[JAVA] Generate java.util class as schema definition instead

Description

When using swagger 2.0, the java.util.Currency class got generated as a seperate definition. But when we generate OpenAPI 3.0, we run into the issue that swagger-core generates it as a property.

Generation of the OpenAPI 3.0 specification from the classes

We have f.e. this class:

import java.util.Currency; 

public class Wrapper {
   private Currency currency;
}

From this code we generate the openapi specification with the following plugin configuration:

      <plugin>
            <groupId>io.swagger.core.v3</groupId>
            <artifactId>swagger-maven-plugin</artifactId>
            <version>2.0.9</version>
            <configuration>
                <outputFileName>openapi</outputFileName>
                <outputFormat>YAML</outputFormat>
                <outputPath>....</outputPath>
                <resourcePackages>...</resourcePackages>
            </configuration>
            <executions>
                <execution>
                    <phase>compile</phase>
                    <goals>
                        <goal>resolve</goal>
                    </goals>
                </execution>
            </executions>
        </plugin>

When generated this will result in this component definition:

components:
  schemas:
    Wrapper:
      type: "object"
      properties:
        currency:
          type: object
          properties:
            currencyCode:
              type: string
            defaultFractionDigits:
              type: integer
              format: int32
            numericCode:
              type: integer
              format: int32
            displayName:
              type: string
            symbol:
              type: string

This in turn when generated by the openapi-generator maven plugin, will result in a WrapperCurrency class. In swagger 2.x we could specify an --import-mappings to map the Currency definition to java.util.Currency. This is not possible on properties.

So, is there any way to f.e. annotate the property in such a way that the java.util.Currency will be generated as a schema? something along the lines of:

components:
  schemas:
    Wrapper:
      type: "object"
      properties:
        currency:
           $ref: "components/schemas/Currency"

    Currency:
      type: object
      properties:
        currencyCode:
          type: string
        defaultFractionDigits:
          type: integer
          format: int32
        numericCode:
            type: integer
            format: int32
        displayName:
          type: string
        symbol:
          type: string

Swagger-codegen version

2.0.9

Command line used for generation

mvn clean install

Start with the swagger-core OpenAPI 3 generation path and reproduce the issue using the supplied Wrapper and java.util.Currency example. Compare its output with Swagger 2.0 behavior; done when Currency is emitted as a reusable component schema that openapi-generator can target with an import mapping.