Optional query string enum parameter - openapi, springboot - spring-boot

Question

I have an OpenApi spec:
paths:
/lessons:
get:
tags:
- lesson
operationId: getLessons
parameters:
- in: query
name: daysOfWeek
schema:
type: array
items:
$ref: '#/components/schemas/DaysOfWeekEnum'
Using swagger codegen this generates an endpoint like:
#ApiOperation(value = "Get a collection lessons", nickname = "getLessons", notes = "", response = LessonDto.class, responseContainer = "List", tags={ "lesson", })
#ApiResponses(value = {
#ApiResponse(code = 200, message = "List of Lessons", response = LessonDto.class, responseContainer = "List") })
#RequestMapping(value = "/lessons",
produces = { "application/json" },
method = RequestMethod.GET)
default ResponseEntity<List<LessonDto>> _getLessons(#ApiParam(removed for brevity) #Valid #RequestParam(value = "daysOfWeek", required = false, defaultValue="new ArrayList<>()") List<DaysOfWeekEnum> daysOfWeek) {
return getLessons(daysOfWeek);
}
I use TestRestTemplate in a test like so:
ResponseEntity<List<LessonDto>> lessonDtos =
testRestTemplate.exchange("/lessons", HttpMethod.GET, null,
new ParameterizedTypeReference<List<LessonDto>>() {
});
This url works:
/lessons?daysOfWeek=THURSDAY
These urls do not:
/lessons
/lessons?daysOfWeek=SOME_INVALID_VALUE
...and I get the following error:
nested exception is
com.fasterxml.jackson.databind.exc.MismatchedInputException: Cannot
deserialize instance of java.util.ArrayList out of START_OBJECT
token
Any help appreciated.

Answer 1

In order to give you an empty list, you need to set the default value to be as an empty string:
#RequestParam(value = "daysOfWeek",
required = false,
defaultValue = "") List<DaysOfWeekEnum> daysOfWeek)

Answer 2

So the issue was related to an openapi-generator bug. Summary being:
When parameters of type array are added to an operation, the generated
Spring code includes an invalid defaultValue in the Spring MVC
parameter annotations
The fix was to upgrade to a later version of openapi-generator - 4.0.0 did the trick for me.
As an aside, the error message:
exception is
com.fasterxml.jackson.databind.exc.MismatchedInputException: Cannot
deserialize instance of java.util.ArrayList out of START_OBJECT token
..was a bit of a red herring and it was actually TestRestTemplate related i.e. the ParameterizedTypeReference part. Changing this to String.class identified the true nature of the error.