Change default executor service implementation to WORKER_THREAD_POOL

documentation/modules/ROOT/pages/writing-tests/parallel-execution.adoc

@@ -164,22 +164,18 @@ concurrently. You can configure which implementation of `HierarchicalTestExecuto
 is used be setting the `junit.jupiter.execution.parallel.config.executor-service`
 configuration parameter to one of the following options:
 
-`fork_join_pool` (default)::
+`worker_thread_pool` (default)::
+Use an executor service that is backed by a `ThreadPoolExecutor`. The
+executor service tries to maintain the desired parallelism but does
+not compensate for test or production code using blocking APIs in the JDK.
+
+`fork_join_pool`::
 Use an executor service that is backed by a `ForkJoinPool` from the JDK. This will cause
 tests to be executed in a `ForkJoinWorkerThread`. In some cases, usages of
 `ForkJoinPool` in test or production code or calls to blocking JDK APIs may cause the
 number of concurrently executing tests to increase. To avoid this situation, please use
 `worker_thread_pool`.
 
-`worker_thread_pool` (experimental)::
-Use an executor service that is backed by a regular thread pool and does not create
-additional threads if test or production code uses `ForkJoinPool` or calls a blocking
-API in the JDK.
-
-WARNING: Using `worker_thread_pool` is currently an _experimental_ feature. You're invited
-to give it a try and provide feedback to the JUnit team so they can improve and eventually
-xref:api-evolution.adoc[promote] this feature.
-
 [[config-strategies]]
 === Strategies
 
@@ -241,7 +237,7 @@ xref:running-tests/configuration-parameters.adoc[] for details on how to set suc
 
 `junit.jupiter.execution.parallel.config.executor-service=fork_join_pool|worker_thread_pool`::
   Type of `HierarchicalTestExecutorService` to use for parallel execution (defaults to
-  `fork_join_pool`).
+  `worker_thread_pool`).
 
 `junit.jupiter.execution.parallel.config.strategy=dynamic|fixed|custom`::
   Execution strategy for desired parallelism, maximum pool size, etc. (defaults to `dynamic`).

documentation/modules/ROOT/partials/release-notes/release-notes-6.2.0-M1.adoc

@@ -46,6 +46,8 @@ repository on GitHub.
   xref:advanced-topics/configuration-parameter-documentation.adoc[Documenting Configuration
   Parameters] for more information.
 * Platform configuration parameters are documented in `META-INF/junit-platform-configuration-metadata.json`
+* Test engines using the `ParallelHierarchicalTestExecutorServiceFactory` now use the
+  `WORKER_THREAD_POOL` executor service by default.
 
 [[v6.2.0-M1-junit-jupiter]]
 === JUnit Jupiter
@@ -81,6 +83,8 @@ repository on GitHub.
 * The `assertTimeout`, `assertTimeoutPreemptively` and methods annotated with `@Timeout`
   now measure the execution time with sub-millisecond precision.
 * Jupiter configuration parameters are documented in `META-INF/junit-platform-configuration-metadata.json`
+* The `junit.jupiter.execution.parallel.config.executor-service` configuration parameter
+  now defaults to `WORKER_THREAD_POOL` when parallel execution is enabled.
 
 [[v6.2.0-M1-junit-vintage]]
 === JUnit Vintage

junit-jupiter-engine/src/main/java/org/junit/jupiter/engine/config/DefaultJupiterConfiguration.java

@@ -29,14 +29,11 @@
 import static org.junit.jupiter.api.Constants.EXTENSIONS_AUTODETECTION_INCLUDE_PROPERTY_NAME;
 import static org.junit.jupiter.api.Constants.EXTENSIONS_TIMEOUT_THREAD_DUMP_ENABLED_PROPERTY_NAME;
 import static org.junit.jupiter.api.Constants.INCLUDE_ALL_EXTENSIONS_PATTERN;
-import static org.junit.jupiter.api.Constants.PARALLEL_CONFIG_EXECUTOR_SERVICE_PROPERTY_NAME;
 import static org.junit.jupiter.api.Constants.PARALLEL_EXECUTION_ENABLED_PROPERTY_NAME;
 import static org.junit.jupiter.api.Defaults.CLOSING_STORED_AUTO_CLOSEABLE_ENABLED_DEFAULT;
 import static org.junit.jupiter.api.Defaults.EXTENSIONS_AUTODETECTION_ENABLED_DEFAULT;
 import static org.junit.jupiter.api.Defaults.PARALLEL_EXECUTION_ENABLED_DEFAULT;
 import static org.junit.jupiter.engine.config.FilteringConfigurationParameterConverter.exclude;
-import static org.junit.platform.engine.support.hierarchical.ParallelHierarchicalTestExecutorServiceFactory.ParallelExecutorServiceType.FORK_JOIN_POOL;
-import static org.junit.platform.engine.support.hierarchical.ParallelHierarchicalTestExecutorServiceFactory.ParallelExecutorServiceType.WORKER_THREAD_POOL;
 
