Ktor is a framework for building asynchronous (with coroutine) servers and clients in Kotlin. It’s made by the same people that made Kotlin.

If you would rather not use a framework, you can check this article on How to use Kotlin for TCP with java sockets

Using Ktor framework

Ktor in addition to HTTP handling, also supports client and server, TCP and UDP raw sockets. It exposes a suspending API that uses NIO (Java New-IO for non-blocking I/O) under the hoods.

You can check there example: Raw Sockets Ktor example.

It still a bit experimental, but it should be good to go soonish~~

Add the dependencies

Use Kotlin 1.4 and ktor 1.6.0, add it to your build.gradle.kts:

plugins {
    kotlin("jvm") version "1.4.32"
}

dependencies {
    implementation("io.ktor:ktor-server-netty:1.6.0")
    implementation("io.ktor:ktor-network:1.6.0")
}

The newer version of ktor requires ktor-network in order to use the raw sockets.

Implementation

Let’s look at a simpler example with one call-flow between the server and the client. Here is the code:

Server

The server is running on port 2323 and write back what the connected client sent. Unlike the ktor example, this one is not multi-bind.

suspend fun server() {
    val server = aSocket(ActorSelectorManager(Dispatchers.IO)).tcp()
        .bind(InetSocketAddress("127.0.0.1", 2323))
    println("Server running: ${server.localAddress}")
    val socket = server.accept()
    println("Socket accepted: ${socket.remoteAddress}")
    val input = socket.openReadChannel()
    val output = socket.openWriteChannel(autoFlush = true)
    val line = input.readUTF8Line()

    println("Server received '$line' from ${socket.remoteAddress}")
    output.writeFully("$line back\r\n".toByteArray())
}

It’s a suspend function because ktor is actively using coroutine, so this run blocking bit of code needs to be in an asynchronous call (⇰ in a coroutine). I added some line to be printed in the terminal to witness the transaction 👀

Client

This client will connect to the server above and send hello. Then it will wait for the server’s response and print it in the log.

suspend fun client() {
    val socket = aSocket(ActorSelectorManager(Dispatchers.IO)).tcp()
        .connect(InetSocketAddress("127.0.0.1", 2323))
    val input = socket.openReadChannel()
    val output = socket.openWriteChannel(autoFlush = true)

    output.writeFully("hello\r\n".toByteArray())
    println("Server said: '${input.readUTF8Line()}'")
}

The autoFlush for the output means that the data doesn’t wait in the buffer, it goes through it, gets sent and then flushed out. Usually in TCP you can have a buffer to accumulate the data and wait until a packet size to send it.

Since "hello" is relative small in terms of bytes, without autoFlush it could get “stucked” in the buffer and not being sent.

Run them both

Since kotr uses coroutine we’ll use it to start our server that will run in the background, however for the client we will use runBlocking which will start the client in a new blocking coroutine to wait and the see the transaction being made.

fun main() {
    CoroutineScope(Dispatchers.Default).launch { server() }
    runBlocking { client() }
}

When you run them, the client will send a message, the server will respond, and you should see something like this:

Server running: /127.0.0.1:2323
Socket accepted: /127.0.0.1:56215
Server received 'hello' from /127.0.0.1:56215
Server said: 'hello back' 

You can see that it’s running on localhost (127.0.0.1) and that the socket’s ip has a random port 56215. Now you should have mastered the basics of socket communication with ktor.

Find more examples on their documentation like this simple echo server

If you like this article or have a question, you can upvote ⬆️ or drop a comment ✍️ on the original stackoverflow answer! 💬