Engines

To run a Ktor server application, you need to create and configure a server first. Server configuration can include different settings: a server engine, various engine-specific options, host and port values, and so on. The following engines are supported:

• Netty

• Jetty

• Tomcat

• CIO (Coroutine-based I/O)

In addition to the engines mentioned above, Ktor provides a special engine type TestEngine for testing application logic. You can learn more about it from Testing.

Add Dependencies

Before using the desired engine, you need to add a corresponding dependency to your build.gradle or pom.xml file:

• ktor-server-netty
• ktor-server-jetty
• ktor-server-tomcat
• ktor-server-cio

Below are examples of adding a dependency for Netty:

Gradle (Groovy)

Gradle (Kotlin)

Maven

implementation "io.ktor:ktor-server-netty:$ktor_version"

tip

For testing, you need to add the ktor-server-test-host dependency. There is also the ktor-server-servlet dependency that allows you to run an application in a servlet container like Jetty or Tomcat. Learn more at Containers.

Configure Engines

A Ktor server application can be run in two ways:

• embeddedServer is a simple way to configure server parameters in code and quickly run an application.

• EngineMain provides more flexibility to configure a server. You can specify server parameters in an application.conf file and change a configuration without recompiling your application. Moreover, you can run your application from a command line and override the required server parameters by passing corresponding command-line arguments.

You can learn more about configuring Ktor from Configurations.

embeddedServer

The embeddedServer function accepts an engine factory used to create an engine of a specific type. In the example below, we pass the Netty factory to run a server with the Netty engine and listen on the 8000 port:

import io.ktor.server.netty.*
fun main() {
    embeddedServer(Netty, port = 8000) {
        routing {
            get ("/") {
                call.respondText("Hello, world!")
            }
        }
    }.start(wait = true)
}

EngineMain

EngineMain represents an engine for running a server. You can use the following engines:

• io.ktor.server.netty.EngineMain
• io.ktor.server.jetty.EngineMain
• io.ktor.server.tomcat.EngineMain
• io.ktor.server.cio.EngineMain

The EngineMain.main function is used to start a server with the selected engine and can accept command-line server parameters. In the example below, we start a server from the application’s main function:

fun main(args: Array<String>): Unit = io.ktor.server.netty.EngineMain.main(args)
fun Application.module(testing: Boolean = false) {
    routing {
        get("/") {
            call.respondText("Hello, world!")
        }
    }
}

If you need to start a server using a build system task, you need to configure the required EngineMain as the main class:

Gradle (Groovy)

Gradle (Kotlin)

Maven

mainClassName = "io.ktor.server.netty.EngineMain"