[BUG][Java Client][Feign] Request headers are sent in body

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• What's the version of OpenAPI Generator used?
• Have you search for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Bounty to sponsor the fix (example)

Description

Generated Feign client methods use wrong parameter names. This causes headers to be sent in request body.

Method generated by the definition provided below looks like this:

  @RequestLine("POST /resource")
  @Headers({
    "Accept: */*",
    "X-Correlation-ID: {xCorrelationID}",
    
    "X-Request-ID: {xRequestID}"
  })
  Object createResource(@Param("X-Correlation-ID") String xCorrelationID, @Param("X-Request-ID") String xRequestID);

And according to OpenFeign docs it should look like this:

  @RequestLine("POST /resource")
  @Headers({
    "Accept: */*",
    "X-Correlation-ID: {xCorrelationID}",
    
    "X-Request-ID: {xRequestID}"
  })
  Object createResource(@Param("xCorrelationID") String xCorrelationID, @Param("xRequestID") String xRequestID);

Note the difference between the @Param annotation values.

openapi-generator version

3.4.0

OpenAPI declaration file content or url

LINK

Command line used for generation

<plugin>
  <groupId>org.openapitools</groupId>
  <artifactId>openapi-generator-maven-plugin</artifactId>
  <version>4.3.0</version>
  <executions>
    <execution>
      <goal>generate</goal>
      <configuration>
        <inputSpec>${project.basedir}/src/main/resources/api/@item@.json</inputSpec>
        <generatorName>java</generatorName>
        <output>target/generated-sources</output>
        <apiPackage>com.company.api</apiPackage>
        <modelPackage>com.company.model</modelPackage>
        <skipValidateSpec>true</skipValidateSpec>
        <generateApis>true</generateApis>
        <generateApiDocumentation>false</generateApiDocumentation>
        <generateSupportingFiles>true</generateSupportingFiles>
        <generateModels>true</generateModels>
        <generateModelTests>false</generateModelTests>
        <generateModelDocumentation>false</generateModelDocumentation>
        <configOptions>
          <sourceFolder>swagger</sourceFolder>
          <java8>true</java8>
          <interfaceOnly>false</interfaceOnly>
          <dateLibrary>java8-localdatetime</dateLibrary>
          <library>feign</library>
          <hideGenerationTimestamp>true</hideGenerationTimestamp>
          <serializableModel>true</serializableModel>
          <invokerPackage>com.company</invokerPackage>
          <allowUnicodeIdentifiers>true</allowUnicodeIdentifiers>
        </configOptions>
      </configuration>
    </execution>
  </executions>
</plugin>

Steps to reproduce

Just generate client code and try to sent a request.

Related issues/PRs

This bug was introduced in 4.2.3 as a result of fixing #3295 (pull request #4883)

Suggest a fix

Reverse the above-mentioned or find a better way of generating headers in the mustache template.

Start with the Java Feign mustache template responsible for generated @Param annotations, then compare the regression described from #3295 and #4883. Reproduce using the linked gist and Maven configuration. Done means generated @Param values match the Java parameter names so Feign sends headers as headers rather than in the request body.