KN-JVM

Note: still in the experimental stage and is not stable for production, It may be changed at any time

Auto JNI binding based on Kotlin Multiplatform. Just need simply declare functions in the common module, and the tool will automatically generate the corresponding Kotlin Native code (based on KSP)

Usage

// 1. declare shared api in common module
expect fun nativePlus(a: Int, b: Int): Int

// 2. declare api/impl in jvm/native sourceset
@JniApi // jvm
actual external fun nativePlus(a: Int, b: Int): Int

@JniImpl // native
actual fun nativePlus(a: Int, b: Int): Int {
    return a + b + 2 // plus 2 makes you believe this from native
}

// 3. load native lib and call the jvm api function
fun main() {
    // load native sourceset as a lib by yourself
    // loadLib()
    println(nativePlus(1, 2)) // 5
}

Custom Type

You need to use kotlinx.serialization for custom type support. We will use ProtoBuf as data transfer format when you want to pass an object as parameter between Kotlin/JVM and Kotlin/Native. Sample below:

expect fun nativePlus(a: Int, b: Foo): Bar

@Serializable // make sure your object is Serializable
class Foo(val v: String)

@Serializable // return type also need Serializable
class Bar(val v: String)

When using it, it is completely same with normal usage, and no additional adaptation is required:

fun main() {
    println(nativePlus(1, Foo("f")).v)
}

Call JVM From Native

Like call Native from JVM, we just need invert annotations, mark @JniApi at Native and mark JniImpl at JVM sourceset.

However, because the external keyword behavior of the Native platform is difficult to control, we need to manually call the Impl function generated by ksp on the Native platform.

// 1. declare shared api in common module
expect fun jvmPlus(a: Int, b: Foo): Bar

// 2. declare impl in jvm sourceset
@JniImpl // jvm
actual fun jvmPlus(a: Int, b: Foo): Bar {
   return Bar(a + b) // write your jvm logic here
}

// 3. declare api in native sourceset
@JniApi // native
actual fun jvmPlus(a: Int, b: Foo): Bar {
   return jvmPlusImpl(a, b) // jvmPlusImpl is generated by ksp
}

// now, you can call jvmPlus in native sourceset!
// it will auto call it's jvm implementation!

Performance

I made some simple performance testing based on my local machine:

Environment: single thread, Windows 11, 32G RAM, AMD Ryzen 9 7940HS, 4700 MHz (47 x 100)

ms/mop: milliseconds per million calls
op/s: operations per seconds

	direct-call	simple-task
operations	52.83 ms/mop (18,927,709.84 op/s)	9607.47 ms/mop (104,085.72 op/s)
description	kotlin function direct call	a very simply implementation, contains 1 basic type and 2 custom types. Can be used to test interaction costs

According to the data of direct-call and simple-task, we can get the extra cost of frequent JNI calls, and serialization, and other operations is about 9.55 ms per thousand calls, which means that on average such a call may consume an extra 10 μs.

But usually we only transfer time-consuming operations to native, or call some native libs, in these cases, 10 μs may be acceptable.

Gradle Setup

Maven Central: kni-api

1. apply KSP to your project:

   plugins {
       kotlin("multiplatform")
       kotlin("plugin.serialization") // for custom type supports
       id("com.google.devtools.ksp")
   }

2. apply kni ksp plugin to different target

   commonMain {
       dependencies {
           implementation("host.bytedance:kni-api:<version>")
       }
   }
   
   dependencies {
       val kniKspPath = "host.bytedance:kni-ksp:<version>"
       // your jvm platform target, here is a sample
       add("kspJvm", kniKspPath)
       // your native platform target, here is windows x64 sample
       add("kspMingwX64", kniKspPath)
   }

3. add configurations:

   ksp {
       // config jni package, For Android platform, usually `platform.android`
       arg("kni-jni-package", "zsu.jni")
   }

   arg	required	defaultValue	description
   kni-jni-package	yes
   kni-generated-package	no	zsu.kni.generated	package name of generated code
   kni-generated-proto-name	no	JniNativeProto	class name of generated proto class
   kni-generated-serializer-name	no	Serializer	class name of generated serializer

or take a look at demo project here: demo-kni

How to run Kotlin/Native codes from jvm?

You need to load native libraries which produced by Kotlin/Native sourceSets manually, and it is not autoload by this plugin, but still can check the sample here: demo-kni

Other

Tasks

• Interact Native from JVM
• More compile asserts to check invalid usages.
• Customization serialization provider
• Interact JVM from Native
• Unit test fixup and performance improvement
• Research Project Panama

(WIP) Tech Solution

We use the Lark to write the technical solution, although its language is Chinese, but you can choose to use Lark document translator

From KMM to Project Panama, or Higher

License

Copyright 2023 zsqw123

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.