Return JSON serialized boolean from Spring Boot RESTful endpoint

I'm writing a Spring Boot backend for a Kotlin multiplatform ktor client. In most cases this works pretty well but I have one issue with serializing and deserialization booleans. This is my endpoint on server-side:

@GetMapping("/api/v1/user/existsByUsername/{username}", produces = [MediaType.APPLICATION_JSON_VALUE])
@ResponseBody
fun userExistsByUsername(@PathVariable("username") username: String) : ResponseEntity<Boolean> {
    val exists = userService.usernameExists(username)
    return ResponseEntity.status(HttpStatus.OK).body(exists)
}

I am expecting to get a result from this endpoint like {true} but the client receives the body true - and can't deserialize it. Can I force the endpoind in a simple way to add the curly brackets? Or is there a way to tell ktor to deserialization basic types as non-JSON?

Solution

In Spring Boot, primitive types (like boolean or integer) are serialized directly as a pure value in the response body, without being wrapped in an object (so you get true and false).

Make a class in your kotlin code

data class BooleanResponse(val value: Boolean)

then, dear @padmalcom, you can write something like

@GetMapping("/api/v1/user/existsByUsername/{username}", produces = [MediaType.APPLICATION_JSON_VALUE])
@ResponseBody
fun userExistsByUsername(@PathVariable("username") username: String): ResponseEntity<BooleanResponse> {
    val exists = userService.usernameExists(username)
    return ResponseEntity.status(HttpStatus.OK).body(BooleanResponse(exists))
}

that uses your class.

So your JSON return (rest endpoint) will be:

{
  "value": true
}

with curly braces.

Ktor side then:

@Serializable
data class BooleanResponse(val value: Boolean)

val response: BooleanResponse = client.get("http://backend/api/v1/user/existsByUsername/someusername")
println(response.value) // here you get something like "true"

Fast & ̶f̶u̶r̶i̶o̶u̶s̶ simple