Using Robolectric Correctly

Using Robolectric Correctly — Android On The JVM, Without The Footguns

Robolectric simulates Android on the JVM by swapping in pure-Java reimplementations of system classes ("shadows"). It is fast (10-100x vs an emulator) but not an emulator: no real RenderThread, no real Binder, no real GPU. This skill locks down the runner choice, the @Config matrix, the looper draining ritual, and the AGP 7.2+ sharedTest gotcha. Compose-on-Robolectric specifics live in ../../../compose/synchronization/controlling-the-test-clock/SKILL.md.

When to use this skill

• The class under test imports android.* (Context, Resources, View, PackageManager, Looper) and the developer wants to test it on the JVM in seconds, not on an emulator in minutes.
• A unit test fails with RuntimeException: Method ... not mocked (the bare-Android-jar default) — the developer needs Robolectric's shadows.
• A unit test fails with Resources$NotFoundException — testOptions.unitTests.includeAndroidResources is missing.
• The developer asks "AndroidJUnit4 vs RobolectricTestRunner — which?".
• A Handler.postDelayed / Looper queue does not advance under Robolectric's default LooperMode.PAUSED.
• The developer asks how to run the same test source on JVM and on a device (sharedTest / androidHostTest+androidDeviceTest).
• Build error after AGP 7.2+: "Source directory ... already added to source set", caused by the classic sharedTest srcDir sharing pattern.

When NOT to use this skill

• The class under test has zero android.* imports. Robolectric pays a 1-3 s class-load tax for nothing — use plain JUnit4. See ../../runner/configuring-junit4-on-android/SKILL.md.
• The test verifies pixel correctness, ripple animations, screenshot diffs, accessibility services, or RenderThread timing. Run on an emulator/device. See ../../../instrumentation/runner/running-instrumented-tests-with-androidjunit4/SKILL.md.
• The test is about coroutines + runTest and does not need framework Android. Use ../../coroutines/testing-coroutines-with-runtest/SKILL.md.
• The test is about Compose UI rendering. The host-test environment is set up by ../../../compose/setup/configuring-test-dependencies/SKILL.md and the clock semantics live in ../../../compose/synchronization/controlling-the-test-clock/SKILL.md.

Prerequisites

testImplementation("junit:junit:4.13.2")
testImplementation("org.robolectric:robolectric:4.x")
testImplementation("androidx.test:core:1.7.0")              // ApplicationProvider works on both runtimes
testImplementation("androidx.test.ext:junit:1.3.0")         // androidx.test.ext.junit.runners.AndroidJUnit4

(docs/CORPUS.md §G.1; R7.)

android {
    testOptions {
        unitTests {
            includeAndroidResources = true                  // non-negotiable
        }
    }
}

includeAndroidResources is required if any test (or any class loaded by any test) touches R.*, Resources, layouts, themes, or anything from res/. Without it, resource lookups throw at test startup (R7).

Robolectric 4.x supports SDKs 21 through the latest stable. The androidx Compose host tests pin a floor of 23 — quote it as a single source of truth:

// androidx/compose/ui/ui-test/src/androidHostTest/.../Constants.kt
internal const val RobolectricMinSdk = 23

(R7.)

Runner choice — AndroidJUnit4 vs RobolectricTestRunner

Two runners work; they read @Config identically.

(R7; docs/CORPUS.md §G.6.)

import androidx.test.ext.junit.runners.AndroidJUnit4
@RunWith(AndroidJUnit4::class)
@Config(minSdk = 23)
class FooTest { /* runs under Robolectric on JVM, real runner on device */ }

import org.robolectric.RobolectricTestRunner
@RunWith(RobolectricTestRunner::class)
@Config(sdk = [33])
class LooperBoundTest { /* uses ShadowLooper APIs directly — JVM only */ }

CRITICAL: androidx.test.runner.AndroidJUnit4 (the runner package, not ext.junit.runners) is @Deprecated (docs/CORPUS.md §G.2). Always import from androidx.test.ext.junit.runners.

@Config — sdk, qualifiers, application

@Config configures the simulated Android environment per class or per method. Method-level overrides class-level; subclasses inherit and may override.

// Single SDK
@Config(sdk = [Build.VERSION_CODES.UPSIDE_DOWN_CAKE])
class FooTest

// SDK matrix — runs once per listed SDK
@Config(sdk = [21, 28, 33])
class BarTest

// Range — runs once at the project's targetSdk if it is >= minSdk
@Config(minSdk = 23)
class BazTest

// Per-method override
class QuxTest {
    @Test @Config(sdk = [Build.VERSION_CODES.TIRAMISU])
    fun onTiramisuOnly() { /* ... */ }

    @Test @Config(qualifiers = "fr-rFR-w820dp")           // locale + screen
    fun frenchLayout() { /* ... */ }

    @Test @Config(qualifiers = "+night")                   // additive: keep, force night
    fun darkColors() { /* ... */ }
}

// Override the Application instance
@Config(application = MyTestApplication::class)
class WithFakeApp

(R7.)

