Locking down the serialVersionUID in Kotlin - kotlin

Question

I've been battling the whole morning to lock down the serialVersionUID in a Kotlin class. I have a BaseModel which is extended by Project
abstract class BaseModel<T>(
var id: Int? = null,
private val fileName: String,
private val data: MutableList<T>,
private val indices: MutableMap<Int, T>
) : Serializable {
...
protected fun writeToDisk() {
val oos = ObjectOutputStream(BufferedOutputStream(FileOutputStream(fetchFileName())) )
oos.writeObject(fetchData());
oos.close();
}
}
And the project class:
class Project(
var name: String = "",
var repo: String = ""
) : BaseModel<Project>(
data = Data.projects,
indices = Data.projectsIndex,
fileName = "data/projects.dat"
), Serializable {
...
override fun toString(): String {
return "Project: id=${id}, name=${name}, repo=${repo}"
}
}
Every time I write to Disk and then change anything in the class and try to read it back again, I would get:
java.io.InvalidClassException: com.jvaas.bob.model.Project; local
class incompatible: stream classdesc serialVersionUID =
4156405178259085766, local class serialVersionUID =
2024101567466310467
I've tried adding:
private val serialVersionUID: Long = 1
to all classes with no effect.
Some examples on StackOverflow were using serialVersionUid which had no effect either (I believe this is intelliJ lowercasing the last two letters for some reason)
#JvmStatic doesn't work here since it's not an object, I've tried making it non-private with no success.

Answer 1

You can define serialVersionUID as a constant in a companion object:
abstract class BaseModel<T> : Serializable {
companion object {
private const val serialVersionUID: Long = -1
}
}
Constants are compiled to fields, and fields of a companion are stored as static fields of the class that contains companion. Therefore you get what you need – a private static field serialVersionUID in your serializable class.

Answer 2

The solution was actually much simpler than I thought, use a companion object. This now serializes perfectly and if I add more fields, it still serializes to disk and deserializes unless I change the serialVersionUID
Base:
abstract class BaseModel<T>(
var id: Int? = null,
private val fileName: String,
private val data: MutableList<T>,
private val indices: MutableMap<Int, T>
) : Serializable {
companion object {
#JvmStatic private val serialVersionUID: Long = 1
}
...
}
Project:
class Project(
var name: String = "",
var repo: String = ""
) : BaseModel<Project>(
data = Data.projects,
indices = Data.projectsIndex,
fileName = "data/projects.dat"
), Serializable {
companion object {
#JvmStatic private val serialVersionUID: Long = 1
}
override fun toString(): String {
return "Project: id=${id}, name=${name}, repo=${repo}"
}
}

Answer 3

Install this plugin: GenerateSerialVersionUID，use plugin to auto generate default serial version uid，usage: Click here.