Integrating Kotlin's Suspend Functions with Java's CompletableFuture

Kotlin's coroutine-based asynchronous model offers suspend functions that make asynchronous programming straightforward. However, when integrating with Java, which does not natively understand these suspend functions, one might face compatibility issues. Thankfully, we can bridge this gap by converting suspend functions into Java's CompletableFuture, which provides a non-blocking way to handle asynchronous operations.

Converting Suspend Functions to CompletableFuture

To facilitate this conversion, we can use the following utility function, suspendToFuture, which takes in an Executor and a suspend function, then returns a CompletableFuture:

Kotlin

fun <T> suspendToFuture(
  executor: Executor,
  block: suspend () -> T
): CompletableFuture<T> {
  val future = CompletableFuture<T>()
  val dispatcher = executor.asCoroutineDispatcher()
  CoroutineScope(dispatcher + SupervisorJob()).launch {
    try {
      future.complete(block())
    } catch (e: Exception) {
      future.completeExceptionally(e)
    }
  }
  return future
}

Usage Example

Let's take a suspend function as an example:

Kotlin

suspend fun add(x: Int, y: Int): Int {
  delay(1000)
  return x + y  
}

To use this function from Java, we first wrap it into a non-blocking function that returns a CompletableFuture:

Kotlin

fun addAsync(
  x: Int,
  y: Int,
  executor: Executor
): CompletableFuture<Int> = suspendToFuture(executor) {
  add(x, y)
}

Subsequently, from Java, we can then execute this function asynchronously and handle the result:

Java

final var executor = Executors.newSingleThreadExecutor();
final var future = addAsync(1, 2, executor);
future.thenAccept(System.out::println);

In the above Java code, the addAsync function is called, and its result (after a delay) will be printed to the console. This approach allows seamless interoperability between Kotlin's coroutine model and Java's asynchronous paradigm.