Is NetworkOnMainThreadException valid for a network call in a coroutine?

Question

I'm putting together a simple demo app in Kotlin for Android that retrieves the title of a webpage with Jsoup. I'm conducting the network call using Dispatchers.Main as context.

My understanding of coroutines is that if I call launch on the Dispatchers.Main it does run on the main thread, but suspends the execution so as to not block the thread.

My understanding of android.os.NetworkOnMainThreadException is that it exists because network operations are heavy and when run on the main thread will block it.

So my question is, given that a coroutine does not block the thread it is run in, is a NetworkOnMainThreadException really valid? Here is some sample code that throws the given Exception at Jsoup.connect(url).get():

class MainActivity : AppCompatActivity() {
    val job = Job()

    val mainScope = CoroutineScope(Dispatchers.Main + job)

    // called from onCreate()
    private fun printTitle() {
        mainScope.launch {
            val url ="https://kotlinlang.org"
            val document = Jsoup.connect(url).get()
            Log.d("MainActivity", document.title())
            // ... update UI with title
        }
    }
}

I know I can simply run this using the Dispatchers.IO context and providing this result to the main/UI thread, but that seems to dodge some of the utility of coroutines.

For reference, I'm using Kotlin 1.3.

Answer 1

My understanding of coroutines is that if I call launch on the Dispatchers.Main it does run on the main thread, but suspends the execution so as to not block the thread.

The only points where execution is suspended so as to not block the thread is on methods marked as suspend - i.e., suspending methods.

As Jsoup.connect(url).get() is not a suspending method, it blocks the current thread. As you're using Dispatchers.Main, the current thread is the main thread and your network operation runs directly on the main thread, causing the NetworkOnMainThreadException.

Blocking work like your get() method can be made suspending by wrapping it in withContext(), which is a suspending method and ensures that the Dispatchers.Main is not blocked while the method runs.

mainScope.launch {
    val url ="https://kotlinlang.org"
    val document = withContext(Dispatchers.IO) {
        Jsoup.connect(url).get()
    }
    Log.d("MainActivity", document.title())
    // ... update UI with title
}