Build fails with NullPointerException during discovery phase

[sbrannen]

Overview

While trying to migrate the Spring Framework build to the TestNG TestEngine (see spring-projects/spring-framework#27406), I encounter a NullPointerException which fails the build.

Exception

org.gradle.api.internal.tasks.testing.TestSuiteExecutionException: Could not complete execution for Gradle Test Executor 168.
	at org.gradle.api.internal.tasks.testing.SuiteTestClassProcessor.stop(SuiteTestClassProcessor.java:63)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:498)
	at org.gradle.internal.dispatch.ReflectionDispatch.dispatch(ReflectionDispatch.java:36)
	at org.gradle.internal.dispatch.ReflectionDispatch.dispatch(ReflectionDispatch.java:24)
	at org.gradle.internal.dispatch.ContextClassLoaderDispatch.dispatch(ContextClassLoaderDispatch.java:33)
	at org.gradle.internal.dispatch.ProxyDispatchAdapter$DispatchingInvocationHandler.invoke(ProxyDispatchAdapter.java:94)
	at com.sun.proxy.$Proxy2.stop(Unknown Source)
	at org.gradle.api.internal.tasks.testing.worker.TestWorker$3.run(TestWorker.java:193)
	at org.gradle.api.internal.tasks.testing.worker.TestWorker.executeAndMaintainThreadName(TestWorker.java:129)
	at org.gradle.api.internal.tasks.testing.worker.TestWorker.execute(TestWorker.java:100)
	at org.gradle.api.internal.tasks.testing.worker.TestWorker.execute(TestWorker.java:60)
	at org.gradle.process.internal.worker.child.ActionExecutionWorker.execute(ActionExecutionWorker.java:56)
	at org.gradle.process.internal.worker.child.SystemApplicationClassLoaderWorker.call(SystemApplicationClassLoaderWorker.java:133)
	at org.gradle.process.internal.worker.child.SystemApplicationClassLoaderWorker.call(SystemApplicationClassLoaderWorker.java:71)
	at worker.org.gradle.process.internal.worker.GradleWorkerMain.run(GradleWorkerMain.java:69)
	at worker.org.gradle.process.internal.worker.GradleWorkerMain.main(GradleWorkerMain.java:74)
Caused by: org.junit.platform.commons.JUnitException: TestEngine with ID 'testng' failed to discover tests
	at org.junit.platform.launcher.core.EngineDiscoveryOrchestrator.discoverEngineRoot(EngineDiscoveryOrchestrator.java:160)
	at org.junit.platform.launcher.core.EngineDiscoveryOrchestrator.discoverSafely(EngineDiscoveryOrchestrator.java:134)
	at org.junit.platform.launcher.core.EngineDiscoveryOrchestrator.discover(EngineDiscoveryOrchestrator.java:108)
	at org.junit.platform.launcher.core.EngineDiscoveryOrchestrator.discover(EngineDiscoveryOrchestrator.java:80)
	at org.junit.platform.launcher.core.DefaultLauncher.discover(DefaultLauncher.java:110)
	at org.junit.platform.launcher.core.DefaultLauncher.execute(DefaultLauncher.java:86)
	at org.junit.platform.launcher.core.DefaultLauncherSession$DelegatingLauncher.execute(DefaultLauncherSession.java:86)
	at org.junit.platform.launcher.core.SessionPerRequestLauncher.execute(SessionPerRequestLauncher.java:53)
	at org.gradle.api.internal.tasks.testing.junitplatform.JUnitPlatformTestClassProcessor$CollectAllTestClassesExecutor.processAllTestClasses(JUnitPlatformTestClassProcessor.java:99)
	at org.gradle.api.internal.tasks.testing.junitplatform.JUnitPlatformTestClassProcessor$CollectAllTestClassesExecutor.access$000(JUnitPlatformTestClassProcessor.java:79)
	at org.gradle.api.internal.tasks.testing.junitplatform.JUnitPlatformTestClassProcessor.stop(JUnitPlatformTestClassProcessor.java:75)
	at org.gradle.api.internal.tasks.testing.SuiteTestClassProcessor.stop(SuiteTestClassProcessor.java:61)
	... 18 more
Caused by: java.lang.NullPointerException
	at java.util.concurrent.ConcurrentHashMap.putVal(ConcurrentHashMap.java:1011)
	at java.util.concurrent.ConcurrentHashMap$KeySetView.add(ConcurrentHashMap.java:4595)
	at org.junit.support.testng.engine.TestClassRegistry.lambda$start$0(TestClassRegistry.java:29)
	at java.util.concurrent.ConcurrentHashMap.computeIfAbsent(ConcurrentHashMap.java:1660)
	at org.junit.support.testng.engine.TestClassRegistry.start(TestClassRegistry.java:27)
	at org.junit.support.testng.engine.DiscoveryListener.onBeforeClass(DiscoveryListener.java:37)
	at org.testng.internal.TestMethodWorker.invokeBeforeClassMethods(TestMethodWorker.java:167)
	at org.testng.internal.TestMethodWorker.run(TestMethodWorker.java:122)
	at java.util.ArrayList.forEach(ArrayList.java:1259)
	at org.testng.TestRunner.privateRun(TestRunner.java:794)
	at org.testng.TestRunner.run(TestRunner.java:596)
	at org.testng.SuiteRunner.runTest(SuiteRunner.java:377)
	at org.testng.SuiteRunner.runSequentially(SuiteRunner.java:371)
	at org.testng.SuiteRunner.privateRun(SuiteRunner.java:332)
	at org.testng.SuiteRunner.run(SuiteRunner.java:276)
	at org.testng.SuiteRunnerWorker.runSuite(SuiteRunnerWorker.java:53)
	at org.testng.SuiteRunnerWorker.run(SuiteRunnerWorker.java:96)
	at org.testng.TestNG.runSuitesSequentially(TestNG.java:1212)
	at org.testng.TestNG.runSuitesLocally(TestNG.java:1134)
	at org.testng.TestNG.runSuites(TestNG.java:1063)
	at org.testng.TestNG.run(TestNG.java:1031)
	at org.junit.support.testng.engine.TestNGTestEngine.configureAndRun(TestNGTestEngine.java:182)
	at org.junit.support.testng.engine.TestNGTestEngine.lambda$discover$1(TestNGTestEngine.java:91)
	at org.junit.support.testng.engine.TestNGTestEngine.withTemporarySystemProperty(TestNGTestEngine.java:190)
	at org.junit.support.testng.engine.TestNGTestEngine.discover(TestNGTestEngine.java:90)
	at org.junit.platform.launcher.core.EngineDiscoveryOrchestrator.discoverEngineRoot(EngineDiscoveryOrchestrator.java:152)
	... 29 more

Example

Executing gradle :spring-test:check in the following branch reproduces the issue.

• https://github.com/sbrannen/spring-framework/tree/issues/gh-27406-TestNG-TestEngine