JSON deserialization

Learn JSON deserialization with kotlinx.serialization.

JSON deserialization

JSON deserialization turns JSON text from the server into a Kotlin object. In Ktor the ContentNegotiation plugin does this with kotlinx.serialization.

You add @Serializable to a data class; when the client calls response.body<User>() the JSON becomes a User object. Example: @Serializable data class User(val name: String, val age: Int). JSON field names must match Kotlin property names. If they differ, use @SerialName to map them.

Reading an object with body()

To read an object from an HttpResponse call response.body<User>(). The generic parameter declares the target type.

Example: val user: User = client.get(url).body(). This call uses the ContentNegotiation plugin; without it the call throws. The return type is checked at compile time; mismatched JSON throws at runtime. For a list response use body<List<User>>(). The plugin maps the JSON shape to the Kotlin type automatically.

Exercise Define a Serializable User class

Define a data class User(val name: String) marked with @Serializable. Inside main, print println("User class ready").

Show example solution

@Serializable
data class User(val name: String)

fun main() {
    println("User class ready")
}

Test yourself

Complete the annotation name

@____
data class User(val name: String)

Show answer

@Serializable
data class User(val name: String)

Which annotation is needed to deserialize JSON to a Kotlin object?
- @Serializable
- @JsonProperty
- @SerializedName
- @Json
Show answer

@Serializable — @Serializable is required for kotlinx.serialization.
Match deserialization parts

@Serializable

Marks the class
data class

Data class
.body()

Gets object
@SerialName

Maps field name

Next lesson: Summary - Ktor HTTP

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