[BUG][jaxrs] server generator does not handle exploded query parameters of type object correctly

[lbilger]

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

I have an API that allows arbitrary query parameters. Following this answer on StackOverflow, I defined the parameter as follows:

        - name: parameters
          in: query
          schema:
            type: object
            additionalProperties:
              type: string
          style: form
          explode: true

The generator (I used jaxrs-resteasy-eap, but also tried some other jaxrs-based ones and got similar results) renders this parameter as @QueryParam("parameters") Map<String, String> parameters. This causes resteasy to complain at startup that it doesn't know how to deserialize a Map<String, String> from a Query Parameter.

I tried implementing my own ParamConverter, which works but is only called if a Query Param called parameters is actually specified in the URL, so it's obviously not the way to go.

In the end I settled for modifying the queryParams.mustache template to change the parameter to @Context javax.ws.rs.core.UriInfo uriInfo, so the implementation can use the UriInfo to access the query parameters.

openapi-generator version

4.3.1

OpenAPI declaration file content or url

openapi: 3.0.1
info:
  title: Test
  version: 1.0.0
paths:
  /something:
    get:
      operationId: getSomething
      parameters:
        - name: parameters
          in: query
          schema:
            type: object
            additionalProperties:
              type: string
          style: form
          explode: true
      responses:
        200:
          description: concatenated parameters
          content:
            application/json:
              schema:
                type: string

Generation Details

          <execution>
            <id>generate-test-server-code</id>
            <phase>generate-sources</phase>
            <goals>
              <goal>generate</goal>
            </goals>
            <configuration>
              <inputSpec>${project.basedir}/src/main/resources/openapi-test.yaml</inputSpec>
              <generatorName>jaxrs-resteasy-eap</generatorName>
              <apiPackage>test</apiPackage>
              <modelPackage>test.model</modelPackage>
              <modelNamePrefix>Api</modelNamePrefix>
              <generateSupportingFiles>false</generateSupportingFiles>
              <configOptions>
                <interfaceOnly>true</interfaceOnly>
                <swaggerDocketConfig>false</swaggerDocketConfig>
                <dateLibrary>java8</dateLibrary>
                <bigDecimalAsString>true</bigDecimalAsString>
                <sourceFolder>src/gen/java</sourceFolder>
              </configOptions>
            </configuration>
          </execution>

Steps to reproduce

Generate code with the above source spec and settings.

Suggest a fix

As described above, you could change the parameter to be the UriInfo, or leave out the parameter entirely and let implementors inject it at class level instead.
If there's a nicer solution to actually specify it as a Map<String, String>, that would be welcome, too, of course.