How to hide Kotlin's lateinit var backing field from Java?

Question

In Kotlin, suppose, I have class:

class MyKotlinClass {
    lateinit var field: String
}

According to docs:

Late-Initialized properties are also exposed as fields. The visibility of the field will be the same as the visibility of lateinit property setter.

I can use in java code either myKotlinClass.field or myKotlinClass.getField(). I want to disable field access and remain only access through getter and setter.

How can I achieve this and remain lateinit modifier?

score 15 · Accepted Answer · edited May 23 '17 at 10:31

15

You can use @JvmSynthetic that hides declarations from Java (and not from Kotlin). Just annotate the backing field of the property:

@field:JvmSynthetic
lateinit var field: String

Though the field will remain public in the bytecode, it will also have the synthetic modifier, which prevents it from being used in Java sources. However, the field seems to be still accessible through reflection at runtime.

See also: another question about @JvmSynthetic (though no definite answer there).

edited May 23 '17 at 10:31

Community

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answered Mar 15 '17 at 11:29

hotkey

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Thank you. That's exactly what I need. – Michael Spitsin Mar 15 '17 at 14:39
I checked the generated byte code with Kotlin 1.0.6, and the backing field was still public – voddan Mar 15 '17 at 17:42
@voddan, but it should now have the `synthetic` modifier, does it? With this modifier, its usages in Java sources should become invalid. – hotkey Mar 15 '17 at 17:46
@voddan, like this: `public synthetic Ljava/lang/String; s`. Or have you found any way to reference the field from Java? – hotkey Mar 15 '17 at 17:50
@hotkey the field appears to have the `ACC_SYNTHETIC` on it. Let me check if I can access it from Java – voddan Mar 15 '17 at 17:55
@hotkey you were absolutely correct, sorry for the `-1`. Please edit your A (add a space somewhere :) so I could upvote it – voddan Mar 15 '17 at 20:04
@voddan, no problem, updated the answer with more details on what this does. :) – hotkey Mar 15 '17 at 20:42
Is there any way to hide the generated getter and setter as well without setting those as private? – stefana Mar 09 '18 at 14:34

score 2 · Answer 2 · answered Mar 15 '17 at 17:50

2

The classical solution to this problem would be to use property delegation:

import kotlin.properties.Delegates

class MyKotlinClass {
    var field: String by Delegates.notNull()
}

This code does exactly what you asked for in the question

answered Mar 15 '17 at 17:50

voddan

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Thank you for an alternative solutions. I've also checked this discussion https://discuss.kotlinlang.org/t/notnull-delegate-vs-lateinit/1923 - to see the actual difference between `lateinit` and `Delegates.notNull()` – Michael Spitsin Mar 16 '17 at 05:18

score 1 · Answer 3 · answered Mar 15 '17 at 15:21

The visibility of lateinit field is derived from the visibility of setter of the corresponding property, so another option is to have a property with non-public setter:

lateinit var field: String
    private set

The drawback of this approach is that the setter itself becomes inaccessible outside of the class.

How to hide Kotlin's lateinit var backing field from Java?

3 Answers3