Testing Kotlin coroutines on Android

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Unit testing code that uses coroutines requires some extra attention, as their execution can be asynchronous and happen across multiple threads. This guide covers how suspending functions can be tested, the testing constructs you need to be familiar with, and how to make your code that uses coroutines testable.

The APIs used in this guide are part of the kotlinx.coroutines.test library. Make sure to add the artifact as a test dependency to your project to have access to these APIs.

dependencies {
    testImplementation "org.jetbrains.kotlinx:kotlinx-coroutines-test:$coroutines_version"
}

Invoking suspending functions in tests

To call suspending functions in tests, you need to be in a coroutine. As JUnit test functions themselves aren't suspending functions, you need to call a coroutine builder inside your tests to start a new coroutine.

runTest is a coroutine builder designed for testing. Use this to wrap any tests that include coroutines. Note that coroutines can be started not only directly in the test body, but also by the objects being used in the test.

suspend fun fetchData(): String {
    delay(1000L)
    return "Hello world"
}

@Test
fun dataShouldBeHelloWorld() = runTest {
    val data = fetchData()
    assertEquals("Hello world", data)
}

In general, you should have one invocation of runTest per test, and using an expression body is recommended.

Wrapping your test’s code in runTest will work for testing basic suspending functions, and it will automatically skip any delays in coroutines, making the test above complete much faster than one second.

However, there are additional considerations to make, depending on what happens in your code under test:

  • When your code creates new coroutines other than the top-level test coroutine that runTest creates, you’ll need to control how those new coroutines are scheduled by choosing the appropriate TestDispatcher.
  • If your code moves the coroutine execution to other dispatchers (for example, by using withContext), runTest will still generally work, but delays will no longer be skipped, and tests will be less predictable as code runs on multiple threads. For these reasons, in tests you should inject test dispatchers to replace real dispatchers.

TestDispatchers

TestDispatchers are CoroutineDispatcher implementations for testing purposes. You’ll need to use TestDispatchers if new coroutines are created during the test to make the execution of the new coroutines predictable.

There are two available implementations of TestDispatcher: StandardTestDispatcher and UnconfinedTestDispatcher, which perform different scheduling of newly-started coroutines. These both use a TestCoroutineScheduler to control virtual time and manage running coroutines within a test.

There should only be one scheduler instance used in a test, shared between all TestDispatchers. See Injecting TestDispatchers to learn about sharing schedulers.

To start the top-level test coroutine, runTest creates a TestScope, which is an implementation of CoroutineScope that will always use a TestDispatcher. If not specified, a TestScope will create a StandardTestDispatcher by default, and use that to run the top-level test coroutine.

runTest keeps track of the coroutines that are queued on the scheduler used by the dispatcher of its TestScope, and will not return as long as there’s pending work on that scheduler.

StandardTestDispatcher

When you start new coroutines on a StandardTestDispatcher, they are queued up on the underlying scheduler, to be run whenever the test thread is free to use. To let these new coroutines run, you need to yield the test thread (free it up for other coroutines to use). This queueing behavior gives you precise control over how new coroutines run during the test, and it resembles the scheduling of coroutines in production code.

If the test thread is never yielded during the execution of the top-level test coroutine, any new coroutines will only run after the test coroutine is done (but before runTest returns):

@Test
fun standardTest() = runTest {
    val userRepo = UserRepository()

    launch { userRepo.register("Alice") }
    launch { userRepo.register("Bob") }

    assertEquals(listOf("Alice", "Bob"), userRepo.getAllUsers()) // ❌ Fails
}

There are several ways to yield the test coroutine to let queued-up coroutines run. All of these calls let other coroutines run on the test thread before returning:

  • advanceUntilIdle: Runs all other coroutines on the scheduler until there is nothing left in the queue. This is a good default choice to let all pending coroutines run, and it will work in most test scenarios.
  • advanceTimeBy: Advances virtual time by the given amount and runs any coroutines scheduled to run before that point in virtual time.
  • runCurrent: Runs coroutines that are scheduled at the current virtual time.

