FunctionalInterface annotation for JVM1.8+ on Kotlin functions

[MukjepScarlet]

All Java built-in function types like BiFunction, IntProvider have this annotation. Also, this annotation is used by the GraalVM Polyglot library. It can only parse types with this annotation to a JavaScript function, that's why I want to add this.

[MukjepScarlet]

By the way, I think fun interface types can also have this in bytecode, but I'm not sure it's appropriate.

[udalov]

Also, this annotation is used by the GraalVM Polyglot library. It can only parse types with this annotation to a JavaScript function

Can you please clarify where it is documented that this is the case? Or maybe there is a sample project where something breaks once you remove the @FunctionalInterface annotation?

[MukjepScarlet]

CCBlueX/LiquidBounce#5454
Here is an example.

At first, I implement them as methods in object:
CCBlueX/LiquidBounce@3cd4546

Then I found that the Polyglot throws an exception on script execution:
[2.js] test2::Test2::enable threw [PolyglotException]: TypeError: execute on net.ccbluex.liquidbounce.script.bindings.api.ScriptContextProvide r$setupContext$1$1 failed due to: Message not supported.

After that I tried another functions like a simple val test: (Int, Int) -> Int = { a, b -> a+b }, this is also not available in script, but the val test2: BiFunction<Int, Int, Int> = BiFunction { a, b -> a+b } works.
So I guess there's difference between the Kotlin generated FunctionN interfaces and Java functional interfaces. It's this annotation.

After replace with a custom interface with this annotation, it works.

[05nelsonm]

I'm wondering how this would affect android API 23 and below as these APIs are not available. Have you tested it?

[MukjepScarlet]

Well, I'm not a Android developer, this is only tested on desktop JDK 8, 17 and 21.

[MukjepScarlet]

Android API 23 seems to not support this annotation (because this is included in stdlib from Java 8)

[MukjepScarlet]

So we might only use this if the class is existing. But as far as I know, latest Kotlin 2.x only supports Java 1.8 and higher versions.