[BUG] [JAVA] Java Spring stub generation generates code that does not compile when using callbacks in the api

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?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

The generation of a Java Spring stub for a json API that contains callbacks seems broken: java code is generated, but does not compile.

openapi-generator version

This is a regression detected in versions 6.4.0 and later.

• 6.1.0 : OK
• 6.3.0 : OK
• 6.4.0 : KO
• 6.6.0 : KO
• 7.0.0-SNAPSHOT (latest master from 2023/05/27): KO

OpenAPI declaration file content or url

The bug has been reproduced with the exact content of https://swagger.io/docs/specification/callbacks/ tutorial:

openapi: 3.0.0
info:
  version: 0.0.0
  title: test
paths:
  /subscribe:
    post:
      summary: Subscribe to a webhook
      requestBody:
        required: true
        content:
          application/json:
            schema:
              type: object
              properties:
                callbackUrl:   # Callback URL
                  type: string
                  format: uri
                  example: https://myserver.com/send/callback/here
              required:
                - callbackUrl
      callbacks:   # Callback definition
        myEvent:   # Event name
          '{$request.body#/callbackUrl}':   # The callback URL,
                                            # Refers to the passed URL
            post:
              requestBody:   # Contents of the callback message
                required: true
                content:
                  application/json:
                    schema:
                      type: object
                      properties:
                        message:
                          type: string
                          example: Some event happened
                      required:
                        - message
              responses:   # Expected responses to the callback message
                '200':
                  description: Your server returns this code if it accepts the callback
      responses:
        '201':
          description: Webhook created

Generation Details

Language: Java
Java version: openjdk 17.0.7 2023-04-18

Detailed command line used for stub generation: java -jar openapi-generator-cli-7.0.0-SNAPSHOT.jar generate -g spring -i apis.yaml -o stub-spring -p generateAliasAsModel=false -p returnSuccessCode=true -p artifactId=spring_stub -p artifactVersion="0.0.1"

Steps to reproduce

• Execute openapi-generator-cli with command line
  java -jar openapi-generator-cli-6.6.0.jar generate -g spring -i apis.json -o stub-spring -p generateAliasAsModel=false -p returnSuccessCode=true -p artifactId=spring_stub -p artifactVersion="0.0.1"

=> The generation produces code , but also emits warnings that looks related:

[main] WARN  o.o.codegen.DefaultCodegen - Empty operationId found for path: post /subscribe. Renamed to auto-generated operationId: subscribePost
[main] WARN  o.o.codegen.DefaultCodegen - Empty operationId found for path: post myEvent_. Renamed to auto-generated operationId: myEvent_Post
[main] WARN  o.o.codegen.DefaultCodegen - The following schema has undefined (null) baseType. It could be due to form parameter defined in OpenAPI v2 spec with incorrect consumes. A correct 'consumes' for form parameters should be 'application/x-www-form-urlencoded' or 'multipart/?'
[main] WARN  o.o.codegen.DefaultCodegen - schema: class ObjectSchema {
    class Schema {
        type: object
        format: null
        $ref: null
        description: null
        title: null
        multipleOf: null
        maximum: null
        exclusiveMaximum: null
        minimum: null
        exclusiveMinimum: null
        maxLength: null
        minLength: null
        pattern: null
        maxItems: null
        minItems: null
        uniqueItems: null
        maxProperties: null
        minProperties: null
        required: [callbackUrl]
        not: null
        properties: {callbackUrl=class StringSchema {
            class Schema {
                type: string
                format: uri
                $ref: null
                description: null
                title: null
                multipleOf: null
                maximum: null
                exclusiveMaximum: null
                minimum: null
                exclusiveMinimum: null
                maxLength: null
                minLength: null
                pattern: null
                maxItems: null
                minItems: null
                uniqueItems: null
                maxProperties: null
                minProperties: null
                required: null
                not: null
                properties: null
                additionalProperties: null
                nullable: null
                readOnly: null
                writeOnly: null
                example: https://myserver.com/send/callback/here
                externalDocs: null
                deprecated: null
                discriminator: null
                xml: null
            }
        }}
        additionalProperties: null
        nullable: null
        readOnly: null
        writeOnly: null
        example: null
        externalDocs: null
        deprecated: null
        discriminator: null
        xml: null
    }
}
[main] WARN  o.o.codegen.DefaultCodegen - codegenModel is null. Default to UNKNOWN_BASE_TYPE

• Try packaging with maven: cd stub-spring && mvn clean package

=> Compilation fails with following errors:

[ERROR] COMPILATION ERROR : 
[ERROR] stub-spring/src/main/java/org/openapitools/api/SubscribeApi.java:[8,30] cannot find symbol
  symbol:   class UNKNOWN_BASE_TYPE
  location: package org.openapitools.model
[ERROR] stub-spring/src/main/java/org/openapitools/api/SubscribeApi.java:[64,103] cannot find symbol
  symbol:   class UNKNOWN_BASE_TYPE
  location: interface org.openapitools.api.SubscribeApi
[ERROR] stub-spring/src/main/java/org/openapitools/api/SubscribeApiController.java:[3,30] cannot find symbol
  symbol:   class UNKNOWN_BASE_TYPE
  location: package org.openapitools.model
[ERROR] Failed to execute goal org.apache.maven.plugins:maven-compiler-plugin:3.10.1:compile (default-compile) on project spring_stub: Compilation failure: Compilation failure: 
[ERROR] stub-spring/src/main/java/org/openapitools/api/SubscribeApi.java:[8,30] cannot find symbol
[ERROR]   symbol:   class UNKNOWN_BASE_TYPE
[ERROR]   location: package org.openapitools.model
[ERROR] stub-spring/src/main/java/org/openapitools/api/SubscribeApi.java:[64,103] cannot find symbol
[ERROR]   symbol:   class UNKNOWN_BASE_TYPE
[ERROR]   location: interface org.openapitools.api.SubscribeApi
[ERROR] stub-spring/src/main/java/org/openapitools/api/SubscribeApiController.java:[3,30] cannot find symbol
[ERROR]   symbol:   class UNKNOWN_BASE_TYPE
[ERROR]   location: package org.openapitools.model
[ERROR] -> [Help 1]
[ERROR] 
[ERROR] To see the full stack trace of the errors, re-run Maven with the -e switch.
[ERROR] Re-run Maven using the -X switch to enable full debug logging.
[ERROR] 
[ERROR] For more information about the errors and possible solutions, please read the following articles:
[ERROR] [Help 1] http://cwiki.apache.org/confluence/display/MAVEN/MojoFailureException

Related issues/PRs

None identified (perhaps #800 ?)

Suggest a fix

Produced code should compile successfully ("BUILD SUCCESS")

Reproduce the issue with the openapi-generator CLI using the callback specification, then inspect the generated SubscribeApi.java and SubscribeApiController.java files for the UNKNOWN_BASE_TYPE references. Run mvn clean package in the generated stub and consider the issue complete when the Spring stub builds successfully without those compilation errors.