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NullPointerException when there are racy resources #2456

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NullPointerException when there are racy resources#2456

Description

@TWiStErRob
TWiStErRob
opened on Dec 18, 2025

Only happened once, cannot reproduce, but reading the code confirms there's a bug here:

android-test/espresso/core/java/androidx/test/espresso/base/UiControllerImpl.java

Lines 553 to 559 in eacf07d

List<String> busyResources = idlingResourceRegistry.getBusyResources();
conditionName =
String.format(
Locale.ROOT,
"%s(busy resources=%s)",
conditionName,
StringJoinerKt.joinToString(busyResources, ","));

android-test/espresso/core/java/androidx/test/espresso/util/StringJoiner.kt

Line 34 in eacf07d

fun joinToString(iterable: Iterable<kotlin.Any>, delimiter: String): String {

the above expects a non-null list, but the below returns null:

android-test/espresso/core/java/androidx/test/espresso/base/IdlingResourceRegistry.java

Lines 366 to 387 in eacf07d

List<String> getBusyResources() {
List<String> busyResourceNames = new ArrayList<>();
List<IdlingState> racyResources = new ArrayList<>();
for (IdlingState state : idlingStates) {
if (!state.idle) {
if (state.resource.isIdleNow()) {
// We have not been notified of a BUSY -> IDLE transition, but the resource is telling us
// its that its idle. Either it's a race condition or is this resource buggy.
racyResources.add(state);
} else {
busyResourceNames.add(state.resource.getName());
}
}
}
if (!racyResources.isEmpty()) {
Message raceBuster =
handler.obtainMessage(POSSIBLE_RACE_CONDITION_DETECTED, TIMEOUT_MESSAGE_TAG);
raceBuster.obj = racyResources;
handler.sendMessage(raceBuster);
return null; 

