OkHttpTest.kt

import kotlinx.coroutines.experimental.runBlocking
import okhttp3.*
import java.io.IOException
import kotlin.coroutines.experimental.suspendCoroutine

fun main(args: Array<String>) {
    val http = OkHttpClient.Builder().build()
    val appid = "mock"
    val secret = "mock"
    val code = "mock"
    val api = "https://api.weixin.qq.com/sns/jscode2session?appid=${appid}&secret=${secret}&js_code=${code}&grant_type=authorization_code"
    val request = Request.Builder().get().url(api).build()

    val call1 = http.newCall(request)
    runBlocking {
        val result =   suspendCoroutine<String?> { co ->
            call1.enqueue(object: Callback {
                override fun onFailure(call: Call, e: IOException) {
                    co.resumeWithException(e)
                }

                override fun onResponse(call: Call, response: Response) {
                    co.resume(response.body()?.string())
                }
            })
        }
        // Exception in thread "main" java.lang.ClassCastException: java.lang.Object cannot be cast to java.lang.String

        println(result)
    }
}