Call Kotlin suspend function in Java class

50
votes

Assume we have the following suspend function:

suspend fun doSomething(): List<MyClass> { ... }

If I want to call this function in one of my existing Java classes (which I'm not able to convert to Kotlin for now) and get its return value I have to provide a Continuation<? super List<MyClass>> as its parameter (Obviously).

My question is, How can I implement one. Specially its getContext getter.

3
javakotlinkotlin-coroutines
I would do anything possible to avoid having to do this; I wouldn't expect it to work very effectively. For example, you could add another Kotlin function to launch the coroutine however you think is appropriate, that isn't a suspend fun, and call that from Java. - Louis Wasserman
I used to have Java code that managed to create an implementation of Continuation and call a suspend fun, but in Kotlin 1.3 Continuation declares resumeWith(Result), where Result is a discriminated union of the result and an internal class Failure and there's just no way to supply that from Java, save for using reflection to access private members in Kotlin implementation. - Marko Topolnik

3 Answers

70
votes

First, add org.jetbrains.kotlinx:kotlinx-coroutines-jdk8 module to your dependencies. In your Kotlin file define the following async function that corresponds to Java style of writing async APIs:

fun doSomethingAsync(): CompletableFuture<List<MyClass>> =
    GlobalScope.future { doSomething() }

Now use doSomethingAsync from Java in the same way as you are using other asynchronous APIs in the Java world.

8
votes

If you dont want to use org.jetbrains.kotlinx:kotlinx-coroutines-jdk8, I have a new idea.

Write below code in your kotlin project.

    @JvmOverloads
    fun <R> getContinuation(onFinished: BiConsumer<R?, Throwable?>, dispatcher: CoroutineDispatcher = Dispatchers.Default): Continuation<R> {
        return object : Continuation<R> {
            override val context: CoroutineContext
                get() = dispatcher

            override fun resumeWith(result: Result<R>) {
                onFinished.accept(result.getOrNull(), result.exceptionOrNull())
            }
        }
    }

I write it in my Coroutines class

Then you can call your suspend function like:

            Coroutines coroutines = new Coroutines();
            UserUtils.INSTANCE.login("user", "pass", coroutines.getContinuation(
                    (tokenResult, throwable) -> {
                        System.out.println("Coroutines finished");
                        System.out.println("Result: " + tokenResult);
                        System.out.println("Exception: " + throwable);
                    }
            ));

login() function is a suspend function.
suspend fun login(username: String, password: String): TokenResult

For your code, you can:

doSomething(getContinuation((result, throwable) -> { 
       //TODO
}));
7
votes

For coroutines 1.3.0 use this:

BuildersKt.launch(GlobalScope.INSTANCE,
                Dispatchers.getMain(),//context to be ran on
                CoroutineStart.DEFAULT,
                (coroutineScope, continuation) -> suspendFunction(arguments)
        );

For java < 8:

BuildersKt.launch(
        GlobalScope.INSTANCE,
        Dispatchers.getMain(),//context to be ran on
        CoroutineStart.DEFAULT,
        new Function2<CoroutineScope, Continuation<? super Unit>, Unit/*or your return type here*/>() {
            @Override
            public Unit/*or your return type here*/ invoke(CoroutineScope coroutineScope, Continuation<? super Unit> continuation) {
                //do what you want
                return Unit.INSTANCE; //or something with the defined type
            }
        }
);

My gradle file:

implementation "org.jetbrains.kotlin:kotlin-stdlib-jdk7:1.3.50"
implementation "org.jetbrains.kotlinx:kotlinx-coroutines-core:1.3.0"
implementation "org.jetbrains.kotlinx:kotlinx-coroutines-android:1.3.0"

Kotlin uses static classes for extension functions, launch is an extension function, so it is defined in BuildersKt. The first parameter is the target of the extension function, the rest are the parameters from the extension functions.