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jvm-native-flag-support.md

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Support for JVM's ACC_NATIVE flag

Goal: enable JNI interop for Kotlin, so that anything that works through JNI in Java could be reproduced 1-to-1 in Kotlin

Syntax

The following definition is present in the library:

package kotlin.jvm

Retention(RetentionPolicy.SOURCE)
public annotation class native

This annotation is applicable to

  • functions
  • property accessors

A declaration marked with this annotation is referred to as a native declaration.

Checks to perform:

  • a native declaration can not be abstract
  • a native declaration can not have a body
  • a native declaration can not be marked inline
  • a native declaration can not have reified type parameters NOTE: this is achieved through prohibiting inline, as reified is only allowed on inline-functions now
  • members of traits can not be native

NOTE: native members can override open (or abstract) members of supertypes

Semantics on the JVM

Intuition: a JVM method whose source declaration is native is marked with the ACC_NATIVE flag, and has no CODE attribute.

Interaction with [platformStatic]

A native member of an object marked native and platformStatic is translated in a straightforward way: there is only one JVM method corresponding to it, and it is marked as ACC_NATIVE.

A member of a companion object of class C marked native and platformStatic yields two JVM methods:

  • static member of C that is marked ACC_NATIVE;
  • instance member of C$object that is not marked ACC_NATIVE and its body delegates to the native static method.

Top-level declarations

A native member of package p yields one JVM method:

  • member of a package-facade class PPackage which is marked with ACC_NATIVE flag.

Native Property Accessors

A property can not be marked native. A property accessor can be marked native if it has no body (i.e. it is a default accessor). In this case the generated code is the same as for a native function defined in the same context as the property.

Example:

val foo: Int
  [native] get

Not implemented (yet)

  • native property accessors
  • frontend: when accessors are default and native, don't require an initializer, don't allow a backing field
  • backend: when accessors are default and native, don't generate a backing field
  • applicability checks: only functions and property accessors