Ktor 1.5.4 Help


In addition to HTTP handling for the server and the client, Ktor supports client and server, TCP and UDP raw sockets. It exposes a suspending API that uses NIO under the hoods.

Add dependencies

To enable Sockets support, you need to include the ktor-network artifact in the build script:

implementation "io.ktor:ktor-network:$ktor_version"
<dependency> <groupId>io.ktor</groupId> <artifactId>ktor-network</artifactId> <version>${ktor_version}</version> </dependency>


In order to create either server or client sockets, you have to use the aSocket builder, with a mandatory ActorSelectorManager: aSocket(selector). For example: aSocket(ActorSelectorManager(Dispatchers.IO)).

Then use:

  • val socketBuilder = aSocket(selector).tcp() for a builder using TCP sockets

  • val socketBuilder = aSocket(selector).udp() for a builder using UDP sockets

This returns a SocketBuilder that can be used to:

  • val serverSocket = aSocket(selector).tcp().bind(address) to listen to an address (for servers)

  • val clientSocket = aSocket(selector).tcp().connect(address) to connect to an address (for clients)

If you need to control the dispatcher used by the sockets, you can instantiate a selector, that uses, for example, a cached thread pool:

val exec = Executors.newCachedThreadPool() val selector = ActorSelectorManager(exec.asCoroutineDispatcher()) val tcpSocketBuilder = aSocket(selector).tcp()

Once you have a socket open by either binding or connecting the builder, you can read from or write to the socket, by opening read/write channels:

val input : ByteReadChannel = socket.openReadChannel() val output: ByteWriteChannel = socket.openWriteChannel(autoFlush = true)


When creating a server socket, you have to bind to a specific SocketAddress to get a ServerSocket:

val server = aSocket(selector).tcp().bind(InetSocketAddress("", 2323))

The server socket has an accept method that returns, one at a time, a connected socket for each incoming connection pending in the backlog:

val socket = server.accept()

Simple Echo Server:

fun main(args: Array<String>) { runBlocking { val server = aSocket(ActorSelectorManager(Dispatchers.IO)).tcp().bind(InetSocketAddress("", 2323)) println("Started echo telnet server at ${server.localAddress}") while (true) { val socket = server.accept() launch { println("Socket accepted: ${socket.remoteAddress}") val input = socket.openReadChannel() val output = socket.openWriteChannel(autoFlush = true) try { while (true) { val line = input.readUTF8Line() println("${socket.remoteAddress}: $line") output.write("$line\r\n") } } catch (e: Throwable) { e.printStackTrace() socket.close() } } } } }

Then you can connect to it using telnet and start typing:

telnet 2323

For each line that you type (you have to press the return key), the server will reply with the same line:

Trying Connected to Escape character is '^]'. Hello Hello World World |


When creating a socket client, you have to connect to a specific SocketAddress to get a Socket:

val socket = aSocket(selector).tcp().connect(InetSocketAddress("", 2323))

Simple Client Connecting to an Echo Server:

fun main(args: Array<String>) { runBlocking { val socket = aSocket(ActorSelectorManager(Dispatchers.IO)).tcp().connect(InetSocketAddress("", 2323)) val input = socket.openReadChannel() val output = socket.openWriteChannel(autoFlush = true) output.write("hello\r\n") val response = input.readUTF8Line() println("Server said: '$response'") } }

Secure sockets (SSL/TLS)

Ktor supports secure sockets. To enable them you will need to include the io.ktor:ktor-network-tls:$ktor_version artifact, and call the .tls() to a connected socket.

Connect to a secure socket:

runBlocking { val socket = aSocket(ActorSelectorManager(Dispatchers.IO)).tcp().connect(InetSocketAddress("google.com", 443)).tls() val w = socket.openWriteChannel(autoFlush = false) w.write("GET / HTTP/1.1\r\n") w.write("Host: google.com\r\n") w.write("\r\n") w.flush() val r = socket.openReadChannel() println(r.readUTF8Line()) }

You can adjust a few optional parameters for the TLS connection:

suspend fun Socket.tls( trustManager: X509TrustManager? = null, randomAlgorithm: String = "NativePRNGNonBlocking", serverName: String? = null, coroutineContext: CoroutineContext = Dispatchers.IO ): Socket
Last modified: 19 April 2021