Constraint: sdk and minSdk/maxSdk cannot coexist on the same annotation. minSdk and maxSdk may be combined.

CI-time hot take: prefer @Config(sdk = [<one SDK>]) on the commit gate; reserve multi-SDK matrices for the merge gate. Each SDK in the matrix re-runs the whole class.

Shadows

Shadows are JVM-friendly fakes for Android system classes. Robolectric ships hundreds; you can write your own with @Implements/@Implementation. Common ones:

Shadows.shadowOf(realObject) returns the shadow instance. Static-import and rely on overload resolution:

import androidx.test.core.app.ApplicationProvider
import org.robolectric.Shadows.shadowOf
import org.robolectric.shadows.ShadowPackageManager

val app: Application = ApplicationProvider.getApplicationContext()
val shadowPm: ShadowPackageManager = shadowOf(app.packageManager)
shadowPm.setSystemFeature(PackageManager.FEATURE_CAMERA_ANY, true)

Looper draining — the most-missed step

Robolectric's main looper is paused in the default LooperMode.PAUSED runtime mode. Handler messages posted by the SUT do NOT run automatically. Drain explicitly:

import android.os.Looper
import org.robolectric.Shadows.shadowOf
import org.robolectric.shadows.ShadowLooper
import java.time.Duration

// Drain everything currently queued on the main looper.
shadowOf(Looper.getMainLooper()).idle()

// Run delayed tasks too (postDelayed et al).
ShadowLooper.runUiThreadTasksIncludingDelayedTasks()

// Advance virtual time by N ms; only handlers due before that time fire.
shadowOf(Looper.getMainLooper()).idleFor(Duration.ofMillis(500))

(R7.)

This complements (and is independent from) Compose's MainTestClock.advanceTimeBy(...). The Compose clock and the platform looper are separate. For Compose-on-Robolectric, runComposeUiTest { … waitForIdle() } drives the looper for you via RobolectricIdlingStrategy — outside Compose, drain manually.

sharedTest / androidHostTest / androidDeviceTest

The classic sharedTest pattern (per the 2021 Robolectric blog post) puts one source directory under both test and androidTest:

// LEGACY — broken on AGP 7.2+
android {
    sourceSets {
        String dir = 'src/sharedTest/'
        test       { java.srcDirs += dir + 'java' }
        androidTest { java.srcDirs += dir + 'java' }
    }
}

CRITICAL: starting with AGP 7.2+, you cannot put the same source directory into multiple source sets (Robolectric GitHub issue #7432; R7). The build fails with "Source directory ... already added to source set".

Modern alternatives (PREFERRED order):

1. AndroidX-style KMP source sets — commonTest -> {androidHostTest, androidDeviceTest} via the androidXMultiplatform plugin. Compose internally migrated to this:

   sourceSets {
       androidCommonTest  { dependsOn(commonTest) }
       androidDeviceTest  { dependsOn(androidCommonTest) }   // instrumented APK
       androidHostTest    { dependsOn(androidCommonTest) }   // Robolectric on JVM
   }
   

2. Separate Gradle module that both test and androidTest depend on as testImplementation / androidTestImplementation.
3. Symlink or copy sources at configuration time (least preferred — fragile).

Both androidHostTest and androidDeviceTest can use @RunWith(AndroidJUnit4::class) unchanged because AndroidJUnit4 is a router. Source-set wiring is ../../../fundamentals/strategies/organizing-test-source-sets/SKILL.md.

When NOT to reach for Robolectric

Patterns

Pattern: WRONG — RobolectricTestRunner for a test that should also run on device

// WRONG
@RunWith(RobolectricTestRunner::class)
@Config(minSdk = 23)
class LoginViewModelTest {
    @Test fun loginFlow() { /* ... */ }
}
// WRONG because: this class CAN run unchanged on a device too — there are no
// Robolectric-only APIs in the body. Pinning to RobolectricTestRunner locks the
// test to the JVM and forfeits the sharedTest pattern. The same class moved to
// src/androidTest/ (or androidDeviceTest) would not run.

