Content negotiation and serialization in Ktor Server

The ContentNegotiation plugin serves two primary purposes:

Negotiating media types between the client and server. For this, it uses the Accept and Content-Type headers.
Serializing/deserializing the content in a specific format. Ktor supports the following formats out-of-the-box: JSON, XML, CBOR, and ProtoBuf.

Add dependencies

ContentNegotiation

To use ContentNegotiation, you need to include the ktor-server-content-negotiation artifact in the build script:

implementation("io.ktor:ktor-server-content-negotiation:$ktor_version")

implementation "io.ktor:ktor-server-content-negotiation:$ktor_version"

        
                    <dependency>
                        <groupId>io.ktor</groupId>
                        <artifactId>ktor-server-content-negotiation-jvm</artifactId>
                        <version>${ktor_version}</version>
                    </dependency>
                    
        

Note that serializers for specific formats require additional artifacts. For example, kotlinx.serialization requires the ktor-serialization-kotlinx-json dependency for JSON.

Serialization

Before using kotlinx.serialization converters, you need to add the Kotlin serialization plugin as described in the Setup section.

JSON

To serialize/deserialize JSON data, you can choose one of the following libraries: kotlinx.serialization, Gson, or Jackson.

Add the ktor-serialization-kotlinx-json artifact in the build script:

implementation("io.ktor:ktor-serialization-kotlinx-json:$ktor_version")

implementation "io.ktor:ktor-serialization-kotlinx-json:$ktor_version"

        
                    <dependency>
                        <groupId>io.ktor</groupId>
                        <artifactId>ktor-serialization-kotlinx-json-jvm</artifactId>
                        <version>${ktor_version}</version>
                    </dependency>
                    
        

Add the ktor-serialization-gson artifact in the build script:

implementation("io.ktor:ktor-serialization-gson:$ktor_version")

implementation "io.ktor:ktor-serialization-gson:$ktor_version"

        
                    <dependency>
                        <groupId>io.ktor</groupId>
                        <artifactId>ktor-serialization-gson-jvm</artifactId>
                        <version>${ktor_version}</version>
                    </dependency>
                    
        

Add the ktor-serialization-jackson artifact in the build script:

implementation("io.ktor:ktor-serialization-jackson:$ktor_version")

implementation "io.ktor:ktor-serialization-jackson:$ktor_version"

        
                    <dependency>
                        <groupId>io.ktor</groupId>
                        <artifactId>ktor-serialization-jackson-jvm</artifactId>
                        <version>${ktor_version}</version>
                    </dependency>
                    
        

XML

To serialize/deserialize XML, add the ktor-serialization-kotlinx-xml in the build script:

implementation("io.ktor:ktor-serialization-kotlinx-xml:$ktor_version")

implementation "io.ktor:ktor-serialization-kotlinx-xml:$ktor_version"

        
                    <dependency>
                        <groupId>io.ktor</groupId>
                        <artifactId>ktor-serialization-kotlinx-xml-jvm</artifactId>
                        <version>${ktor_version}</version>
                    </dependency>
                    
        

CBOR

To serialize/deserialize CBOR, add the ktor-serialization-kotlinx-cbor in the build script:

implementation("io.ktor:ktor-serialization-kotlinx-cbor:$ktor_version")

implementation "io.ktor:ktor-serialization-kotlinx-cbor:$ktor_version"

        
                    <dependency>
                        <groupId>io.ktor</groupId>
                        <artifactId>ktor-serialization-kotlinx-cbor-jvm</artifactId>
                        <version>${ktor_version}</version>
                    </dependency>
                    
        

ProtoBuf

To serialize/deserialize ProtoBuf, add the ktor-serialization-kotlinx-protobuf in the build script:

implementation("io.ktor:ktor-serialization-kotlinx-protobuf:$ktor_version")

implementation "io.ktor:ktor-serialization-kotlinx-protobuf:$ktor_version"

        
                    <dependency>
                        <groupId>io.ktor</groupId>
                        <artifactId>ktor-serialization-kotlinx-protobuf-jvm</artifactId>
                        <version>${ktor_version}</version>
                    </dependency>
                    
        

Install ContentNegotiation

To install the ContentNegotiation plugin to the application, pass it to the install function in the specified module. The code snippets below show how to install ContentNegotiation...

... inside the embeddedServer function call.
... inside the explicitly defined module, which is an extension function of the Application class.

                    import io.ktor.server.engine.*
                    import io.ktor.server.netty.*
                    import io.ktor.server.application.*
                    import io.ktor.server.plugins.contentnegotiation.*

                    fun main() {
                        embeddedServer(Netty, port = 8080) {
                            install(ContentNegotiation)
                            // ...
                        }.start(wait = true)
                    }
                

                    import io.ktor.server.application.*
                    import io.ktor.server.plugins.contentnegotiation.*
                    // ...
                    fun Application.module() {
                        install(ContentNegotiation)
                        // ...
                    }
                

Configure a serializer

Ktor supports the following formats out-of-the-box: JSON, XML, CBOR. You can also implement your own custom serializer.

