[BUG] Description

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?
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Description

Spring Validation throws binding error for class with JsonNullable(from openAPI)

openapi-generator version

org.openapi.generator=5.3.0

OpenAPI declaration file content or url

paths:
  patch:
    summary: Update user properties
    description: Allow to update user properties
    operationId: updateUserInfo
    tags:
    - user info
    requestBody:
      content:
        'application/json':
          schema:
            $ref: '#/components/schemas/UserDetails'
        schemas:
        UserDetails:
          description: Details of the user
          type: object
          properties:
            address:
              nullable: true
              allOf:
              - $ref: '#/components/schemas/Address'
            name:
              type: string
              description: name of the user
              example: "John"
            phone:
              type: string
              description: Phone number of the user
              example: "8795678633"
              nullable: true
        Address:
          type: object
          required:
          - country
          properties:
            country:
              type: string
              example: "France"
            state:
              type: string
              example: "Lyon"
            district:
              type: string
              example: 'LL'

Generation Details

There are two classes generated classes from openAPI,

UserDetails class holds,

@JsonProperty("address")
private JsonNullable<Address> address = JsonNullable.undefined();

@JsonProperty("name")
private String name;

@JsonProperty("phone")
private JsonNullable<String> phone = JsonNullable.undefined();

Address class holds

@JsonProperty("country")
private String country;

@JsonProperty("state")
private String state;

@JsonProperty("district")
private String district;

@ApiModelProperty(example = "France", required = true, value = "")
@NotNull
public String getCountry() {
	return country;
}

In short, we have country as not null and address holding the country fields is wrapped by JsonNullable.

Steps to reproduce

We have a rest endpoint that takes in the UserDetails as the payload. When we send a payload without the country value in address, similar to the below

{
    "name": "John",
    "address": {
        "state": "Lyon",
        "district": "LL"
    },
    "phone" : "+44232309923"
}

We get a spring binding error,
JSR-303 validated property 'address.country' does not have a corresponding accessor for Spring data binding - check your DataBinder's configuration (bean property versus direct field access)

What happened behind the scenes?

1. Spring identifies the required constraint violation for country.
2. In order to bind the violation to the field variable, Spring searches for the wrapper class of country.
3. Spring search happens in two steps
   a. Fetches the bean wrapper class
   b. Fetches the getter method in the wrapper class for the field violating the constraint
4. In an ideal scenario, it was expecting Address class which is the wrapper class in our case.
5. Since Address is wrapped by JsonNullable, spring returns JsonNullable as the wrapper instead of Address.
6. The getter for country is obviously not within the JsonNullable class, and so the error is thrown.

Is there a workaround for this? Or am I missing anything here?

Start by reproducing the Spring binding failure with the provided OpenAPI schema, generated UserDetails and Address classes, and the JSON payload that omits address.country. Trace how JsonNullable wraps Address during validation; done means the required nested country violation is handled without the reported accessor error.