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Knee ships with a native runtime that deals with type conversions and other sorts of boilerplate logic, while also providing some nice utilities for low-level JNI invocations.

Init calls

To do so, the runtime must be initialized with a JniEnvironment (a CPointer<JNIEnvVar>) as soon as possible in the application lifecycle. Any Knee-related calls that happen before initialization will likely lead to a crash.

kotlin logokotlin
val environment: CPointer<JNIEnvVar> = ... initKnee(environment)

Such a pointer can be retrieved in multiple ways.

Using JNI_OnLoad

The JNI_OnLoad function is called when the binary is loaded by the JVM using System.loadLibrary. A reference to the JVM is passed down as well, and it can provide an environment for Knee.

kotlin logokotlin
@CName(externName = "JNI_OnLoad") fun onLoad(vm: JavaVirtualMachine): Int { vm.useEnv { } return 0x00010006 // JNI_VERSION_1_6 }

Use carefully: only one library can export the JNI_OnLoad symbol. If you are developing a library to be consumed by others, this strategy is not recommended as they may have their own JNI_OnLoad. Prefer other strategies or modules.

Using JVM calls

After the binary is loaded with System.loadLibrary, you can use any external function to invoke native code and a JniEnvironment will be passed as well. That can be handed over to Knee:

kotlin logokotlin
// Kotlin/JVM package com.example class KneeInitializer { external fun initializeKnee() } // Kotlin/Native @CName(externName = "Java_com_example_KneeInitializer_initializeKnee") fun initializeKnee(env: JniEnvironment) { }

Note that this boilerplate (external fun, @CName...) is exactly what Knee will solve for all your other calls.


All Kotlin Modules (e.g. Gradle projects) using Knee must be initialized. Knee supports module hierarchies in two different ways. You can pick the one you find more appropriate to your codebase.

Initialize in every module

The simplest, but verbose, way of initializing all modules is to simply call initKnee separately in all of them. This means that every module should add one initialization call and deal with Knee internally.

kotlin logokotlin
// module A, at some point... // module B, at some point...

As long as the underlying JavaVM is the same, the runtime will be able to progressively load all modules this way, meaning that the init call from a given module won't interfere with the one from other modules.

Declare a KneeModule

For more flexibility, libraries can avoid calling initKnee and declare a public object extending Then, the consumer module can add a module dependency in their init call or in their module definition.

For example, we may have a root module, Lib1, an intermediate module Lib2 depending on Lib1, and the application module App. In Lib1, simply declare a module:

kotlin logokotlin
object Lib1Module : KneeModule()

In Lib2, again, declare a module, but declare the Lib1 dependency:

kotlin logokotlin
object Lib2Module : KneeModule(Lib1Module) // vararg

In the App module, pass the dependency to the initialization call:

kotlin logokotlin
initKnee(environment, Lib2Module) // vararg

This way, the initialization call will also initialize the whole graph of modules that were declared. It is even possible for your library to receive an initialization callback:

kotlin logokotlin
object LibModule : KneeModule({ initialize { environment -> // Perform initialization logic (e.g. cache a jclass) } })

Exporting types

Another benefit of declaring a KneeModule in multi-module hierarchies is type exporting. You will learn in features that Knee allows you to mark specific classes or interfaces with annotations like @KneeClass, @KneeInterface, @KneeEnum.

The presence of that annotation allows that type to travel through the JNI interface (in both ways) seamlessly.

Sometimes, when creating library modules (for example, :Lib), such types should also be used by dependent modules (for example, :App) and are supposed to pass through their JNI interface. By default this creates an error, because :App does not know how to serialize and deserialize a type declared in the dependency module :Lib!

Knee allows you to mark such types as exported through the KneeModule builder:

kotlin logokotlin
// :Lib module object LibModule : KneeModule({ export<SomeType>() export<SomeOtherType>() export<SomeTypeWithGenerics<ChildType>>() })

This way you'll be able to serialize and deserialize types like SomeType in the app module:

kotlin logokotlin
// :App module fun initialize(env: JniEnvironment) { initKnee(env, LibModule) } @Knee fun doSomething(someType: SomeType): SomeOtherType { // this is now allowed! ... }