An example echo server written in Kotlin.

KotlinEchoServer.kt

import java.io.BufferedReader
import java.io.InputStreamReader
import java.io.PrintWriter
import java.net.ServerSocket
import kotlin.concurrent.thread

const val SERVER_PORT = 1337

fun main(args: Array<String>) {

  println("Hello  World!")
  val socketListener = ServerSocket(SERVER_PORT)
  var clientNumber = 0

  socketListener.use {
    while (true) {
      val socket = socketListener.accept()
      thread(start = true) {
        clientNumber++
        val thisClient = clientNumber
        val output = PrintWriter(socket.getOutputStream(), true)
        val input = BufferedReader(InputStreamReader(socket.getInputStream()))
        output.println("Welcome to the Kotlin echo server. You are client #$thisClient.")
        output.println("End this conversation by sending a single '.' on a new line.")
        while (true) {
          val line = input.readLine()
          if (line == ".") {
            println("Client #$thisClient closed connection.")
            output.println("Kthxbai!")
            socket.close()
            return@thread
          }
          println("Client #$thisClient said: $line")
          output.println("I heard you say: " + line)
        }
      }
    }
  }

}