How to easily consume a Kotlin channel producer in Java?

Question

As part of working on the development of a new API, I am learning to use Kotlin. Initially I want the Kotlin API to be used within a Java (Android) project, but in the long term I hope to adopt Kotlin entirely.

As part of improving the implementation of a long-running process, I want to use coroutines. Specifically, a channel producer from the kotlinx.coroutines package.

For example:

fun exampleProducer() = produce {
    send("Hello")
    delay(1000)
    send("World")
}

What is the best way to consume this in Java? I am okay with adding temporary 'helper' functions to Kotlin and/or Java.

score 2 · Accepted Answer · answered Nov 11 '17 at 10:32

The easiest way to interop channels with Java is via Reactive Streams. Both Rx and Project Reactor are supported out-of-the-box. For example, add kotlinx-coroutines-rx2 to your dependicies and you'll be able to use rxFlowable builder:

fun exampleFlowable() = rxFlowable<String> {
    send("Hello")
    delay(1000)
    send("World")
}

This function returns an instance of Flowable, which is specifically designed for ease-of-use from Java, for example, you can do in Java:

exampleFlowable().subscribe(t -> System.out.print(t));

Works perfectly, although I opted to use `rxObservable` in my case (I do not expect backpressure). — Steven Jeuris, Nov 11 '17 at 15:39

Steven Jeuris · Answer 2 · 2017-11-10T18:16:28.077

1

Currently, assuming Java 8 is used and lambdas are available, I rely on a helper function defined in Kotlin which allows passing a callback to consume incoming results.

The helper method in Kotlin:

fun exampleProducerCallback( callback: (String) -> Unit ) = runBlocking {
    exampleProducer().consumeEach { callback( it ) }
}

This is then consumed in Java as:

ApiKt.exampleProducerCallback( text -> {
    System.out.print( text );
    return Unit.INSTANCE; // Needed since there is no void in Kotlin.
} );

Explanation on why return Unit.INSTANCE is needed can be found in this answer.

edited Nov 10 '17 at 18:16

answered Nov 10 '17 at 17:43

Steven Jeuris

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I am totally new to Kotlin and have no idea whether or not this is idiomatic. I merely posted my current solution to let you know where I am at right now. Perhaps there are better alternatives available? – Steven Jeuris Nov 10 '17 at 17:44
1

An obvious improvement is using `callback: Consumer` instead. – Alexey Romanov Nov 10 '17 at 20:17

How to easily consume a Kotlin channel producer in Java?

2 Answers2