To fix the previous test, advanceUntilIdle can be used to let the two pending coroutines perform their work before continuing to the assertion:

@Test
fun standardTest() = runTest {
    val userRepo = UserRepository()

    launch { userRepo.register("Alice") }
    launch { userRepo.register("Bob") }
    advanceUntilIdle() // Yields to perform the registrations

    assertEquals(listOf("Alice", "Bob"), userRepo.getAllUsers()) // ✅ Passes
}

UnconfinedTestDispatcher

When new coroutines are started on an UnconfinedTestDispatcher, they are started eagerly on the current thread. This means that they’ll start running immediately, without waiting for their coroutine builder to return. In many cases, this dispatching behavior results in simpler test code, as you don’t need to manually yield the test thread to let new coroutines run.

However, this behavior is different from what you’ll see in production with non-test dispatchers. If your test focuses on concurrency, prefer using StandardTestDispatcher instead.

To use this dispatcher for the top-level test coroutine in runTest instead of the default one, create an instance and pass it in as a parameter. This will make new coroutines created within runTest execute eagerly, as they inherit the dispatcher from the TestScope.

@Test
fun unconfinedTest() = runTest(UnconfinedTestDispatcher()) {
    val userRepo = UserRepository()

    launch { userRepo.register("Alice") }
    launch { userRepo.register("Bob") }

    assertEquals(listOf("Alice", "Bob"), userRepo.getAllUsers()) // ✅ Passes
}

In this example, the launch calls will start their new coroutines eagerly on the UnconfinedTestDispatcher, which means that each call to launch will only return after the registration is completed.

Remember that UnconfinedTestDispatcher starts new coroutines eagerly, but this doesn’t mean that it’ll run them to completion eagerly as well. If the new coroutine suspends, other coroutines will resume executing.

For example, the new coroutine launched within this test will register Alice, but then it suspends when delay is called. This lets the top-level coroutine proceed with the assertion, and the test fails as Bob is not registered yet:

@Test
fun yieldingTest() = runTest(UnconfinedTestDispatcher()) {
    val userRepo = UserRepository()

    launch {
        userRepo.register("Alice")
        delay(10L)
        userRepo.register("Bob")
    }

    assertEquals(listOf("Alice", "Bob"), userRepo.getAllUsers()) // ❌ Fails
}

Injecting test dispatchers

Code under test might use dispatchers to switch threads (using withContext) or to start new coroutines. When code is executed on multiple threads in parallel, tests can become flaky. It can be difficult to perform assertions at the correct time or to wait for tasks to complete if they’re running on background threads that you have no control over.

In tests, replace these dispatchers with instances of TestDispatchers. This has several benefits:

  • The code will run on the single test thread, making tests more deterministic
  • You can control how new coroutines are scheduled and executed
  • TestDispatchers use a scheduler for virtual time, which skips delays automatically and lets you advance time manually

Using dependency injection to provide dispatchers to your classes makes it easy to replace the real dispatchers in tests.

TestDispatchers will, by default, create a new scheduler when they’re instantiated. Inside runTest, you can access the testScheduler property of the TestScope and pass it in to any newly created TestDispatchers. This will share their understanding of virtual time, and methods like advanceUntilIdle will run coroutines on all test dispatchers to completion.

In the following example, you can see a Repository class that creates a new coroutine using the IO dispatcher in its initialize method and switches the caller to the IO dispatcher in its fetchData method:

// Example class demonstrating dispatcher use cases
class Repository(private val ioDispatcher: CoroutineDispatcher = Dispatchers.IO) {
    private val scope = CoroutineScope(ioDispatcher)
    val initialized = AtomicBoolean(false)

    // A function that starts a new coroutine on the IO dispatcher
    fun initialize() {
        scope.launch {
            initialized.set(true)
        }
    }

    // A suspending function that switches to the IO dispatcher
    suspend fun fetchData(): String = withContext(ioDispatcher) {
        require(initialized.get()) { "Repository should be initialized first" }
        delay(500L)
        "Hello world"
    }
}

In tests, you can inject a TestDispatcher implementation to replace the IO dispatcher.

In the example below, we inject a StandardTestDispatcher into the repository, and use advanceUntilIdle to make sure that the new coroutine started in initialize completes before proceeding.

fetchData will also benefit from running on a TestDispatcher, as it will run on the test thread and skip the delay it contains during the test.

class RepositoryTest {
    @Test
    fun repoInitWorksAndDataIsHelloWorld() = runTest {
        val dispatcher = StandardTestDispatcher(testScheduler)
        val repository = Repository(dispatcher)

        repository.initialize()
        advanceUntilIdle() // Runs the new coroutine
        assertEquals(true, repository.initialized.get())

        val data = repository.fetchData() // No thread switch, delay is skipped
        assertEquals("Hello world", data)
    }
}

New coroutines started on a TestDispatcher can be advanced manually as shown above with initialize. Note, however, that this would not be possible or desirable in production code. Instead, this method should be redesigned to be either suspending (for sequential execution), or to return a Deferred value (for concurrent execution).

