Use Auto Content Negotiation

Example showing how to combine multiple body Lenses into a single facade that will simplify content negotiation for inbound and outbound messages.

Gradle setup

Auto Content Negotiation is available in the core http4k module.

dependencies {

But it also integrates with the contract module.

dependencies {


Standard Body lenses work great in APIs that use a single message format (such as the ubiquitous JSON API), but there are scenarios where you may want to offer the user their choice of format (e.g. XML, YAML, JSON).

While it is possible to use the ACCEPT and CONTENT_TYPE lenses to manually select the inbound and outbound body lenses, the AutoContentNegotiator can do this for you.

Using Auto Content Negotiation

The AutoContentNegotiator starts with your selection of body lenses, and wraps them together. It can then be used to: - Unmarshall Request bodies based on the CONTENT_TYPE header - Select an outbound BodyLens, based on the ACCEPT header - Add all the request and body formats to your contract

If the CONTENT_TYPE and ACCEPT headers are not present, or if there is no lens for the requested format, then a default lens is used.


package guide.howto.use_auto_content_negotiation

import org.http4k.contract.ContractRoute
import org.http4k.contract.contract
import org.http4k.contract.meta
import org.http4k.contract.openapi.ApiInfo
import org.http4k.contract.openapi.v3.OpenApi3
import org.http4k.core.HttpHandler
import org.http4k.core.Method.POST
import org.http4k.core.Request
import org.http4k.core.Response
import org.http4k.core.Status.Companion.OK
import org.http4k.core.with
import org.http4k.format.JacksonXml
import org.http4k.format.Moshi
import org.http4k.format.MoshiYaml
import org.http4k.lens.ContentNegotiation
import java.util.UUID

// define a Person to be our request/response body
private class Person(val id: UUID, val name: String)

fun main() {
    val jsonLens = Moshi.autoBody<Person>().toLens()
    val xmlLens = JacksonXml.autoBody<Person>().toLens()
    val yamlLens = MoshiYaml.autoBody<Person>().toLens()

     * Create a content negotiator, which will handle lens selection for request and
     * response bodies.
     * Since the json lens is first, it will be the fallback when no accepted lens is found.
    val negotiator =, xmlLens, yamlLens)

    // This sample will be used to populate our contract
    val samplePerson = Person(UUID.fromString("7eb9a4b7-2e50-40d7-b4d2-16ce500a0245"), "john")

    // Create a contract route that will accept a person, and echo it back.
    val echoPerson: ContractRoute = "/echo" meta {
        summary = "Echo Person"
        receiving(negotiator to samplePerson) // add request bodies to contract
        returning(OK, negotiator to samplePerson) // add response bodies to contract
    } bindContract POST to { req: Request ->
        // Unmarshall the body based on the CONTENT-TYPE header
        val person = negotiator(req)

        // select the appropriate outbound lens based on the ACCEPT header
        val outboundLens =
        Response(OK).with(outboundLens of person)

    // Create an HttpHandler
    val handler: HttpHandler = contract {
        routes += echoPerson
        renderer = OpenApi3(
            ApiInfo("Content Negotiator Sample API", "v1.0")

    // Send a request with a json request body, accepting a YAML response body
    Request(POST, "/echo")
        .with(jsonLens of samplePerson)
        .header("ACCEPT", yamlLens.contentType.toHeaderValue())
        .also { println(it.bodyString()) }

    // Send a request with an xml request body, which will return the default json response body
    Request(POST, "/echo")
        .with(xmlLens of samplePerson)
        .also { println(it.bodyString()) }