[spring] Invalid default values for arrays and maps on API method declarations

Created by: rubms

Description

When parameters of type array are added to an operation, the generated Spring code includes an invalid defaultValue in the Spring MVC parameter annotations. For instance, an array of strings query parameter would generate a Spring code like the following:

@Valid @RequestParam(value = "stringArrayParam", required = false, default="new ArrayList<>()") List<String> stringArrayParam

That generated defaultValue is invalid: the Spring MVC application will throw an error when a request is performed with no value specified for the parameter.

openapi-generator version

Reproduced with openapi-generator v3.3.3 and previous versions.

OpenAPI declaration file content or url

openapi: 3.0.0
info:
  description: "Spec for testing the default value of array parameters"
  version: 1.0.0
  title: Array parameters test case.
paths:
  /pets:
    get:
      operationId: getPets
      summary: Pets.
      description: "Get pets."
      parameters:
      - name: stringArrayParam
        in: query
        description: "Test array query parameter"
        required: false
        schema:
          type: array
          items:
            type: string
      tags:
      - Pets
      responses:
        200:
          description: Successful request.
components:
  schemas:
    ExampleObject:
      type: object
      properties:
        arrayProperty:
          type: array
          items:
            type: string

Command line used for generation

java -jar openapi-generator-cli-3.3.3.jar generate -g spring -i test.yml -o test

Additionally, the configuration property library is set to spring-mvc:

{
	"library": "spring-mvc"
}

Steps to reproduce

1. Generate a Spring-MVC server application from the previous specification.
2. Run the application.
3. Throw a GET request to the /pets path (getPets operation) leaving the stringArrayParam query parameter blank or unspecified.

The server will throw the following error:

Related issues/PRs

This is related to #890, though not the same issue.

Suggest a fix/enhancement

The problem is that there are 2 different cases with respect to default values:

• The default value specified by the user, that should be included as default value in the Spring MVC API method annotations.
• The default variable initializer, which may be affected by the default value specified by the user but differs in data-types like arrays or maps.

However the Spring code generator uses the same value (the default variable initializer, in my opinion) for both: initializing variables and specifying Spring MVC annotations.

The Spring MVC code generator must use a different "default value" for both cases.