// RIGHT
@RunWith(AndroidJUnit4::class)
@Config(minSdk = RobolectricMinSdk)                      // pin via single source of truth
class LoginViewModelTest {
    @Test fun loginFlow() { /* ... */ }
}

Pattern: WRONG — assert before draining the looper

// WRONG
@RunWith(AndroidJUnit4::class)
class HandlerTest {
    @Test fun postedRuns() {
        var fired = false
        Handler(Looper.getMainLooper()).post { fired = true }
        assertTrue(fired)                                 // FAIL: runnable is queued, not run
    }
}
// WRONG because: Robolectric's default LooperMode.PAUSED does not auto-run posted
// runnables. The post is queued; assertTrue runs before the queue is drained.

// RIGHT
@RunWith(AndroidJUnit4::class)
class HandlerTest {
    @Test fun postedRuns() {
        var fired = false
        Handler(Looper.getMainLooper()).post { fired = true }
        shadowOf(Looper.getMainLooper()).idle()           // drain queue
        assertTrue(fired)
    }
}

Pattern: WRONG — missing includeAndroidResources

// WRONG
android {
    testOptions {
        unitTests { /* includeAndroidResources missing */ }
    }
}

Symptom: android.content.res.Resources$NotFoundException: String resource ID #0x7f0c0001 at test startup. The fix:

// RIGHT
android {
    testOptions {
        unitTests {
            includeAndroidResources = true                // Groovy
            // isIncludeAndroidResources = true           // Kotlin DSL
        }
    }
}

Pattern: WRONG — sharedTest srcDir sharing on AGP 7.2+

// WRONG
android {
    sourceSets {
        test       { java.srcDirs += 'src/sharedTest/java' }
        androidTest { java.srcDirs += 'src/sharedTest/java' }
    }
}
// WRONG because: AGP 7.2+ rejects sharing a single source directory across
// source sets (Robolectric issue #7432). Build fails with "Source directory
// already added to source set".

// RIGHT — AndroidX-style KMP source sets
sourceSets {
    androidCommonTest  { dependsOn(commonTest) }
    androidDeviceTest  { dependsOn(androidCommonTest) }
    androidHostTest    { dependsOn(androidCommonTest) }
}

Pattern: ParameterizedRobolectricTestRunner — when matrix is the point

// androidx/compose/ui/ui-test/.../ViewVisibilityRobolectricTest.kt (paraphrased)
@RunWith(ParameterizedRobolectricTestRunner::class)
@Config(minSdk = RobolectricMinSdk)
class ViewVisibilityRobolectricTest(private val visibility: Int) {
    companion object {
        @JvmStatic
        @Parameters(name = "visibility={0}")
        fun params() = listOf(View.VISIBLE, View.INVISIBLE, View.GONE)
    }
    @Test fun behaves() { /* uses `visibility` */ }
}

This is the one place to pin RobolectricTestRunner (its parameterized subclass) — there is no AndroidJUnit4 parameterized variant.

Pattern: production code branching on Robolectric — read with caution

// androidx/compose/ui/ui-test/.../RobolectricIdlingStrategy.android.kt
internal val HasRobolectricFingerprint
    get() = Build.FINGERPRINT.lowercase() == "robolectric"

This idiom is widespread inside androidx (Compose accessibility, idling, graphics layers). Library code under test SHOULD avoid copying it unless the branch is genuinely test-only — misdetection silently changes behavior.

Mandatory rules

• MUST import AndroidJUnit4 from androidx.test.ext.junit.runners. MUST NOT import from androidx.test.runner — that one is @Deprecated (docs/CORPUS.md §G.2).
• MUST prefer @RunWith(AndroidJUnit4::class) over @RunWith(RobolectricTestRunner::class). The latter only when the test uses Robolectric-specific APIs (e.g. shadowOf(Looper.getMainLooper()).idle()) directly, or needs ParameterizedRobolectricTestRunner.
• MUST set testOptions.unitTests.includeAndroidResources = true for any project whose unit tests touch R.* / Resources / layouts / themes.
• MUST drain the looper with shadowOf(Looper.getMainLooper()).idle() (or ShadowLooper.runUiThreadTasksIncludingDelayedTasks()) before asserting anything that depends on a Handler.post* callback. Default LooperMode.PAUSED does NOT auto-run.
• MUST pin a class-level @Config(minSdk = …) (or sdk = […]) — do not depend on the project's default SDK varying in CI.
• MUST NOT use the legacy srcDirs sharing pattern test.java.srcDirs += 'src/sharedTest/java' on AGP 7.2+. Migrate to androidHostTest + androidDeviceTest (KMP source sets) or a separate Gradle module.
• MUST NOT add Robolectric to a test class that has zero android.* imports. The class-load tax is 1-3 s for nothing.
• MUST NOT use Robolectric for screenshot tests, ripple animations, accessibility services, or RenderThread timing — use an emulator/device.
• PREFERRED: quote RobolectricMinSdk = 23 from androidx/compose/ui/ui-test/src/androidHostTest/.../Constants.kt as the single source of truth for the floor SDK in androidHostTest modules.
• PREFERRED: group related tests into the same class to amortize the per-class class-load tax. Avoid sprinkling @RunWith(RobolectricTestRunner::class) across many small classes.
• PREFERRED: single-SDK @Config(sdk = [33]) on the commit gate; multi-SDK matrices on the merge gate.

