How validation is done? for example we need to check that a property length should not be more than 100 characters

You can do it in

init

blocks which are executed after deserialization

usually on deserialization we want to collect all validation errors and return them in response, in this case validation about not null and other validation will be separated, so we can not collect all errors at once is this correct?

not-null validation is performed automatically by the framework

I mean for example we have a class

class Data(val name: String, val age: Int, val email: String)

when we send a json

{"email": "<mailto:xyz@xyz.com|xyz@xyz.com>", "name": "A"}

we should return a response where to indicate that

age

can not be null and

name

size must be greater than 2 Is this possible?

I think yes. If you throw, say

ValidationException

from init block of

Data

, you can write

try {
   deserialize(request)
} catch (e: MissingFieldException) {
...
} catch (e: ValidationException) {
...
}

but how we will pass the constructor if we do not have value for

age

field?

Ah, I got it. Yes, you will get only first exception in this case, unfortunately

Oh, bad 😞 BTW, what do you think about to do validation before object creation? We had an interesting thread today on similar issue https://kotlinlang.slack.com/conversation/C0B8Q383C/p1545658515001900

It would be complicated to design, because validation is natural to do in

init

block, and its execution likely requires already constructed object...