Add support for Kotlin Value Classes

[mp911de]

We now support instantiation and mutation of types using Kotlin Value (@JvmInline) classes:

@JvmInline
value class MyValueClass(val id: String)

data class WithMyValueClass(val id: MyValueClass) { }

class WithMyValueClass {
  private final String id;

  WithMyValueClass(String id){…}
  // getters, setter, copy
}

Value classes are typically flattened into their value type (component type) if the type at the declaration site does not change. Nullable value classes will be boxed.

@JvmInline
value class MyNullableValue(val id: String?)

data class WithMyNullableValue(val id: MyNullableValue?) { }

class MyNullableValue {
  private final MyNullableValue id;

  WithMyValueClass(MyNullableValue id){…}
  // getters, setter, copy
}

Boxed values result in being recognized as entities.

The following components are affected by this change:

• KotlinBeanInfoFactory
• Reflective and class-generating EntityInstantiator
• Reflective and class-generating PersistentPropertyAccessorFactory
  • Kotlin reflection (or the inline-class compilation) suffers from a bug preventing the use of copy methods: https://youtrack.jetbrains.com/issue/KT-57357

Closes #2806

[christophstrobl]

@sdeleuze what's your take on moving this one to core framework?

[sdeleuze]

I have in my todo to have a broader look to value class support in Framework, I will share my feedback on this specific point after and comment here.

[odrotbohm]

I wonder if more of the underlying Kotlin class analysis code should / would need to move to Spring Framework, so that the standard framework binding subsystem was enabled to populate these types as well?

[christophstrobl]

merged to main development line.

[udalov]

@mp911de Sorry for trespassing, but I believe this might work incorrectly in case there are unrelated properties in the class. For example:

@JvmInline
value class Z(val value: String)

data class D(val x: Int) {
    val unrelated1 = Z("1")

    val Any.unrelated2: Z get() = Z("2")
}

hasValueClassProperty will return true for the class D, but it probably should be false (if I understood call sites correctly).

I suppose you're interested in finding only if any property in the primary constructor has a value class type? For that you need to take only non-extension properties (i.e. KClass.memberProperties) whose names are equal to the parameters of the primary constructor.

[udalov]

@mp911de Similarly to the above, this should use memberProperties, otherwise it'll fail on something like this:

@JvmInline
value class Z(val value: String) {
    val String.unrelated: String get() = this
}

[udalov]

@mp911de There's no guarantee that the first constructor is the primary one, nor that it has any parameters, so this can fail on something like this:

@JvmInline
value class Z(val value: String) {
    constructor() : this("Fail")
}

I recommend to use primaryConstructor instead.

[udalov]

@mp911de You can use jvmErasure for this.