A demo of creating a Java NIO one threaded, non-blocking server coroutine API.

nio_server_coroutine.kt

interface NioSocketServer {
  fun stop()
  suspend fun SelectableChannel.awaitSelection(ops: Int): SelectionKey
}

fun nioSocketServer(port: Int, acceptor: suspend NioSocketServer.(SocketChannel) -> Unit) {
  var isStopped = false
  val serverChannel = ServerSocketChannel.open().apply {
    configureBlocking(false)
    bind(InetSocketAddress(port))
  }
  val selector = Selector.open()
  val dispatcher = object : CoroutineDispatcher() {
    override fun dispatch(context: CoroutineContext, block: Runnable) { block.run() }
  }
  val server = object : NioSocketServer {
    override suspend fun SelectableChannel.awaitSelection(ops: Int) =
      suspendCoroutine<SelectionKey> { cont -> register(selector, ops, cont) }
    override fun stop() { isStopped = false }
  }
  runBlocking(dispatcher) {
    while (!isStopped) {
      if (selector.selectNow() > 0) {
        selector.selectedKeys().onEach {
          it.cancel()
          (it.attachment() as Continuation<SelectionKey>).resume(it)
        }.clear()
      }
      serverChannel.accept()?.let {
        it.configureBlocking(false)
        async { server.acceptor(it) }
      }
    }
  }
}

fun main() {
  nioSocketServer(80) { channel ->
    val buffer = ByteBuffer.allocate(128)
    val selectionKey = channel.awaitSelection(SelectionKey.OP_READ)
    channel.read(buffer)
    buffer.flip()
    val byteArray = ByteArray(128)
    buffer.get(byteArray)
    println(byteArray.toString(Charsets.UTF_8))
    channel.socket().close()
  }
}