[BUG] content-type application/json using iso-8859-1 instead of utf-8 (accent and special char not supported)

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• [ x 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?
• What's the actual output vs expected output?
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Description

Using the plugin org.openapitools:openapi-generator-maven-plugin:6.0.1 to generator a Java client from an openapi3.yml spec file, I have noticed requestBody content-type application/json generate a Java Api that does not support accent or special characters.

For example )çàç!è!§è(‘“é‘(§‘“é&“’(§è!çà send to the Java API is transformed to )???!?!??(????(????&??(??! in the HTTP call (wireshark inspection).

Problem : request content-type application/json is parsed as application/json with no charset. Therfore, using apache-httpclient, the generator use org.apache.httpcomponents:httpcore where the default charset iso-8859-1 which does not support accent.

application/json with ISO-8859-1 it is not compliant with RFC4627. In my case it lead to unwanted behavior in Java HTTP call such as unexpected character replacement.

openapi-generator version

org.openapitools:openapi-generator-maven-plugin:6.0.1
library : apache-httpclient

OpenAPI declaration file content or url

openapi: 3.0.1
info:
  version: 1.0.0
  title: Example
paths:
  /test-body:
    put:
      tags:
        - Test
      requestBody:
        required: true
        content:
          application/json:
            schema:
              $ref: '#/components/schemas/TestBodys'
      responses:
        200:
          description: OK
          content:
            text/plain:
              schema:
                type: string
                example: pong
components:
  schemas:
    TestBodys:
      type: object
      properties:
        test_body:
          type: array
          items:
            $ref: '#/components/schemas/TestBody'

    TestBody:
      type: object
      properties:
        code:
          type: string
        type:
          type: string
        value:
          type: object

Generation Details

pom.xml

<plugin>
    <groupId>org.openapitools</groupId>
    <artifactId>openapi-generator-maven-plugin</artifactId>
    <version>6.0.1</version>
    <executions>
        <execution>
            <phase>generate-sources</phase>
            <goals>
                <goal>generate</goal>
            </goals>
            <configuration>
                <generatorName>java</generatorName>
                <inputSpec>${project.basedir}/src/main/resources/openapi/connect/openapi3.yml</inputSpec>
                <!-- api final path: output + sourceFolder + apiPackage -->
                <apiPackage>com.test.client.generated.api</apiPackage>
                <!-- model final path: output + sourceFolder + modelPackage -->
                <modelPackage>com.test.client.generated.model</modelPackage>
                <!-- disable unused code generation -->
                <generateApiTests>false</generateApiTests>
                <generateModelTests>false</generateModelTests>
                <generateApiDocumentation>false</generateApiDocumentation>
                <generateModelDocumentation>false</generateModelDocumentation>
                <typeMappings>
                    <typeMapping>OffsetDateTime=java.time.Instant</typeMapping>
                </typeMappings>
                <!-- disable pom.xml and other unwanted file generation -->
                <supportingFilesToGenerate>
                    ApiClient.java,ServerConfiguration.java,ServerVariable.java,JavaTimeFormatter.java,StringUtil.java,Authentication.java,HttpBasicAuth.java,HttpBearerAuth.java,ApiKeyAuth.java,ApiException.java,Configuration.java,Pair.java,auth/Authentication.java,RFC3339DateFormat.java
                </supportingFilesToGenerate>
                <configOptions>
                    <library>apache-httpclient</library>
                    <dateLibrary>java8</dateLibrary>
                    <sourceFolder>src/main/java/</sourceFolder>
                </configOptions>
                <output>${project.basedir}</output>
            </configuration>
        </execution>
    </executions>
</plugin>

Steps to reproduce

Generator the Java client using the OpenApi plugin generator.
Then use the ApiClient TestBody api to send special character such as )çàç!è!§è(‘“é‘(§‘“é&“’(§è!çà
Using wireshark or another tool inspect the payload request content.
=> Special caracters has been replaces by interrogation point.

Related issues/PRs

Not found

Suggest a fix

Use UTF-8 charset proposed by apache httpcore such as here in httpcore.

Inside generated ApiClient.java, the getContentType method is :

private ContentType getContentType(String headerValue) throws ApiException {
        try {
            return ContentType.parse(headerValue);
        } catch (org.apache.http.ParseException var3) {
            throw new ApiException("Could not parse content type " + headerValue);
        }
    }

And can be replaced by the following to retrieve an utf8 charset inside the content-type :

private ContentType getContentType(String headerValue) {
       return ContentType.getByMimeType(headerValue);
    }

Start with the generated ApiClient.java getContentType method and the Java client produced from the supplied openapi3.yml and pom.xml configuration. Reproduce the request with accented characters using the apache-httpclient library, then verify that the generated client sends those characters without replacement characters and add coverage for the corrected content type handling.