[java, spring]: codegen.v3:swagger-codegen-maven-plugin does not create @JsonTypeInfo and @JsonSubTypes annotations if polymorphism is used

Description

We are using polymorphism like the famous Pet/Cat/Dog example. In effect, the problem occurs even if you use the example code exactly as provided (see below in "Swagger generation file"

In order to use these classes correctly as body-input in Rest-class, the base class (Pet) must be annotated with @Json-related annotations like this

@JsonTypeInfo(use = JsonTypeInfo.Id.NAME, property = "type")
@JsonSubTypes({
@JsonSubTypes.Type(value = Cat.class, name = "Cat"),
@JsonSubTypes.Type(value = Dog.class, name = "Dog")})

The io.swagger.codegen.v3:swagger-codegen-maven-plugin simply doesn't create this annotations.

The interesting thing (which I figured out after several hours of research and Try-and-Error): if I use the (older) io.swagger:swagger-codegen-maven-plugin, it works.

Swagger-codegen version

We tested several versions of io.swagger.codegen.v3:swagger-codegen-maven-plugin, latest was 3.0.19. But also 3.0.3 doesn't do it better.

The latest tested version of io.swagger:swagger-codegen-maven-plugin was 2.4.17. As I said, this one works.

Swagger declaration file content or url

      "Pet": {
      "type": "object",
      "discriminator": "petType",
      "properties": {
        "name": {
          "type": "string"
        },
        "petType": {
          "type": "string"
        }
      },
      "required": [
        "name",
        "petType"
      ]
    },
    "Cat": {
      "description": "A representation of a cat",
      "allOf": [
        {
          "$ref": "#/definitions/Pet"
        },
        {
          "type": "object",
          "properties": {
            "huntingSkill": {
              "type": "string",
              "description": "The measured skill for hunting",
              "default": "lazy",
              "enum": [
                "clueless",
                "lazy",
                "adventurous",
                "aggressive"
              ]
            }
          },
          "required": [
            "huntingSkill"
          ]
        }
      ]
    },
    "Dog": {
      "description": "A representation of a dog",
      "allOf": [
        {
          "$ref": "#/definitions/Pet"
        },
        {
          "type": "object",
          "properties": {
            "packSize": {
              "type": "integer",
              "format": "int32",
              "description": "the size of the pack the dog is from",
              "default": 0,
              "minimum": 0
            }
          },
          "required": [
            "packSize"
          ]
        }
      ]
    },

Command line used for generation

           <plugin>
                <groupId>io.swagger.codegen.v3</groupId>
                <artifactId>swagger-codegen-maven-plugin</artifactId>
                <version>3.0.19</version>
                <executions>
                    <execution>
                        <id>api</id>
                        <phase>process-sources</phase>
                        <goals>
                            <goal>generate</goal>
                        </goals>
                        <configuration>
                            <inputSpec>
                                ${file.location}
                            </inputSpec>
                            <language>java</language>
                            <configOptions>
                                <library>jersey2</library>
                                <interfaceOnly>true</interfaceOnly>
                                <useTags>true</useTags>
                                <dateLibrary>java8</dateLibrary>
                            </configOptions>
                            <apiPackage>${default.package}</apiPackage>
                            <modelPackage>${default.package}.model</modelPackage>
                        </configuration>
                    </execution>
                </executions>
            </plugin>

Note that for <language> I also tested "spring". For <library>, I tested several values given with '<configHelp>true</configHelp>'

Steps to reproduce

call mvn generate and see the result Pet.java class

Related issues/PRs

Suggest a fix/enhancement

Start by running mvn generate with the provided Swagger declaration and plugin configuration, then inspect the generated Pet.java class. Trace the Java generator's model and template handling for discriminator-based polymorphism. Done means generated base models include the expected Jackson polymorphism annotations and the result is covered by a reproducible generation check.