Preparing the request

Estimated reading time: 5 minutes

Making request

After client configuration we’re ready to perform our first request. Most of the simple requests are made with pattern

val response = client.'http-method'<'ResponseType'>("url-string")

or even simpler form(due to kotlin generic type inference):

val response: ResponseType = client.'http-method'("url-string")

For example to perform a GET request fully reading a String:

val htmlContent = client.get<String>("")
// same as
val content: String = client.get("")

And in the case you are interested in the raw bits, you can read a ByteArray:

val channel: ByteArray = client.get("")

Or get full HttpResponse:

val response: HttpResponse = client.get("")

The HttpResponse is downloaded in memory by default. To learn how to download response partially or work with a stream data consult with the Streaming section.

And even your data class using Json feature:

data class User(val id: Int)

val response: User = client.get("")

Please note that some of response types are Closeable and can hold resources.

Customizing requests

We cannot live only on get requests, Ktor allows you to build complex requests with any of the HTTP verbs, with the flexibility to process responses in many ways.

Default http methods

Similar to request, there are several extension methods to perform requests with the most common HTTP verbs: GET, POST, PUT, DELETE, PATCH, HEAD and OPTIONS.

val text =<String>("")

When calling request methods, you can provide a lambda to build the request parameters like the URL, the HTTP method(verb), the body, or the headers:

val text =<String>("") {
    header("Hello", "World")

The HttpRequestBuilder looks like this:

class HttpRequestBuilder : HttpMessageBuilder {
    var method: HttpMethod

    val url: URLBuilder
    fun url(block: URLBuilder.(URLBuilder) -> Unit)

    val headers: HeadersBuilder
    fun header(key: String, value: String)
    fun headers(block: HeadersBuilder.() -> Unit)

    var body: Any = EmptyContent

    val executionContext: CompletableDeferred<Unit>
    fun setAttributes(block: Attributes.() -> Unit)
    fun takeFrom(builder: HttpRequestBuilder): HttpRequestBuilder

The HttpClient class only offers some basic functionality, and all the methods for building requests are exposed as extensions.
You can check the standard available HttpClient build extension methods.

Customize method

In addition to call, there is a request method for performing a typed request, receiving a specific type like String, HttpResponse, or an arbitrary class. You have to specify the URL and the method when building the request.

val call = client.request<String> {
    method = HttpMethod.Get

Posting forms

There are a couple of convenience extension methods for submitting form information. The detailed refrence is listed here.

The submitForm method:

    formParameters: Parameters = Parameters.Empty,
    encodeInQuery: Boolean = false,
    block: HttpRequestBuilder.() -> Unit = {}

It allows requesting with the Parameters encoded in the query string(GET by default) or requesting with the Parameters encoded as multipart(POST by default) depending on the encodeInQuery parameter.

The submitFormWithBinaryData method:

    formData: List<PartData>,
    block: HttpRequestBuilder.() -> Unit = {}
): T

It allows to generate a multipart POST request from a list of PartData. PartData can be PartData.FormItem, PartData.BinaryItem or PartData.FileItem.

To build a list of PartData, you can use the formData builder:

val data: List<PartData> = formData {
    // Can append: String, Number, ByteArray and Input.
    append("hello", "world")
    append("number", 10)
    append("ba", byteArrayOf(1, 2, 3, 4))
    appendInput("input", size = knownSize.orNull()) { openInputStream().asInput() }
    // Allow to set headers to the part:
    append("hello", "world", headersOf("X-My-Header" to "MyValue"))

Specifying custom headers

When building requests with HttpRequestBuilder, you can set custom headers. There is a final property val headers: HeadersBuilder that inherits from StringValuesBuilder. You can add or remove headers using it, or with the header convenience methods.

// this : HttpMessageBuilder

// Convenience method to add a header
header("My-Custom-Header", "HeaderValue")

// Calls methods from the headers: HeadersBuilder to manipulate the headers
headers.append("My-Custom-Header", "HeaderValue")
headers.appendAll("My-Custom-Header", listOf("HeaderValue1", "HeaderValue2"))

// Applies the headers with the `headers` convenience method
headers { // this: HeadersBuilder
    append("My-Custom-Header", "HeaderValue")
    appendAll("My-Custom-Header", listOf("HeaderValue1", "HeaderValue2"))

Complete HeadersBuilder API is listed here.

Specifying a body for requests

For POST and PUT requests, you can set the body property:<Unit> {
    body = // ...

The HttpRequestBuilder.body property can be a subtype of OutgoingContent as well as a String instance:

  • body = "HELLO WORLD!"
  • body = TextContent("HELLO WORLD!", ContentType.Text.Plain)
  • body = ByteArrayContent("HELLO WORLD!".toByteArray(Charsets.UTF_8))
  • body = LocalFileContent(File("build.gradle"))
  • body = JarFileContent(File("myjar.jar"), "test.txt", ContentType.fromFileExtension("txt").first())
  • body = URIFileContent("")

If you install the JsonFeature, and set the content type to application/json you can use arbitrary instances as the body, and they will be serialized as JSON:

data class HelloWorld(val hello: String)

val client = HttpClient(Apache) {
    install(JsonFeature) {
        serializer = GsonSerializer {
            // Configurable .GsonBuilder
}<Unit> {
    body = HelloWorld(hello = "world")

Alternatively (using the integrated JsonSerializer):

val json = io.ktor.client.features.json.defaultSerializer()<Unit>() {
    body = json.write(HelloWorld(hello = "world")) // Generates an OutgoingContent

Or using Jackson (JVM only):

val json = jacksonObjectMapper()<Unit> {
    body = TextContent(json.writeValueAsString(userData), contentType = ContentType.Application.Json)

Remember that your classes must be top-level to be recognized by Gson.
If you try to send a class that is inside a function, the feature will send a null.

Uploading multipart/form-data

Ktor HTTP Client has support for making MultiPart requests. The idea is to use the MultiPartFormDataContent(parts: List<PartData>) as OutgoingContent for the body of the request.

The easiest way is to use the submitFormWithBinaryData method.

Alternatively, you can set the body directly:

val request = client.request {
    method = HttpMethod.Post
    body = MultiPartFormDataContent(formData {
        append("key", "value")