IllegalStateException when launching on Dispatchers.Main: which artifact is actually required?

Answer the question first

For Kotlin Coroutines 1.7 and newer, Dispatchers.Main is provided by a *platform* artifact that registers a MainDispatcherFactory via ServiceLoader. The core library deliberately contains no Main implementation. The required artifact depends on the target platform:

• Android JVM – kotlinx-coroutines-android
• Swing desktop – kotlinx-coroutines-swing
• JavaFX desktop – kotlinx-coroutines-javafx
• Kotlin/JS – kotlinx-coroutines-core-js already includes a Main dispatcher; no extra artifact is needed.
• Plain JVM (console, server, unit tests) – the core library intentionally has no Main dispatcher. In this case you should either use a non‑UI dispatcher (e.g. Dispatchers.Default, IO, Unconfined) or, for tests, set a test dispatcher with Dispatchers.setMain from kotlinx-coroutines-test.

How to satisfy Dispatchers.Main in a mixed‑environment module

When a module is shared between an Android app and plain JVM unit tests, keep the platform dependency out of the common implementation configuration:

dependencies {
    implementation("org.jetbrains.kotlinx:kotlinx-coroutines-core:1.7.3")

    // Android source set only
    androidMainImplementation("org.jetbrains.kotlinx:kotlinx-coroutines-android:1.7.3")

    // JVM test source set only
    testImplementation("org.jetbrains.kotlinx:kotlinx-coroutines-test:1.7.3")
}

In your JVM test setup, call:

import kotlinx.coroutines.test.*

@Before
fun setUp() {
    Dispatchers.setMain(StandardTestDispatcher())
}

This satisfies Dispatchers.Main without pulling a UI toolkit into the test classpath.

Version skew and its effects

Each platform artifact depends on a specific version of kotlinx-coroutines-core. If Gradle or Maven resolves two different core versions, the MainDispatcherFactory may try to load classes that no longer exist, resulting in:

• IllegalStateException: Main dispatcher is not available (most common)
• Or a linkage error such as NoSuchMethodError or ClassNotFoundException

To avoid this, declare the same version for both artifacts explicitly. Do not rely on transitive resolution to keep them in sync.

Practical verification steps

1. Run ./gradlew dependencies --configuration runtimeClasspath (or Maven dependency:tree) and confirm that kotlinx-coroutines-core and the platform artifact share the same version.
2. In a minimal main class, launch a coroutine on Dispatchers.Main and observe the exception message; it should mention the missing dispatcher if no factory is loaded.
3. Print the classpath of the JVM test runtime and verify that kotlinx-coroutines-test is present and that Dispatchers.setMain is called before any coroutine starts.

Ask for clarification

Could you confirm which build system you use and whether the Android source set already includes kotlinx-coroutines-android while the JVM tests include kotlinx-coroutines-test? This will ensure that the recommendation matches your actual configuration.