For example, you can use async to start a new coroutine and create a Deferred:

class BetterRepository(private val ioDispatcher: CoroutineDispatcher = Dispatchers.IO) {
    private val scope = CoroutineScope(ioDispatcher)

    fun initialize() = scope.async {
        // ...
    }
}

This lets you safely await the completion of this code in both tests and production code:

@Test
fun repoInitWorks() = runTest {
    val dispatcher = StandardTestDispatcher(testScheduler)
    val repository = BetterRepository(dispatcher)

    repository.initialize().await() // Suspends until the new coroutine is done
    assertEquals(true, repository.initialized.get())
    // ...
}

runTest will wait for pending coroutines to complete before returning if the coroutines are on a TestDispatcher that it shares a scheduler with. It will also wait for coroutines that are children of the top-level test coroutine, even if they’re on other dispatchers (up to a timeout specified by the dispatchTimeoutMs parameter, which is 60 seconds by default).

Setting the Main dispatcher

In local unit tests, the Main dispatcher that wraps the Android UI thread will be unavailable, as these tests are executed on a local JVM and not an Android device. If your code under test references the main thread, it’ll throw an exception during unit tests.

In some cases, you can inject the Main dispatcher the same way as other dispatchers, as described in the previous section, allowing you to replace it with a TestDispatcher in tests. However, some APIs such as viewModelScope use a hardcoded Main dispatcher under the hood.

Here’s an example of a ViewModel implementation that uses viewModelScope to launch a coroutine that loads data:

class HomeViewModel : ViewModel() {
    private val _message = MutableStateFlow("")
    val message: StateFlow<String> get() = _message

    fun loadMessage() {
        viewModelScope.launch {
            _message.value = "Greetings!"
        }
    }
}

To replace the Main dispatcher with a TestDispatcher in all cases, use the Dispatchers.setMain and Dispatchers.resetMain functions.

class HomeViewModelTest {
    @Test
    fun settingMainDispatcher() = runTest {
        val testDispatcher = UnconfinedTestDispatcher(testScheduler)
        Dispatchers.setMain(testDispatcher)

        try {
            val viewModel = HomeViewModel()
            viewModel.loadMessage() // Uses testDispatcher, runs its coroutine eagerly
            assertEquals("Greetings!", viewModel.message.value)
        } finally {
            Dispatchers.resetMain()
        }
    }
}

If the Main dispatcher has been replaced with a TestDispatcher, any newly-created TestDispatchers will automatically use the scheduler from the Main dispatcher, including the StandardTestDispatcher created by runTest if no other dispatcher is passed to it.

This makes it easier to ensure that there is only a single scheduler in use during the test. For this to work, make sure to create all other TestDispatcher instances after calling Dispatchers.setMain.

A common pattern to avoid duplicating the code that replaces the Main dispatcher in each test is to extract it into a JUnit test rule:

// Reusable JUnit4 TestRule to override the Main dispatcher
class MainDispatcherRule(
    val testDispatcher: TestDispatcher = UnconfinedTestDispatcher(),
) : TestWatcher() {
    override fun starting(description: Description) {
        Dispatchers.setMain(testDispatcher)
    }

    override fun finished(description: Description) {
        Dispatchers.resetMain()
    }
}

class HomeViewModelTestUsingRule {
    @get:Rule
    val mainDispatcherRule = MainDispatcherRule()

    @Test
    fun settingMainDispatcher() = runTest { // Uses Main’s scheduler
        val viewModel = HomeViewModel()
        viewModel.loadMessage()
        assertEquals("Greetings!", viewModel.message.value)
    }
}

This rule implementation uses an UnconfinedTestDispatcher by default, but a StandardTestDispatcher can be passed in as a parameter if the Main dispatcher shouldn’t execute eagerly in a given test class.

