[BUG][Java] additionalProperties with properties generates incorrect/broken model

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

When a type in schema has a property that has both additionalProperties and properties, the Java engine generates broken code. For the given example, the following code was generated:

  public Pet putFoobarItem(String key, String foobarItem) {
    if (this.foobar == null) {
      this.foobar = ;
    }
    this.foobar.put(key, foobarItem);
    return this;
  }

It is obvious that the code is broken.

For other engines, the generated code also does not do what the schema specifies, i.e. typechecks for specified properties, and the presence check for required properties.

openapi-generator version

5.4.0

OpenAPI declaration file content or url

components:
  schemas:
    Pet:
      type: object
      required:
        - id
        - name
      properties:
        id:
          type: integer
          format: int64
        name:
          type: string
        foobar:
          type: object
          additionalProperties:
            type: string
          properties:
            fooint:
              type: integer
            fooboolean:
              type: boolean
            foostring:
              type: string
            barint:
              type: integer
          required:
            - fooint
            - fooboolean
            - foostring

Full YAML file: https://gist.github.com/visualage/202b8865fb684097fb5a8d9bd7c7b2ff

Generation Details

java -jar openapi-generator-cli-5.4.0.jar generate -g java -i bad.yaml -o java

Steps to reproduce

Use the command line above. Check the generated file java/src/main/java/org/openapitools/client/model/Pet.java.

Related issues/PRs

None.

Suggest a fix

We should generate a specific model class when the property type is object and has properties defined, regardless if additionalProperties is there or not. If additionalProperties is there, we need to enhance the class so that any unrecognized properties would be put into a special map property. This can be done in Java Jackson by @JsonAnyGetter and @JsonAnySetter annotations. I am not familiar with other engine/language/libraries, so can't speak for them.

Reproduce the issue with the provided YAML and the Java generation command, then inspect java/src/main/java/org/openapitools/client/model/Pet.java for the broken initialization and missing property handling. Done means the generated model correctly represents declared properties, required-property checks, and additional properties without broken Java code.