
    }
}

现在，编写一个简单的测试验证FallbackFactory选项：

@Test(expected = NotFoundException.class)
public void whenFileUploadClientFallbackFactory_thenFileUploadError() throws IOException {
    ClassLoader classloader = Thread.currentThread().getContextClassLoader();
    File file = new File(classloader.getResource(FILE_NAME).getFile());
    Assert.assertTrue(file.exists());
    FileInputStream input = new FileInputStream(file);
    MultipartFile multipartFile = new MockMultipartFile("file", file.getName(), "text/plain",
      IOUtils.toByteArray(input));
    uploadService.uploadFileWithFallbackFactory(multipartFile);
} 

6. 总结

在这篇文章中，我们展示了如何在Feign中处理异常。如往常一样，本教程的所有代码示例都可以在GitHub上找到。