[BUG][JAVA][RESTTEMPLATE] Name conflict when using parameter named Content-Type

Created by: desmese

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs? I noticed #9565 (closed) too late, because i started working on the bugfix yesterday, and didn't search again for new issues today.
• What's the actual output vs expected output?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

I tried to generate a new client using the java resttemplate library. The OpenAPI specification I am using has got a header specified which is named 'Content-Type'. The generated parameter is called contentType. This clashes with the contentType variable defined in the api.mustache. The local variable should have got the localVar prefix.

openapi-generator version

5.1.1

OpenAPI declaration file content or url

openapi: "3.0.0"
info:
  title: Hello World
  version: "1.0"
servers:
  - url: http://localhost:9090/v1.0

paths:
  /greeting:
    get:
      summary: Generate greeting
      description: Generates a greeting message.
      operationId: hello.post_greeting
      responses:
        200:
          description: greeting response
          content:
            text/plain:
              schema:
                type: string
                example: "hello dave!"
      parameters:
        - name: Content-Type
          in: header
          description: Name of the person to greet.
          required: true
          schema:
            type: string
            example: "dave"

Generation Details

I am generating the client using the openapi-generator-maven-plugin. The bug however is found in the template, so it should shall exists in all generators.

    <build>
        <plugins>
            <plugin>
                <groupId>org.openapitools</groupId>
                <artifactId>openapi-generator-maven-plugin</artifactId>
                <!-- RELEASE_VERSION -->
                <version>5.1.0</version>
                <!-- /RELEASE_VERSION -->
                <executions>
                    <execution>
                        <goals>
                            <goal>generate</goal>
                        </goals>
                        <configuration>
                            <inputSpec>${project.basedir}/src/main/resources/openapi.yaml</inputSpec>
                            <generatorName>java</generatorName>
                            <library>resttemplate</library>
                            <configOptions>
                                <dateLibrary>java8</dateLibrary>
                            </configOptions>
                        </configuration>
                    </execution>
                </executions>
            </plugin>
        </plugins>
    </build>

Steps to reproduce

The bug can be reproduced by generating a client using the java generator and the resttemplate library. You could use the above configuration for the maven-plugin, and the openapi file specified above.

Related issues/PRs

https://github.com/OpenAPITools/openapi-generator/issues/9565

Suggest a fix

I created a PR which adds the localVar prefix before the local variable called contentType. I will link it to this issue.