前言

在spring cloud的项目中用到了feign组件，简单配置过后即可完成请求的调用。又因为有向请求添加Header头的需求，查阅了官方示例后，就觉得很简单，然后一顿操作之后调试报错...

按官方修改的示例：

#MidServerClient.java
import feign.Param;
import org.springframework.cloud.openfeign.FeignClient;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RequestMethod;

@FeignClient(value = "edu-mid-server")
public interface MidServerClient {

    @RequestMapping(value = "/test/header", method = RequestMethod.GET)
    @Headers({"userInfo:{userInfo}"})
    Object headerTest(@Param("userInfo") String userInfo);
}

提示错误:

java.lang.IllegalArgumentException: method GET must not have a request body.

分析

通过断点debug发现feign发请求时把userInfo参数当成了requestBody来处理，而okhttp3会检测get请求不允许有body（其他类型的请求哪怕不报错，但因为不是设置到请求头，依然不满足需求）。

查阅官方文档里是通过Contract（Feign.Contract.Default）来解析注解的：

Feign annotations define the Contract between the interface and how the underlying client should work. Feign's default contract defines the following annotations:

Annotation	Interface Target	Usage
@RequestLine	Method	Defines the HttpMethod and UriTemplate for request. Expressions, values wrapped in curly-braces {expression} are resolved using their corresponding @Param annotated parameters.
@Param	Parameter	Defines a template variable, whose value will be used to resolve the corresponding template Expression, by name.
@Headers	Method, Type	Defines a HeaderTemplate; a variation on a UriTemplate. that uses @Param annotated values to resolve the corresponding Expressions. When used on a Type, the template will be applied to every request. When used on a Method, the template will apply only to the annotated method.
@QueryMap	Parameter	Defines a Map of name-value pairs, or POJO, to expand into a query string.
@HeaderMap	Parameter	Defines a Map of name-value pairs, to expand into Http Headers
@Body	Method	Defines a Template, similar to a UriTemplate and HeaderTemplate, that uses @Param annotated values to resolve the corresponding Expressions.

从自动配置类找到使用的是spring cloud的SpringMvcContract（用来解析@RequestMapping相关的注解）,而这个注解并不会处理解析上面列的注解

@Configuration
public class FeignClientsConfiguration {
	···
	@Bean
	@ConditionalOnMissingBean
	public Contract feignContract(ConversionService feignConversionService) {
		return new SpringMvcContract(this.parameterProcessors, feignConversionService);
	}
	···

解决

原因找到了：spring cloud使用了自己的SpringMvcContract来解析注解，导致默认的注解解析方式失效。解决方案自然就是重新解析处理feign的注解，这里通过自定义Contract继承SpringMvcContract再把Feign.Contract.Default解析逻辑般过来即可（重载的方法是在SpringMvcContract基础上做进一步解析，否则Feign对RequestMapping相关对注解解析会失效）

代码如下(此处只对@Headers、@Param重新做了解析)：

#FeignCustomContract.java

import feign.Headers;
import feign.MethodMetadata;
import feign.Param;
import org.springframework.cloud.openfeign.AnnotatedParameterProcessor;
import org.springframework.cloud.openfeign.support.SpringMvcContract;
import org.springframework.core.convert.ConversionService;

import java.lang.annotation.Annotation;
import java.lang.reflect.Method;
import java.util.*;

import static feign.Util.checkState;
import static feign.Util.emptyToNull;

public class FeignCustomContract extends SpringMvcContract {


    public FeignCustomContract(List<AnnotatedParameterProcessor> annotatedParameterProcessors, ConversionService conversionService) {
        super(annotatedParameterProcessors, conversionService);
    }

    @Override
    protected void processAnnotationOnMethod(MethodMetadata data, Annotation methodAnnotation, Method method) {
        //解析mvc的注解
        super.processAnnotationOnMethod(data, methodAnnotation, method);
        //解析feign的headers注解
        Class<? extends Annotation> annotationType = methodAnnotation.annotationType();
        if (annotationType == Headers.class) {
            String[] headersOnMethod = Headers.class.cast(methodAnnotation).value();
            checkState(headersOnMethod.length > 0, "Headers annotation was empty on method %s.", method.getName());
            data.template().headers(toMap(headersOnMethod));
        }
    }

    @Override
    protected boolean processAnnotationsOnParameter(MethodMetadata data, Annotation[] annotations, int paramIndex) {
        boolean isMvcHttpAnnotation = super.processAnnotationsOnParameter(data, annotations, paramIndex);

        boolean isFeignHttpAnnotation = false;
        for (Annotation annotation : annotations) {
            Class<? extends Annotation> annotationType = annotation.annotationType();
            if (annotationType == Param.class) {
                Param paramAnnotation = (Param) annotation;
                String name = paramAnnotation.value();
                checkState(emptyToNull(name) != null, "Param annotation was empty on param %s.", paramIndex);
                nameParam(data, name, paramIndex);
                isFeignHttpAnnotation = true;
                if (!data.template().hasRequestVariable(name)) {
                    data.formParams().add(name);
                }
            }
        }
        return isMvcHttpAnnotation || isFeignHttpAnnotation;

    }

    private static Map<String, Collection<String>> toMap(String[] input) {
        Map<String, Collection<String>> result =
                new LinkedHashMap<String, Collection<String>>(input.length);
        for (String header : input) {
            int colon = header.indexOf(':');
            String name = header.substring(0, colon);
            if (!result.containsKey(name)) {
                result.put(name, new ArrayList<String>(1));
            }
            result.get(name).add(header.substring(colon + 1).trim());
        }
        return result;
    }
}

#FeignCustomConfiguration.java

import feign.Contract;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.autoconfigure.condition.ConditionalOnProperty;
import org.springframework.cloud.openfeign.AnnotatedParameterProcessor;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.core.convert.ConversionService;

import java.util.ArrayList;
import java.util.List;

@Configuration
public class FeignCustomConfiguration {

    ···
    @Autowired(required = false)
    private List<AnnotatedParameterProcessor> parameterProcessors = new ArrayList<>();

    @Bean
    @ConditionalOnProperty(name = "feign.feign-custom-contract", havingValue = "true", matchIfMissing = true)
    public Contract feignContract(ConversionService feignConversionService) {
        return new FeignCustomContract(this.parameterProcessors, feignConversionService);
    }
    ···

改完马上进行新一顿的操作, 看请求日志已经设置成功，响应OK！:

请求： {"type":"OKHTTP_REQ","uri":"/test/header","httpMethod":"GET","header":"{"accept":["/"],"userinfo":["{"userId":"sssss","phone":"13544445678],"x-b3-parentspanid":["e49c55484f6c19af"],"x-b3-sampled":["0"],"x-b3-spanid":["1d131b4ccd08d964"],"x-b3-traceid":["9405ce71a13d8289"]}","param":""}

响应 {"type":"OKHTTP_RESP","uri":"/test/header","respStatus":0,"status":200,"time":5,"header":"{"cache-control":["no-cache,no-store,max-age=0,must-revalidate"],"connection":["keep-alive"],"content-length":["191"],"content-type":["application/json;charset=UTF-8"],"date":["Fri,11Oct201913:02:41GMT"],"expires":["0"],"pragma":["no-cache"],"x-content-type-options":["nosniff"],"x-frame-options":["DENY"],"x-xss-protection":["1;mode=block"]}"}

feign官方链接

spring cloud feign 链接

（另一种实现请求头的方式：实现RequestInterceptor，但是存在hystrix线程切换的坑）