[BUG][JAVA] IllegalArgumentException while parsing responses with unknown fields ❗

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

API client libraries generated with OpenAPI Generator should ignore response fields that were not existing at the time the code was generated, instead of raising the following error:

java.lang.IllegalArgumentException: The field `updated` in the JSON string is not defined in the `Dog` properties. JSON: {"name":"Luna","created":"2021-04-23T17:32:28Z","updated":"2021-04-23T17:32:28Z"}

openapi-generator version

OpenAPI declaration file content or url

• sample-v1.yaml
• sample-v2.yaml

Download ⤓

Generation Details

• java -jar openapi-generator-cli-5.4.0.jar generate -g java -i sample-v1.yaml -c config.json -o api_client_5.4
• java -jar openapi-generator-cli-6.0.0.jar generate -g java -i sample-v1.yaml -c config.json -o api_client_6.0

{ 
    "groupId" : "com.acme.app",
    "artifactId" : "sample-api-client",
    "artifactVersion" : "1.0.0",
    "invokerPackage" : "com.acme.app",
    "apiPackage" : "com.acme.app.api",
    "modelPackage" : "com.acme.app.model"
}

Steps to reproduce

1. Generate a client from sample-v1.yaml:

java -jar openapi-generator-cli-6.0.0.jar generate -g java -i sample-v1.yaml -c config.json -o api_client_6.0

2. Update DogsApiTest:

public class DogsApiTest {

    @Test
    public void addDogTest() throws ApiException {
        // GIVEN
        ApiClient client = new ApiClient();
        client.setDebugging(true);
        client.setBasePath("http://localhost:4010");
        NewDog newDog = new NewDog()
                .name("Bella");

        // WHEN
        Dog dog = new DogsApi(client).addDog(newDog);

        // THEN
        assertNotNull(dog);
    }

}

3. Run prism.exe mock sample-v1.yaml
4. Run addDogTest => ✔️
5. Run prism.exe mock sample-v2.yaml
6. Run addDogTest => ❌

java.lang.IllegalArgumentException: The field `updated` in the JSON string is not defined in the `Dog` properties. JSON: {"name":"Luna","created":"2021-04-23T17:32:28Z","updated":"2021-04-23T17:32:28Z"}

	at com.acme.app.model.Dog.validateJsonObject(Dog.java:217)
	at com.acme.app.model.Dog$CustomTypeAdapterFactory$1.read(Dog.java:253)
	at com.acme.app.model.Dog$CustomTypeAdapterFactory$1.read(Dog.java:243)
	at com.google.gson.TypeAdapter$1.read(TypeAdapter.java:199)
	at com.google.gson.Gson.fromJson(Gson.java:991)
	at com.google.gson.Gson.fromJson(Gson.java:956)
	at com.google.gson.Gson.fromJson(Gson.java:905)
	at com.acme.app.JSON.deserialize(JSON.java:151)
	at com.acme.app.ApiClient.deserialize(ApiClient.java:845)
	at com.acme.app.ApiClient.handleResponse(ApiClient.java:1053)
...

Start by generating the Java client from sample-v1.yaml and sample-v2.yaml, then inspect the generated Dog.java validateJsonObject path and the DogsApiTest reproduction. Trace the failure through JSON.deserialize and ApiClient.deserialize. Done means the generated client accepts the added updated response field without raising IllegalArgumentException.