When you need a TestDispatcher instance in the test body, you can reuse the testDispatcher from the rule, as long as it’s the desired type. If you want to be explicit about the type of TestDispatcher used in the test, or if you need a TestDispatcher that’s a different type than the one used for Main, you can create a new TestDispatcher within runTest. As the Main dispatcher is set to a TestDispatcher, any newly created TestDispatchers will share its scheduler automatically.

class DispatcherTypesTest {
    @get:Rule
    val mainDispatcherRule = MainDispatcherRule()

    @Test
    fun injectingTestDispatchers() = runTest { // Uses Main’s scheduler
        // Use the UnconfinedTestDispatcher from the Main dispatcher
        val unconfinedRepo = Repository(mainDispatcherRule.testDispatcher)

        // Create a new StandardTestDispatcher (uses Main’s scheduler)
        val standardRepo = Repository(StandardTestDispatcher())
    }
}

Creating dispatchers outside a test

In some cases, you might need a TestDispatcher to be available outside the test method. For example, during the initialization of a property in the test class:

class Repository(private val ioDispatcher: CoroutineDispatcher) { /* ... */ }

class RepositoryTestWithRule {
    private val repository = Repository(/* What TestDispatcher? */)

    @get:Rule
    val mainDispatcherRule = MainDispatcherRule()

    @Test
    fun someRepositoryTest() = runTest {
        // Test the repository...
    }
}

If you’re replacing the Main dispatcher as shown in the previous section, TestDispatchers created after the Main dispatcher has been replaced will automatically share its scheduler.

This isn’t the case, however, for TestDispatchers created as properties of the test class or TestDispatchers created during the initialization of properties in the test class. These are initialized before the Main dispatcher is replaced. Therefore, they would create new schedulers.

To make sure that there’s only one scheduler in your test, create the MainDispatcherRule property first. Then reuse its dispatcher (or its scheduler, if you need a TestDispatcher of a different type) in the initializers of other class-level properties as needed.

class RepositoryTestWithRule {
    @get:Rule
    val mainDispatcherRule = MainDispatcherRule()

    private val repository = Repository(mainDispatcherRule.testDispatcher)

    @Test
    fun someRepositoryTest() = runTest { // Takes scheduler from Main
        // Any TestDispatcher created here also takes the scheduler from Main
        val newTestDispatcher = StandardTestDispatcher()

        // Test the repository...
    }
}

Note that both runTest and TestDispatchers created within the test will still automatically share the scheduler of the Main dispatcher.

If you’re not replacing the Main dispatcher, create your first TestDispatcher (which creates a new scheduler) as a property of the class. Then, manually pass that scheduler to each runTest invocation and each new TestDispatcher created, both as properties and within the test:

class RepositoryTest {
    // Creates the single test scheduler
    private val testDispatcher = UnconfinedTestDispatcher()
    private val repository = Repository(testDispatcher)

    @Test
    fun someRepositoryTest() = runTest(testDispatcher.scheduler) {
        // Take the scheduler from the TestScope
        val newTestDispatcher = UnconfinedTestDispatcher(this.testScheduler)
        // Or take the scheduler from the first dispatcher, they’re the same
        val anotherTestDispatcher = UnconfinedTestDispatcher(testDispatcher.scheduler)

        // Test the repository...
    }
}

In this sample, the scheduler from the first dispatcher is passed to runTest. This will create a new StandardTestDispatcher for the TestScope using that scheduler. You could also pass in the dispatcher to runTest directly to run the test coroutine on that dispatcher.

Creating your own TestScope

Like with TestDispatchers, you might need to access a TestScope outside the test body. While runTest creates a TestScope under the hood automatically, you can also create your own TestScope to use with runTest.

When doing this, make sure to call runTest on the TestScope you’ve created:

class SimpleExampleTest {
    val testScope = TestScope() // Creates a StandardTestDispatcher

    @Test
    fun someTest() = testScope.runTest {
        // ...
    }
}

The code above creates a StandardTestDispatcher for the TestScope implicitly, as well as a new scheduler. These objects can all also be created explicitly. This can be useful if you need to integrate it with dependency injection setups.

class ExampleTest {
    val testScheduler = TestCoroutineScheduler()
    val testDispatcher = StandardTestDispatcher(testScheduler)
    val testScope = TestScope(testDispatcher)

    @Test
    fun someTest() = testScope.runTest {
        // ...
    }
}

Additional resources