@PostMapping(value = "/test/post", consumes = "application/json")
 String test(@RequestBody String name);

@PostMapping(value = "/test/post", produces= "application/json")

@FeignClient(name = "a-service", configuration = FeignConfiguration.class)
public interface ACall {
 
    @RequestMapping(method = RequestMethod.POST, value = "/api/xxx/{A}", headers = {"Content-Type=application/json"})
    void updateAToB(@PathVariable("A") final String A, @RequestParam("B") final String B) throws Exception;
}

<dependency>
    <groupId>com.netflix.feign</groupId>
    <artifactId>feign-httpclient</artifactId>
    <version>8.18.0</version>
</dependency>

@Override
  public Response execute(Request request, Request.Options options) throws IOException {
    HttpUriRequest httpUriRequest;
    try {
      httpUriRequest = toHttpUriRequest(request, options);
    } catch (URISyntaxException e) {
      throw new IOException("URL '" + request.url() + "' couldn't be parsed into a URI", e);
    }
    HttpResponse httpResponse = client.execute(httpUriRequest);
    return toFeignResponse(httpResponse);
  }
 
  HttpUriRequest toHttpUriRequest(Request request, Request.Options options) throws
          UnsupportedEncodingException, MalformedURLException, URISyntaxException {
    RequestBuilder requestBuilder = RequestBuilder.create(request.method());
 
    //per request timeouts
    RequestConfig requestConfig = RequestConfig
            .custom()
            .setConnectTimeout(options.connectTimeoutMillis())
            .setSocketTimeout(options.readTimeoutMillis())
            .build();
    requestBuilder.setConfig(requestConfig);
 
    URI uri = new URIBuilder(request.url()).build();
 
    requestBuilder.setUri(uri.getScheme() + "://" + uri.getAuthority() + uri.getRawPath());
 
    //request query params
    List<NameValuePair> queryParams = URLEncodedUtils.parse(uri, requestBuilder.getCharset().name());
    for (NameValuePair queryParam: queryParams) {
      requestBuilder.addParameter(queryParam);
    }
 
    //request headers
    boolean hasAcceptHeader = false;
    for (Map.Entry<String, Collection<String>> headerEntry : request.headers().entrySet()) {
      String headerName = headerEntry.getKey();
      if (headerName.equalsIgnoreCase(ACCEPT_HEADER_NAME)) {
        hasAcceptHeader = true;
      }
 
      if (headerName.equalsIgnoreCase(Util.CONTENT_LENGTH)) {
        // The 'Content-Length' header is always set by the Apache client and it
        // doesn't like us to set it as well.
        continue;
      }
 
      for (String headerValue : headerEntry.getValue()) {
        requestBuilder.addHeader(headerName, headerValue);
      }
    }
    //some servers choke on the default accept string, so we'll set it to anything
    if (!hasAcceptHeader) {
      requestBuilder.addHeader(ACCEPT_HEADER_NAME, "*/*");
    }
 
    //request body
    if (request.body() != null) {
 
    //body為空，則HttpEntity為空
 
      HttpEntity entity = null;
      if (request.charset() != null) {
        ContentType contentType = getContentType(request);
        String content = new String(request.body(), request.charset());
        entity = new StringEntity(content, contentType);
      } else {
        entity = new ByteArrayEntity(request.body());
      }
 
      requestBuilder.setEntity(entity);
    }
 
    //調用org.apache.http.client.methods.RequestBuilder#build方法
    return requestBuilder.build();
  }

public HttpUriRequest build() {
        final HttpRequestBase result;
        URI uriNotNull = this.uri != null ? this.uri : URI.create("/");
        HttpEntity entityCopy = this.entity;
        if (parameters != null && !parameters.isEmpty()) {
    // 這里：如果HttpEntity為空，并且為POST請求或者為PUT請求時，這個方法會將查詢參數取出來封裝成了HttpEntity
    // 就是在這里查詢參數被丟棄了，準確的說是被轉換位置了
            if (entityCopy == null && (HttpPost.METHOD_NAME.equalsIgnoreCase(method)
                    || HttpPut.METHOD_NAME.equalsIgnoreCase(method))) {
                entityCopy = new UrlEncodedFormEntity(parameters, charset != null ? charset : HTTP.DEF_CONTENT_CHARSET);
            } else {
                try {
                    uriNotNull = new URIBuilder(uriNotNull)
                      .setCharset(this.charset)
                      .addParameters(parameters)
                      .build();
                } catch (final URISyntaxException ex) {
                    // should never happen
                }
            }
        }
        if (entityCopy == null) {
            result = new InternalRequest(method);
        } else {
            final InternalEntityEclosingRequest request = new InternalEntityEclosingRequest(method);
            request.setEntity(entityCopy);
            result = request;
        }
        result.setProtocolVersion(this.version);
        result.setURI(uriNotNull);
        if (this.headergroup != null) {
            result.setHeaders(this.headergroup.getAllHeaders());
        }
        result.setConfig(this.config);
        return result;
    }

HttpUriRequest toHttpUriRequest(Request request, Request.Options options) throws
          UnsupportedEncodingException, MalformedURLException, URISyntaxException {
    RequestBuilder requestBuilder = RequestBuilder.create(request.method());
 
   //省略部分代碼
    //request body
    if (request.body() != null) {
      //省略部分代碼
    } else {
    // 此處，如果為null，則會塞入一個byte數組為0的對象
      requestBuilder.setEntity(new ByteArrayEntity(new byte[0]));
    }
 
    return requestBuilder.build();
  }

<dependency>
    <groupId>io.github.openfeign</groupId>
    <artifactId>feign-httpclient</artifactId>
    <version>9.5.1</version>
</dependency>

<dependency>
    <groupId>io.github.openfeign</groupId>
    <artifactId>feign-okhttp</artifactId>
    <version>9.5.1</version>
</dependency>