java.lang.RuntimeException: java.util.concurrent.ExecutionException: java.lang.NullPointerException: Parameter specified as non-null is null: method androidx.test.espresso.util.StringJoinerKt.joinToString, parameter iterable
    at androidx.test.espresso.Espresso.onIdle(Espresso.java:357)
    at androidx.test.espresso.Espresso.onIdle(Espresso.java:378)
    at androidx.compose.ui.test.EspressoLink_androidKt.runEspressoOnIdle(EspressoLink.android.kt:88)
    at androidx.compose.ui.test.EspressoLink.runUntilIdle(EspressoLink.android.kt:82)
    at androidx.compose.ui.test.AndroidComposeUiTestEnvironment.waitForIdle(ComposeUiTest.android.kt:596)
    at androidx.compose.ui.test.AndroidComposeUiTestEnvironment.access$waitForIdle(ComposeUiTest.android.kt:391)
    at androidx.compose.ui.test.AndroidComposeUiTestEnvironment$AndroidTestOwner.getRoots(ComposeUiTest.android.kt:822)
    at androidx.compose.ui.test.TestOwnerKt.getAllSemanticsNodes(TestOwner.kt:81)
    at androidx.compose.ui.test.SemanticsNodeInteraction.fetchSemanticsNodes$ui_test_release(SemanticsNodeInteraction.kt:64)
    at androidx.compose.ui.test.SemanticsNodeInteraction.fetchSemanticsNodes$ui_test_release$default(SemanticsNodeInteraction.kt:59)
    at androidx.compose.ui.test.AndroidAssertions_androidKt.checkIsDisplayed(AndroidAssertions.android.kt:29)
    at androidx.compose.ui.test.AssertionsKt.isDisplayed(Assertions.kt:351)
    at androidx.compose.ui.test.AndroidComposeUiTestEnvironment$AndroidComposeUiTestImpl.waitUntil(ComposeUiTest.android.kt:747)
    at androidx.compose.ui.test.junit4.AndroidComposeTestRule.waitUntil(AndroidComposeTestRule.android.kt:362)
    at com.example.MyTest.myTest(MyTest.kt:78)
    at java.lang.reflect.Method.invoke(Native Method)
    at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:59)
    at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
    at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:56)
    at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
    at androidx.test.internal.runner.junit4.statement.RunBefores.evaluate(RunBefores.java:80)
    at org.junit.rules.ExternalResource$1.evaluate(ExternalResource.java:54)
    at androidx.test.rule.GrantPermissionRule$RequestPermissionStatement.evaluate(GrantPermissionRule.java:150)
    at org.junit.rules.ExternalResource$1.evaluate(ExternalResource.java:54)
    at org.junit.rules.ExternalResource$1.evaluate(ExternalResource.java:54)
    at org.junit.rules.ExternalResource$1.evaluate(ExternalResource.java:54)
    at androidx.compose.ui.test.junit4.AndroidComposeTestRule$apply$testWithDisposal$1.evaluate(AndroidComposeTestRule.android.kt:306)
    at org.junit.rules.ExternalResource$1.evaluate(ExternalResource.java:54)
    at androidx.compose.ui.test.junit4.AndroidComposeTestRule$apply$1$evaluate$1.invokeSuspend(AndroidComposeTestRule.android.kt:318)
    at androidx.compose.ui.test.junit4.AndroidComposeTestRule$apply$1$evaluate$1.invoke(Unknown Source:8)
    at androidx.compose.ui.test.junit4.AndroidComposeTestRule$apply$1$evaluate$1.invoke(Unknown Source:4)
    at androidx.compose.ui.test.AndroidComposeUiTestEnvironment$runTest$1$1$1$1$1.invokeSuspend(ComposeUiTest.android.kt:569)
    at androidx.compose.ui.test.AndroidComposeUiTestEnvironment$runTest$1$1$1$1$1.invoke(Unknown Source:8)
    at androidx.compose.ui.test.AndroidComposeUiTestEnvironment$runTest$1$1$1$1$1.invoke(Unknown Source:2)
    at androidx.compose.ui.test.EspressoLink.withStrategy(EspressoLink.android.kt:69)
    at androidx.compose.ui.test.AndroidComposeUiTestEnvironment$runTest$1$1.invokeSuspend(ComposeUiTest.android.kt:566)
    at androidx.compose.ui.test.AndroidComposeUiTestEnvironment$runTest$1$1.invoke(Unknown Source:8)
    at androidx.compose.ui.test.AndroidComposeUiTestEnvironment$runTest$1$1.invoke(Unknown Source:4)
    at kotlinx.coroutines.test.TestBuildersKt__TestBuildersKt$runTest$2$1$1.invokeSuspend(TestBuilders.kt:317)
    at kotlin.coroutines.jvm.internal.BaseContinuationImpl.resumeWith(ContinuationImpl.kt:34)
    at kotlinx.coroutines.DispatchedTask.run(DispatchedTask.kt:100)
    at kotlinx.coroutines.test.TestDispatcher.processEvent$kotlinx_coroutines_test(TestDispatcher.kt:24)
    at kotlinx.coroutines.test.TestCoroutineScheduler.tryRunNextTaskUnless$kotlinx_coroutines_test(TestCoroutineScheduler.kt:99)
    at kotlinx.coroutines.test.TestBuildersKt__TestBuildersKt$runTest$2$1$workRunner$1.invokeSuspend(TestBuilders.kt:326)
    at kotlin.coroutines.jvm.internal.BaseContinuationImpl.resumeWith(ContinuationImpl.kt:34)
