custom_gson.kt

package myapp

import com.google.gson.GsonBuilder
import org.http4k.core.Body
import org.http4k.core.Response
import org.http4k.core.Status
import org.http4k.core.with
import org.http4k.lens.Header
import org.http4k.lens.zonedDateTime
import java.time.ZonedDateTime
import java.io.IOException
import com.google.gson.TypeAdapter
import com.google.gson.stream.JsonReader
import com.google.gson.stream.JsonWriter
import org.http4k.format.ConfigurableGson
import myapp.MyCustomGson.auto


object MyCustomGson : ConfigurableGson(GsonBuilder()
    .registerTypeAdapter(ZonedDateTime::class.java, object : TypeAdapter<ZonedDateTime>() {
        @Throws(IOException::class)
        override fun write(out: JsonWriter, value: ZonedDateTime) {
            out.value(value.toString())
        }

        @Throws(IOException::class)
        override fun read(`in`: JsonReader): ZonedDateTime {
            return ZonedDateTime.parse(`in`.nextString())
        }
    })
    .enableComplexMapKeySerialization()
)


class SimpleObject(val id: String, val attrs: Map<String, Any?>,
                   val created: ZonedDateTime,
                   val updated: ZonedDateTime
)

fun main(args: Array<String>) {

    val obj = SimpleObject("foo", emptyMap(), ZonedDateTime.now(), ZonedDateTime.now())

    val response = Response(Status.OK)
        .with(Body.auto<SimpleObject>().toLens() of obj)
        .with(Header.zonedDateTime().required("last-modified") of obj.created)

    println(response)
    
    GsonBuilder().serializeNulls()
}