JSON serializer

To register the JSON serializer in your application, call the json method:

import io.ktor.server.plugins.contentnegotiation.*
import io.ktor.serialization.kotlinx.json.*

install(ContentNegotiation) {
    json()
}

The json method also allows you to adjust serialization settings provided by JsonBuilder, for example:

    install(ContentNegotiation) {
        json(Json {
            prettyPrint = true
            isLenient = true
        })
    }

To register the Gson serializer in your application, call the gson method:

import io.ktor.server.plugins.contentnegotiation.*
import io.ktor.serialization.gson.*

install(ContentNegotiation) {
    gson()
}

The gson method also allows you to adjust serialization settings provided by GsonBuilder, for example:

    install(ContentNegotiation) {
        gson {
            registerTypeAdapter(LocalDate::class.java, LocalDateAdapter())
            setDateFormat(DateFormat.LONG, DateFormat.SHORT)
            setPrettyPrinting()
        }

To register the Jackson serializer in your application, call the jackson method:

import io.ktor.server.plugins.contentnegotiation.*
import io.ktor.serialization.jackson.*

install(ContentNegotiation) {
    jackson()
}

The jackson method also allows you to adjust serialization settings provided by ObjectMapper, for example:

    install(ContentNegotiation) {
        jackson {
            configure(SerializationFeature.INDENT_OUTPUT, true)
            setDefaultPrettyPrinter(DefaultPrettyPrinter().apply {
                indentArraysWith(DefaultPrettyPrinter.FixedSpaceIndenter.instance)
                indentObjectsWith(DefaultIndenter("  ", "\n"))
            })
            registerModule(JavaTimeModule())  // support java.time.* types
        }
    }

XML serializer

To register the XML serializer in your application, call the xml method:

import io.ktor.server.plugins.contentnegotiation.*
import io.ktor.serialization.kotlinx.xml.*

install(ContentNegotiation) {
    xml()
}

The xml method also allows you to access XML serialization settings, for example:

import io.ktor.server.plugins.contentnegotiation.*
import io.ktor.serialization.kotlinx.xml.*
import nl.adaptivity.xmlutil.*
import nl.adaptivity.xmlutil.serialization.*

install(ContentNegotiation) {
    xml(format = XML {
        xmlDeclMode = XmlDeclMode.Charset
    })
}

CBOR serializer

To register the CBOR serializer in your application, call the cbor method:

import io.ktor.server.plugins.contentnegotiation.*
import io.ktor.serialization.kotlinx.cbor.*

install(ContentNegotiation) {
    cbor()
}

The cbor method also allows you to access CBOR serialization settings provided by CborBuilder, for example:

import io.ktor.server.plugins.contentnegotiation.*
import io.ktor.serialization.kotlinx.cbor.*
import kotlinx.serialization.cbor.*

install(ContentNegotiation) {
    cbor(Cbor {
        ignoreUnknownKeys = true
    })
}

ProtoBuf serializer

To register the ProtoBuf serializer in your application, call the protobuf method:

import io.ktor.server.plugins.contentnegotiation.*
import io.ktor.serialization.kotlinx.protobuf.*

install(ContentNegotiation) {
    protobuf()
}

The protobuf method also allows you to access ProtoBuf serialization settings provided by ProtoBufBuilder, for example:

import io.ktor.server.plugins.contentnegotiation.*
import io.ktor.serialization.kotlinx.protobuf.*
import kotlinx.serialization.protobuf.*

install(ContentNegotiation) {
    protobuf(ProtoBuf {
        encodeDefaults = true
    })
}

Custom serializer

To register a custom serializer for a specified Content-Type, you need to call the register method. In the example below, two custom serializer are registered to deserialize application/json and application/xml data:

install(ContentNegotiation) {
    register(ContentType.Application.Json, CustomJsonConverter())
    register(ContentType.Application.Xml, CustomXmlConverter())
}

Receive and send data

Create a data class

To deserialize received data into an object, you need to create a data class, for example:

data class Customer(val id: Int, val firstName: String, val lastName: String)

If you use kotlinx.serialization, make sure that this class has the @Serializable annotation:

import kotlinx.serialization.*

@Serializable
data class Customer(val id: Int, val firstName: String, val lastName: String)

Serializing/deserializing of the following types is supported by the kotlinx.serialization library:

Builtin classes
Deserializing of Sequences
Serializing of Flows

Receive data

To receive and convert a content for a request, call the receive method 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)
        }

The Content-Type of the request will be used to choose a serializer for processing the request. The example below shows a sample HTTP client request containing JSON or XML data that is converted to a Customer object on the server side:

POST http://0.0.0.0:8080/customer
Content-Type: application/json

{
  "id": 3,
  "firstName": "Jet",
  "lastName": "Brains"
}

POST http://0.0.0.0:8080/customer
Content-Type: application/xml

<Customer id="3" firstName="Jet" lastName="Brains"/>

You can find the full example here: json-kotlinx.

Send data

To pass a data object in a response, you can use the respond method:

        get("/customer/{id}") {
            val id = call.parameters["id"]
            val customer: Customer = customerStorage.find { it.id == id!!.toInt() }!!
            call.respond(customer)
        }

In this case, Ktor uses the Accept header to choose the required serializer. You can find the full example here: json-kotlinx.

Implement a custom serializer

In Ktor, you can write your own serializer for serializing/deserializing data. To do this, you need to implement the ContentConverter interface:

interface ContentConverter {
    suspend fun serialize(contentType: ContentType, charset: Charset, typeInfo: TypeInfo, value: Any): OutgoingContent?
    suspend fun deserialize(charset: Charset, typeInfo: TypeInfo, content: ByteReadChannel): Any?
}

Take a look at the GsonConverter class as an implementation example.

Last modified: 06 February 2025