What's the use for Kotlin's @JvmSynthetic on a file target?-CodePudding

The @JvmSynthetic annotation is allowed to be used on a file, but I can't figure out what the purpose of this would be.

I was hoping I could hide a file containing a bunch of Kotlin-only extension methods from Java users, but that doesn't seem to be the case:

// Extensions.kt

@file:JvmSynthetic

@JvmSynthetic
fun Foo.mySyntheticExtension() = ...
fun Foo.myExtension() = ...

// Java usage

// This doesn't compile (as expected)
Extensions.mySyntheticExtension(foo);

// This compiles fine, so @JvmSynthetic on a file does not trickle down to all its functions
Extensions.myExtension(foo);

Even without the non-synthetic method Java users still see the cluttering ExtensionsKt class, although it appears empty to them.

If @file:JvmSynthetic doesn't hide the file('s generated class) from Java, nor trickles down the synthetic status to all functions in it, what is its intended purpose?

CodePudding user response：

The original proposal that caused this annotation target to be added was KT-41884:

The rationale given was:

This would apply to the synthesized class which encapsulates top-level members. This allows hiding those members from Java when they are internal visibility.

For example:

// ManyInternals.kt, in module A
@file:JvmSynthetic

internal fun foo() {

}

internal fun bar() {

}

// Main.java, in module B
public class Main {
    public static void main(String[] args) {
        ManyInternalsKt.foo(); // error
    }
}