Add the following dependency to your project’s pom.xml file:

<dependency>
  <groupId>io.quarkiverse.openapi.generator</groupId>
  <artifactId>quarkus-openapi-generator-server</artifactId>
  <version>3.0.0-SNAPSHOT</version>
</dependency>

Note that since this extension has not been yet released, you’ll need a local build of the dependency.

You will also need to add or update the quarkus-maven-plugin configuration with the following:

You probably already have this configuration if you created your application with Code Quarkus. That said, double-check your configuration not to add another plugin entry. 
<plugin>
  <groupId>io.quarkus</groupId>
  <artifactId>quarkus-maven-plugin</artifactId>
  <extensions>true</extensions>
  <executions>
    <execution>
      <goals>
        <goal>build</goal>
        <goal>generate-code</goal>
        <goal>generate-code-tests</goal>
      </goals>
    </execution>
  </executions>
</plugin>

Now, create the directory openapi under your src/main/resources path and add the OpenAPI spec files there. We support JSON, YAML and YML extensions. You have to define the specification used for code generation with the following property:

quarkus.openapi.generator.spec=petstore-openapi.json

If you want to change the directory where OpenAPI files must be found, use the property quarkus.openapi.input-base-dir. IMPORTANT: it is relative to the project base directory. For example, if you have a project called MyJavaProject and decide to place them in MyJavaProject/openapi-definitions, use the following property:

quarkus.openapi.generator.input-base-dir=openapi-definitions

If a base package name is not provided, it will be used the default io.apicurio.api. You can customize it with the following property:

quarkus.openapi.generator.base-package=io.petstore

By default, the extension generates non-reactive code. If you would like to change it, you can do it as follows:

quarkus.openapi.generator.reactive=true

Run mvn compile to generate your classes in target/generated-sources/jaxrs path:

- io.petstore
  - beans
    - Address.java
    - ApiResponse.java
    - Category.java
    - Customer.java
    - Order.java
    - Pet.java
    - Tag.java
    - User.java
  - PetResource.java
  - StoreResource.java
  - UserResource.java

You can reference the generated code in your project, for example:

package io.petstore;

import java.io.InputStream;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

import io.petstore.beans.ApiResponse;
import io.petstore.beans.Pet;

public class PetStoreImpl implements PetResource {

    private static final Map<Long, Pet> PETS = new HashMap<>();

    @Override
    public Pet updatePet(Pet data) {
        return PETS.put(data.getId(), data);
    }

    @Override
    public Pet addPet(Pet data) {
        return PETS.put(data.getId(), data);
    }

    @Override
    public List<Pet> findPetsByStatus(String status) {
        return null;
    }

    @Override
    public List<Pet> findPetsByTags(List<String> tags) {
        return null;
    }

    @Override
    public Pet getPetById(long petId) {
        return PETS.get(petId);
    }

    @Override
    public void updatePetWithForm(long petId, String name, String status) {

    }

    @Override
    public void deletePet(String apiKey, long petId) {
        PETS.remove(petId);
    }

    @Override
    public ApiResponse uploadFile(long petId, String additionalMetadata, InputStream data) {
        return null;
    }
}

See the integration-tests module for more information of how to use this extension. Please be advised that the extension is on experimental, early development stage.