```mongoOperations.findAndModify(searchQuery, upda...

# getting-started

[DMITRY.]

mongoOperations.findAndModify(searchQuery, update, T::class.java)

Cannot use 'T' as reified type parameter. Use a class instead.

How make it work?

[Roukanken]

where are you calling it from?

you can't get class of type parameter, unless it's reified in inline function

so if you are calling it from generic function, consider calling making it reified inline
if you are calling it from a generic class or very complex function, you need to pass KClass<T> as parameter to it too, and use that instead of T::class

[DMITRY.]

I tried this:

@PatchMapping(value = ["{taskId}"], consumes = ["application/json"])
    fun updateTask(@PathVariable("taskId") taskId: ObjectId, @RequestBody task: T, taskClass: KClass<T>): ResponseEntity<Void> {
        val document = Document()
        mongoOperations.converter.write(task, document)
        val update = Update()
        document.forEach { key: String, value: Any -> update[key] = value }
        mongoOperations.findAndModify(Query(Criteria("_id").`is`(taskId)), update, taskClass.java)
        return ResponseEntity<Void>(HttpStatus.NO_CONTENT)
    }

[DMITRY.]

And then on the line with findAndModify I get this exception 😕

java.lang.ClassCastException: class jdk.proxy3.$Proxy99 cannot be cast to class kotlin.jvm.internal.ClassBasedDeclarationContainer (jdk.proxy3.$Proxy99 is in module jdk.proxy3 of loader org.springframework.boot.devtools.restart.classloader.RestartClassLoader @2a01e51d; kotlin.jvm.internal.ClassBasedDeclarationContainer is in unnamed module of loader 'app')

[Roukanken]

not sure what's the error, but Spring won't fill that taskClass argument properly, will it...
won't it just pass null there?

[DMITRY.]

yeah

println(taskClass)
{}

[Roukanken]

I assume the T is generics of the class containing this
how exactly are you using this? Since no idea wth Spring does with generic controllers...

is it just some base controller that you are extending from others?

[Roukanken]

eg I assume this is in class GenericController<T>
and then you have classes like class AbcController : GenericController<Abc>() ?

[DMITRY.]

Yeah

I have an abstract class

abstract class Task(
    val id: ObjectId?,
    var deadline: java.time.LocalDateTime?,
)

Then classes like this

class Change(
    id: ObjectId?,
    deadline: LocalDateTime?,
    worker: ObjectId?,
) : Task(id, deadline)

Interface:

interface TaskController<T: Any> {
    val mongoOperations: MongoOperations

    @PatchMapping(value = ["{taskId}"], consumes = ["application/json"])
    fun updateTask(@PathVariable("taskId") taskId: ObjectId, @RequestBody task: T, taskClass: KClass<T>): ResponseEntity<Void> {
...
        mongoOperations.findAndModify(Query(Criteria("_id").`is`(taskId)), update, taskClass.java)
...
    }
}

And then controller:

@RestController
@RequestMapping("change")
class ChangeController(
    override val taskRepository: ChangeRepository,
    override val mongoOperations: MongoOperations,
) : TaskController<Change>

[DMITRY.]

It works If I specify directly inside interface

mongoOperations.findAndModify(Query(Criteria("_id").`is`(taskId)), update, Change::class.java)

[Roukanken]

well one solution I can think of it to change TaskController

abstract class TaskController<T: Any> (
    private val taskClass: KClass<T>
) {
    // ...
}

then use it like this from the controllers that extend it:

class ChangeController : TaskController<Change> (Change::class) {

}

tiny bit of repeating yourself, but it would definitely work

[Roukanken]

or if you really want to leave it interface: (ofc, more repeating)

interface TaskController<T: Any> {
    val taskClass: KClass<T>
}

class ChangeController : TaskController<Change> {
    override val taskClass = Change::class
}

tho I think it's definitelly an abstract class in spirit, even in your code now, not just an interface

[DMITRY.]

Oh wow, it works!

[DMITRY.]

Thank you very much!

[Roukanken]

basically the idea is, that the generic class has to request the T's class from w/e uses it (by constructor, or property, or function, or...)
if user was using it, you could hide it in constructor, using reified inline factory function

but here you can just specify it explicitly instead