Verification

• No import of androidx.test.runner.AndroidJUnit4 (use androidx.test.ext.junit.runners.AndroidJUnit4). grep -rn 'androidx.test.runner.AndroidJUnit4' src/test src/androidTest returns empty.
• testOptions.unitTests.includeAndroidResources = true is set in the module's build.gradle(.kts).
• No @RunWith(RobolectricTestRunner::class) exists on a test class that does not actually use Robolectric-specific APIs.
• Every test that posts to a Handler / triggers Looper work calls shadowOf(Looper.getMainLooper()).idle() (or runUiThreadTasksIncludingDelayedTasks) before asserting.
• Every Robolectric test class declares @Config(minSdk = …) or @Config(sdk = […]).
• Source-set wiring uses androidHostTest + androidDeviceTest (or a shared module), not legacy srcDirs += sharedTest.
• No screenshot/ripple/accessibility-service test runs under Robolectric — those live in src/androidTest/ or androidDeviceTest.
• ./gradlew :module:test passes; ./gradlew :module:connectedDebugAndroidTest passes for any test in androidCommonTest.

References

• Robolectric — getting started: http://robolectric.org/getting-started/
• Robolectric — configuring: http://robolectric.org/configuring/
• Robolectric — sharedTest blog post: http://robolectric.org/blog/2021/10/06/sharedTest/
• Android Developers — Build local unit tests: https://developer.android.com/training/testing/local-tests
• Android Developers — Robolectric: https://developer.android.com/training/testing/local-tests/robolectric
• Robolectric GitHub issue #7432 — AGP 7.2 sharedTest breakage: https://github.com/robolectric/robolectric/issues/7432
• Research: tasks/research/R7-robolectric.md.
• docs/CORPUS.md §G.1 / §G.6 — Gradle coordinates and runner choice.
• androidx/compose/ui/ui-test/src/androidHostTest/.../Constants.kt — RobolectricMinSdk = 23 floor.
• androidx/compose/ui/ui-test/.../RobolectricComposeTest.kt — canonical @RunWith(AndroidJUnit4::class) @Config(minSdk = RobolectricMinSdk) cross-runtime test.
• androidx/compose/ui/ui-test/.../PrefetchNotHangingMainThreadTest.kt — @RunWith(RobolectricTestRunner::class) with explicit shadowOf(Looper.getMainLooper()).idle().
• androidx/compose/ui/ui-test/.../ViewVisibilityRobolectricTest.kt — ParameterizedRobolectricTestRunner example.
• androidx/compose/ui/ui-test/.../RobolectricIdlingStrategy.android.kt — Build.FINGERPRINT.lowercase() == "robolectric" detection idiom.
• androidx/compose/ui/ui/.../MediaQueryIntegrationTest.kt — ShadowPackageManager / ShadowInputManager in an @RunWith(AndroidJUnit4::class) test.
• Sibling: ../../coroutines/testing-coroutines-with-runtest/SKILL.md — runTest + MainDispatcherRule (independent of Robolectric).
• Sibling: ../../coroutines/testing-flows-with-turbine/SKILL.md — Flow assertions; Turbine works under Robolectric unchanged.
• Sibling: ../../runner/configuring-junit4-on-android/SKILL.md — JUnit4 plumbing surrounding Robolectric.
• Sibling: ../../mocking/mocking-with-mockito/SKILL.md — mocking Android types under Robolectric.
• Sibling: ../../mocking/mocking-with-mockk/SKILL.md — mockkStatic(Build::class) for fingerprint branches.
• Cross-set: ../../../fundamentals/strategies/applying-testing-strategies/SKILL.md — Robolectric is "medium" tests in Google's three-size framing.
• Cross-set: ../../../fundamentals/strategies/organizing-test-source-sets/SKILL.md — src/test/ vs src/androidTest/ vs androidHostTest/androidDeviceTest.
• Cross-set: ../../../compose/synchronization/controlling-the-test-clock/SKILL.md — Compose's MainTestClock is separate from ShadowLooper.
• Cross-set: ../../../instrumentation/runner/running-instrumented-tests-with-androidjunit4/SKILL.md — when the same test source needs to run on a device.