Fatal exception with Kotlin Coroutine without proper error message

Question

I'm learning Kotlin Coroutines and I'm trying to build a simple app with some API requests. Unfortunately I've stumbled upon an error which is not really talkative, this is all I have in the logs:

FATAL EXCEPTION: main 
Process: com.tests.myapp, PID: 14743

This is my simple coroutine which would simply call an API endpoint. I've copied the syntax from this tutorial.

  CoroutineScope(Dispatchers.Main).launch {
       API.call().registration();
  }

For Kotlin Coroutines I use this version:

implementation 'org.jetbrains.kotlinx:kotlinx-coroutines-core:1.6.4'

And for the networking library I have Retrofit like this:

object API {

    private const val BASE_URL = "http://my-test-url-comes-here.com"

    private val okHttpClient = OkHttpClient()
        .newBuilder()
        .addInterceptor(RequestInterceptor)
        .build()

    private fun getClient(): Retrofit =
        Retrofit.Builder()
            .client(okHttpClient)
            .baseUrl(BASE_URL)
            .addConverterFactory(GsonConverterFactory.create())
            .build()


    fun call(): Endpoints {
      return  getClient().create(Endpoints::class.java)
    }
}

Any insights?

Hey @Adam Varhegyi! What does your Endpoints.kt file look like? — karllindmark
– karllindmark, Commented Feb 13, 2023 at 18:48
There are a couple of errors in that tutorial’s explanation of coroutines. Not a good source for learning them. 1) In practice, you would never create a CoroutineScope and leave it for dead after launching a coroutine with it. The only reason to create a CoroutineScope instead of using an existing one is so you can specifically manage its lifecycle. 2) You don’t need to defensively use withContext(Dispatchers.IO) everywhere. In their example, the function fetches a lot of data but doesn’t process it. It’s trivial to pass some references around, even if the reference is to a large object. — Tenfour04
– Tenfour04, Commented Feb 13, 2023 at 19:02
So no need to introduce unnecessary thread switching, complicating the code and slowing things down. If you were processing a lot of the data, Dispatchers.Default would be more appropriate, provided you provide adequate yielding in the code. — Tenfour04
– Tenfour04, Commented Feb 13, 2023 at 19:04

TG. Kahsay · Accepted Answer · 2023-02-13 19:22:46Z

1

I think you should use Dispatchers.IO because you are calling a functon that uses network. by passing Dispatcher.Main, you are asking coroutinScope to use UI thread. that gives a Network on Main thread Exception. so,

  CoroutineScope(Dispatchers.IO /* replace Main with IO here */).launch {
       API.call().registration();
  }

answered Feb 13, 2023 at 19:22

TG. Kahsay

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1 Comment

Tenfour04 Over a year ago

This is incorrect if the network function is a suspend function. A suspend function must never block. If it contains blocking code, it must internally specify a dispatcher with withContext.

Rob · Accepted Answer · 2023-02-13 19:56:34Z

try to use Dispatchers.IO and also I would suggest you to create your api service only once like this and check you manifest file - you should add internet permission to your app, to make network calls

object API {

   private const val BASE_URL = "http://my-test-url-comes-here.com"

   private val okHttpClient = OkHttpClient()
      .newBuilder()
      .addInterceptor(RequestInterceptor)
      .build()

   private val client by lazy {
      Retrofit.Builder()
          .client(okHttpClient)
          .baseUrl(BASE_URL)
          .addConverterFactory(GsonConverterFactory.create())
          .build()
   }


    val api by lazy {
       client.create(Endpoints::class.java)
    }

}

NOTE: the lazy initializer I wrote here is just sample you can also you non-lazy initializer

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