OjbkHttp

main.kt

fun main() {

    data class Foobar(

            @OjbkHttp.FormField("foo")
            val foo: String,

            @OjbkHttp.FormField("bar")
            val bar: Int = 0

    )

    val bean = Foobar("hello,world", 1234)
    val form = OjbkHttp.toFormBody(bean)
    // TODO: post the form

}

OjbkHttp.kt

import okhttp3.FormBody
import java.beans.Introspector
import java.lang.reflect.Method

object OjbkHttp {

    @Target(AnnotationTarget.FIELD)
    @Retention(AnnotationRetention.RUNTIME)
    annotation class FormField (
            val fieldName: String
    )

    private val metadataCache = HashMap<Class<Any>, HashMap<String, Method>>()

    private fun getFormMetadata(clazz: Class<Any>): HashMap<String, Method> {
        synchronized(clazz) {
            var entry = metadataCache[clazz]
            if(entry == null) {
                entry = HashMap()
                for(pd in Introspector.getBeanInfo(clazz).propertyDescriptors) {
                    if(pd.readMethod == null) { continue }
                    try {
                        val ffa = clazz.getDeclaredField(pd.name).getAnnotation(FormField::class.java)
                        if(ffa != null) {
                            entry[ffa.fieldName] = pd.readMethod
                        }
                    } catch (ignored: Exception) {}
                }
                metadataCache[clazz] = entry
            }
            return entry
        }
    }

    fun toFormBody(value: Any): FormBody {
        val fbb = FormBody.Builder()
        getFormMetadata(value.javaClass).forEach { (fieldName, reader) ->
            val fieldValue = reader.invoke(value)
            if(fieldValue != null) {
                fbb.add(fieldName, fieldValue.toString())
            }
        }
        return fbb.build()
    }

}