Kotlin' coroutines and retrofit2: Program blocks for one minute after getting http response

Question

I need to use Kotlin' coroutines and the retrofit2 library to make http requests.
When I run the following code, the http request works as expected (posts is printed), but after this, the program hangs (blocks) and only exits after one minute.

My questions are:

Why doesn’t the program exit immediately after retrieving the data via HTTP?
How can I make the program exit when the data is retrieved?

import retrofit2.http.GET
import retrofit2.Retrofit
import retrofit2.converter.gson.GsonConverterFactory
import kotlinx.coroutines.*

data class Post(
    val userId: Int,
    val id: Int,
    val title: String
)

interface ApiService {
    @GET("posts")
    suspend fun getPosts(): List<Post>
}

object RetrofitClient {
    private const val BASE_URL = "https://jsonplaceholder.typicode.com/"

    val instance: ApiService by lazy {
        Retrofit.Builder()
            .baseUrl(BASE_URL)
            .addConverterFactory(GsonConverterFactory.create())
            .build()
            .create(ApiService::class.java)
    }
}

fun main() = runBlocking {
    val posts = RetrofitClient.instance.getPosts()
    posts.forEach { println(it) }
}

kotlinlang.org/api/kotlinx.coroutines/kotlinx-coroutines-core/… — Crebain
– Crebain, Commented Jul 7 at 7:52
@Crebain yes, I know that runBlocking "blocks the current thread until its completion"... But why the thread is still blocked after the HTTP requested returned? — Berk7871
– Berk7871, Commented Jul 7 at 17:24

Geba · Accepted Answer · 2025-08-17 09:45:41Z

Why doesn’t the program exit immediately after retrieving the data via HTTP?

The application doesn't exit because some non-daemon threads are still running.

Retrofit uses OkHttpClient, which creates threads that stay alive even after requests finish.
These threads are typically kept alive for connection reuse.

You can see these threads by adding this code at the end of the main function:

Thread.getAllStackTraces()
    .keys
    .filter { !it.isDaemon }
    .forEach { println(it.name) }

How can I make the program exit when the data is retrieved?

I would suggest 2 options:

1) Use an `OkHttpClient` that doesn’t keep non-daemon threads running

You need to build such OkHttpClient and pass it when building Retrofit.
In your original code, you don’t call client(httpClient), so Retrofit uses the default OkHttpClient.

object RetrofitClient {
    private const val BASE_URL = "https://jsonplaceholder.typicode.com/"

    val httpClient by lazy {
        val dispatcher = Dispatcher(
            Executors.newCachedThreadPool {
                Thread(it).apply { isDaemon = true }
            }
        )
        OkHttpClient.Builder()
            .dispatcher(dispatcher)
            .connectionPool(ConnectionPool(0, 1, TimeUnit.MILLISECONDS))
            .build()
    }
    val instance: ApiService by lazy {
        Retrofit.Builder()
            .client(httpClient)
            .baseUrl(BASE_URL)
            .addConverterFactory(GsonConverterFactory.create())
            .build()
            .create(ApiService::class.java)
    }
}

fun main() = runBlocking {
    val posts = RetrofitClient.instance.getPosts()
    posts.forEach { println(it) }
}

2) Explicitly shut down `OkHttpClient`'s internal resources

Write a closeHttpClient() function that closes OkHttpClient,
and call it at the end of the main function.

The implementation of closeHttpClient() follows the guidance from "Shutdown isn't necessary" of OkHttpClient documentation.

object RetrofitClient {
    private const val BASE_URL = "https://jsonplaceholder.typicode.com/"

    private val httpClient by lazy { OkHttpClient.Builder().build() }

    val instance: ApiService by lazy {
        Retrofit.Builder()
            .client(httpClient)
            .baseUrl(BASE_URL)
            .addConverterFactory(GsonConverterFactory.create())
            .build()
            .create(ApiService::class.java)
    }

    fun closeHttpClient() {
        httpClient.dispatcher().executorService().shutdown()
        httpClient.connectionPool().evictAll()
        httpClient.cache()?.close()
    }
}

fun main() = runBlocking {
    val posts = RetrofitClient.instance.getPosts()
    posts.forEach { println(it) }
    RetrofitClient.closeHttpClient()
}

Collectives™ on Stack Overflow

Kotlin' coroutines and retrofit2: Program blocks for one minute after getting http response

1 Answer 1

Why doesn’t the program exit immediately after retrieving the data via HTTP?

How can I make the program exit when the data is retrieved?

1) Use an `OkHttpClient` that doesn’t keep non-daemon threads running

2) Explicitly shut down `OkHttpClient`'s internal resources

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Collectives™ on Stack Overflow

1 Answer 1

Why doesn’t the program exit immediately after retrieving the data via HTTP?

How can I make the program exit when the data is retrieved?

1) Use an OkHttpClient that doesn’t keep non-daemon threads running

2) Explicitly shut down OkHttpClient's internal resources

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1) Use an `OkHttpClient` that doesn’t keep non-daemon threads running

2) Explicitly shut down `OkHttpClient`'s internal resources