Handling requests

Ktor allows you to handle incoming requests and send responses inside route handlers. You can perform various actions when handling requests:

Get request information, such as headers, cookies, and so on.
Get path parameter values.
Get parameters of a query string.
Receive body contents, for example, data objects, form parameters, and files.

General request information

Inside a route handler, you can get access to a request using the call.request property. This returns the ApplicationRequest instance and provides access to various request parameters. For example, the code snippet below shows how to get a request URI:

routing {
    get("/") {
        val uri = call.request.uri
        call.respondText("Request uri: $uri")
    }
}

The ApplicationRequest object allows you to get access to various request data, for example:

Headers

To access all request headers, use the ApplicationRequest.headers property. You can also get access to specific headers using dedicated extension functions, such as acceptEncoding, contentType, cacheControl, and so on.
Cookies

The ApplicationRequest.cookies property provides access to cookies related to a request. To learn how to handle sessions using cookies, see the Sessions section.
Connection details

Use the ApplicationRequest.local property to get access to connection details such as a host name, port, scheme, and so on.
X-Forwarded- headers

To get information about a request passed through an HTTP proxy or a load balancer, install the Forwarded headers plugin and use the ApplicationRequest.origin property.

Path parameters

When handling requests, you can get access to path parameter values using the call.parameters property. For example, call.parameters["login"] in the code snippet below will return admin for the /user/admin path:

get("/user/{login}") {
    if (call.parameters["login"] == "admin") {
        // ...
    }
}

Query parameters

To get access to parameters of a query string, you can use the ApplicationRequest.queryParameters property. For example, if a request is made to /products?price=asc, you can access the price query parameter in this way:

get("/products") {
    if (call.request.queryParameters["price"] == "asc") {
        // Show products from the lowest price to the highest
    }
}

You can also obtain the entire query string using the ApplicationRequest.queryString function.

Body contents

This section shows how to receive body contents sent with POST, PUT, or PATCH.

Raw payload

To access the raw body payload and parse it manually, use the ApplicationCall.receive function that accepts a type of payload to be received. Suppose you have the following HTTP request:

POST http://localhost:8080/text
Content-Type: text/plain

Hello, world!

You can receive the body of this request as an object of the specified type in one of the following ways:

String
To receive a request body as a String value, use call.receive<String>(). You can also use receiveText to achieve the same result:
post("/text") { val text = call.receiveText() call.respondText(text) }
ByteArray
To receive the body of a request as a byte array, call call.receive<ByteArray>():
post("/bytes") { val bytes = call.receive<ByteArray>() call.respond(String(bytes)) }
ByteReadChannel
You can use call.receive<ByteReadChannel>() or receiveChannel to receive ByteReadChannel that enables asynchronous reading of byte sequences:
post("/channel") { val readChannel = call.receiveChannel() val text = readChannel.readRemaining().readText() call.respondText(text) }
The sample below shows how to use ByteReadChannel to upload a file:
post("/upload") { val file = File("uploads/ktor_logo.png") call.receiveChannel().copyAndClose(file.writeChannel()) call.respondText("A file is uploaded") }

You can find the full example here: post-raw-data.

Objects

Ktor provides a ContentNegotiation plugin to negotiate the media type of request and deserialize content to an object of a required type. To receive and convert content for a request, call the ApplicationCall.receive function that accepts a data class as a parameter:

        post("/customer") {
            val customer = call.receive<Customer>()
            customerStorage.add(customer)
            call.respondText("Customer stored correctly", status = HttpStatusCode.Created)
        }

You can learn more from Content negotiation and serialization in Ktor Server.

Form parameters

Ktor allows you to receive form parameters sent with both x-www-form-urlencoded and multipart/form-data types using the receiveParameters function. The example below shows an HTTP client POST request with form parameters passed in a body:

POST http://localhost:8080/signup
Content-Type: application/x-www-form-urlencoded

username=JetBrains&email=example@jetbrains.com&password=foobar&confirmation=foobar

You can obtain parameter values in code as follows:

        post("/signup") {
            val formParameters = call.receiveParameters()
            val username = formParameters["username"].toString()
            call.respondText("The '$username' account is created")
        }

You can find the full example here: post-form-parameters.

Multipart form data

To receive a file sent as a part of a multipart request, call the .receiveMultipart() function and then loop over each part as required.

Multipart request data is processed sequentially, so you can't directly access a specific part of it. Additionally, these requests can contain different types of parts, such as form fields, files, or binary data, which need to be handled differently.

The example demonstrates how to receive a file and save it to file system:

import io.ktor.server.application.*
import io.ktor.http.content.*
import io.ktor.server.request.*
import io.ktor.server.response.*
import io.ktor.server.routing.*
import io.ktor.util.cio.*
import io.ktor.utils.io.*
import java.io.File

fun Application.main() {
    routing {
        var fileDescription = ""
        var fileName = ""

        post("/upload") {
            val multipartData = call.receiveMultipart(formFieldLimit = 1024 * 1024 * 100)

            multipartData.forEachPart { part ->
                when (part) {
                    is PartData.FormItem -> {
                        fileDescription = part.value
                    }

                    is PartData.FileItem -> {
                        fileName = part.originalFileName as String
                        val file = File("uploads/$fileName")
                        part.provider().copyAndClose(file.writeChannel())
                    }

                    else -> {}
                }
                part.dispose()
            }

            call.respondText("$fileDescription is uploaded to 'uploads/$fileName'")
        }
    }
}

Default file size limit

By default, the allowed size for binary and file items that can be received is limited to 50MB. If a received file or binary item exceeds the 50MB limit, an IOException is thrown.

To override the default form field limit, pass the formFieldLimit parameter when calling .receiveMultipart():

val multipartData = call.receiveMultipart(formFieldLimit = 1024 * 1024 * 100)

In this example, the new limit is set to 100MB.

Form fields

PartData.FormItem represents a form field, which values can be accessed through the value property:

                when (part) {
                    is PartData.FormItem -> {
                        fileDescription = part.value
                    }
                }

File uploads

PartData.FileItem represents a file item. You can handle file uploads as byte streams:

                when (part) {
                    is PartData.FileItem -> {
                        fileName = part.originalFileName as String
                        val file = File("uploads/$fileName")
                        part.provider().copyAndClose(file.writeChannel())
                    }
                }

The .provider() function returns a ByteReadChannel, which allows you to read data incrementally. Using the .copyAndClose() function, you then write the file content to the specified destination while ensuring proper resource cleanup.

To determine the uploaded file size, you can get the Content-Length header value inside the post handler:

post("/upload") {
    val contentLength = call.request.header(HttpHeaders.ContentLength)
    // ...
}

Resource cleanup

Once the form processing is complete, each part is disposed of using the .dispose() function to free resources.

part.dispose()

To learn how to run this sample, see upload-file.

Last modified: 06 February 2025