    at kotlinx.coroutines.DispatchedTask.run(DispatchedTask.kt:100)
    at kotlinx.coroutines.EventLoopImplBase.processNextEvent(EventLoop.common.kt:263)
    at kotlinx.coroutines.BlockingCoroutine.joinBlocking(Builders.kt:94)
    at kotlinx.coroutines.BuildersKt__BuildersKt.runBlocking(Builders.kt:70)
    at kotlinx.coroutines.BuildersKt.runBlocking(Unknown Source:1)
    at kotlinx.coroutines.BuildersKt__BuildersKt.runBlocking$default(Builders.kt:48)
    at kotlinx.coroutines.BuildersKt.runBlocking$default(Unknown Source:1)
    at kotlinx.coroutines.test.TestBuildersJvmKt.createTestResult(TestBuildersJvm.kt:10)
    at kotlinx.coroutines.test.TestBuildersKt__TestBuildersKt.runTest-8Mi8wO0(TestBuilders.kt:309)
    at kotlinx.coroutines.test.TestBuildersKt.runTest-8Mi8wO0(Unknown Source:1)
    at kotlinx.coroutines.test.TestBuildersKt__TestBuildersKt.runTest-8Mi8wO0(TestBuilders.kt:167)
    at kotlinx.coroutines.test.TestBuildersKt.runTest-8Mi8wO0(Unknown Source:1)
    at androidx.compose.ui.test.AndroidComposeUiTestEnvironment.runTest(ComposeUiTest.android.kt:549)
    at androidx.compose.ui.test.junit4.AndroidComposeTestRule$apply$1.evaluate(AndroidComposeTestRule.android.kt:318)
    at org.junit.runners.ParentRunner$3.evaluate(ParentRunner.java:306)
    at org.junit.runners.BlockJUnit4ClassRunner$1.evaluate(BlockJUnit4ClassRunner.java:100)
    at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:366)
    at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:103)
    at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:63)
    at org.junit.runners.ParentRunner$4.run(ParentRunner.java:331)
    at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:79)
    at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:329)
    at org.junit.runners.ParentRunner.access$100(ParentRunner.java:66)
    at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:293)
    at org.junit.runners.ParentRunner$3.evaluate(ParentRunner.java:306)
    at org.junit.runners.ParentRunner.run(ParentRunner.java:413)
    at org.junit.runners.Suite.runChild(Suite.java:128)
    at org.junit.runners.Suite.runChild(Suite.java:27)
    at org.junit.runners.ParentRunner$4.run(ParentRunner.java:331)
    at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:79)
    at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:329)
    at org.junit.runners.ParentRunner.access$100(ParentRunner.java:66)
    at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:293)
    at org.junit.runners.ParentRunner$3.evaluate(ParentRunner.java:306)
    at org.junit.runners.ParentRunner.run(ParentRunner.java:413)
    at org.junit.runner.JUnitCore.run(JUnitCore.java:137)
    at org.junit.runner.JUnitCore.run(JUnitCore.java:115)
    at androidx.test.internal.runner.TestExecutor.execute(TestExecutor.java:68)
    at androidx.test.internal.runner.TestExecutor.execute(TestExecutor.java:59)
    at androidx.test.runner.AndroidJUnitRunner.onStart(AndroidJUnitRunner.java:467)
    at android.app.Instrumentation$InstrumentationThread.run(Instrumentation.java:2594)
Caused by: java.util.concurrent.ExecutionException: java.lang.NullPointerException: Parameter specified as non-null is null: method androidx.test.espresso.util.StringJoinerKt.joinToString, parameter iterable
    at java.util.concurrent.FutureTask.report(FutureTask.java:122)
    at java.util.concurrent.FutureTask.get(FutureTask.java:191)
    at androidx.test.espresso.Espresso.onIdle(Espresso.java:349)
... 100 more
Caused by: java.lang.NullPointerException: Parameter specified as non-null is null: method androidx.test.espresso.util.StringJoinerKt.joinToString, parameter iterable
    at androidx.test.espresso.util.StringJoinerKt.joinToString(Unknown Source:2)
    at androidx.test.espresso.base.UiControllerImpl.loopUntil(UiControllerImpl.java:559)
    at androidx.test.espresso.base.UiControllerImpl.loopMainThreadUntilIdle(UiControllerImpl.java:434)
    at androidx.test.espresso.Espresso.lambda$onIdle$2(Espresso.java:340)
    at androidx.test.espresso.Espresso$$ExternalSyntheticLambda1.call(D8$$SyntheticClass:0)
    at java.util.concurrent.FutureTask.run(FutureTask.java:264)
    at android.os.Handler.handleCallback(Handler.java:959)
    at android.os.Handler.dispatchMessage(Handler.java:100)
    at android.os.Looper.loopOnce(Looper.java:232)
    at android.os.Looper.loop(Looper.java:317)
    at android.app.ActivityThread.main(ActivityThread.java:8705)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:580)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:886)

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