 import java.util.List;
 import java.util.Optional;
@@ -129,17 +126,6 @@ private void validateConfigurationParameters(DiscoveryIssueReporter issueReporte
 							Please remove it from your configuration.""".formatted(key));
 					issueReporter.reportIssue(warning);
 				}));
-		if (isParallelExecutionEnabled()
-				&& configurationParameters.get(PARALLEL_CONFIG_EXECUTOR_SERVICE_PROPERTY_NAME).isEmpty()) {
-			var info = DiscoveryIssue.create(Severity.INFO,
-				"Parallel test execution is enabled but the default ForkJoinPool-based executor service will be used. "
-						+ "Please give the new implementation based on a regular thread pool a try by setting the '"
-						+ PARALLEL_CONFIG_EXECUTOR_SERVICE_PROPERTY_NAME + "' configuration parameter to '"
-						+ WORKER_THREAD_POOL + "' and report any issues to the JUnit team. "
-						+ "Alternatively, set the configuration parameter to '" + FORK_JOIN_POOL
-						+ "' to hide this message and keep using the original implementation.");
-			issueReporter.reportIssue(info);
-		}
 	}
 
 	@Override

junit-platform-engine/src/main/java/org/junit/platform/engine/support/hierarchical/ParallelHierarchicalTestExecutorServiceFactory.java

@@ -10,7 +10,6 @@
 
 package org.junit.platform.engine.support.hierarchical;
 
-import static org.apiguardian.api.API.Status.EXPERIMENTAL;
 import static org.apiguardian.api.API.Status.MAINTAINED;
 
 import java.util.Locale;
@@ -36,7 +35,7 @@ public final class ParallelHierarchicalTestExecutorServiceFactory {
 	 * Default value for {@value #EXECUTOR_SERVICE_PROPERTY_NAME} is {@value}.
 	 */
 	@API(status = MAINTAINED, since = "6.2")
-	public static final String EXECUTOR_SERVICE_DEFAULT = "FORK_JOIN_POOL";
+	public static final String EXECUTOR_SERVICE_DEFAULT = "WORKER_THREAD_POOL";
 
 	/**
 	 * Property name used to determine the desired
@@ -117,7 +116,7 @@ public enum ParallelExecutorServiceType {
 		 * Indicates that {@link WorkerThreadPoolHierarchicalTestExecutorService}
 		 * should be used.
 		 */
-		@API(status = EXPERIMENTAL, since = "6.1")
+		@API(status = MAINTAINED, since = "6.2")
 		WORKER_THREAD_POOL;
 
 		private static ParallelExecutorServiceType parse(String value) {

junit-platform-engine/src/main/java/org/junit/platform/engine/support/hierarchical/WorkerThreadPoolHierarchicalTestExecutorService.java

@@ -15,7 +15,7 @@
 import static java.util.Objects.requireNonNull;
 import static java.util.concurrent.CompletableFuture.completedFuture;
 import static java.util.concurrent.TimeUnit.SECONDS;
-import static org.apiguardian.api.API.Status.EXPERIMENTAL;
+import static org.apiguardian.api.API.Status.MAINTAINED;
 import static org.junit.platform.commons.util.ExceptionUtils.throwAsUncheckedException;
 import static org.junit.platform.engine.support.hierarchical.ExclusiveResource.GLOBAL_READ_WRITE;
 import static org.junit.platform.engine.support.hierarchical.Node.ExecutionMode.SAME_THREAD;
@@ -67,7 +67,7 @@
  * @see ParallelExecutorServiceType#WORKER_THREAD_POOL
  * @see DefaultParallelExecutionConfigurationStrategy
  */
-@API(status = EXPERIMENTAL, since = "6.1")
+@API(status = MAINTAINED, since = "6.2")
 public final class WorkerThreadPoolHierarchicalTestExecutorService implements HierarchicalTestExecutorService {
 
 	/*

jupiter-tests/src/test/java/org/junit/jupiter/engine/config/DefaultJupiterConfigurationTests.java

@@ -46,7 +46,6 @@
 import org.junit.jupiter.params.provider.EnumSource;
 import org.junit.platform.engine.ConfigurationParameters;
 import org.junit.platform.engine.DiscoveryIssue;
-import org.junit.platform.engine.DiscoveryIssue.Severity;
 import org.junit.platform.engine.support.discovery.DiscoveryIssueReporter;
 import org.junit.platform.engine.support.hierarchical.ParallelHierarchicalTestExecutorServiceFactory.ParallelExecutorServiceType;
 import org.junit.platform.launcher.core.ConfigurationParametersFactoryForTests;
@@ -187,22 +186,6 @@ void doesNotReportAnyIssuesIfParallelExecutionIsEnabledAndConfigurationParameter
 		assertThat(issues).isEmpty();
 	}
 
-	@Test
-	void asksUsersToTryWorkerThreadPoolHierarchicalExecutorServiceIfParallelExecutionIsEnabled() {
-		var parameters = Map.of(Constants.PARALLEL_EXECUTION_ENABLED_PROPERTY_NAME, true);
-		List<DiscoveryIssue> issues = new ArrayList<>();
-
-		new DefaultJupiterConfiguration(configurationParameters(parameters), dummyOutputDirectoryCreator(),
-			DiscoveryIssueReporter.collecting(issues)).getDefaultTestInstanceLifecycle();
-
-		assertThat(issues).containsExactly(DiscoveryIssue.create(Severity.INFO, """
-				Parallel test execution is enabled but the default ForkJoinPool-based executor service will be used. \
-				Please give the new implementation based on a regular thread pool a try by setting the \
-				'junit.jupiter.execution.parallel.config.executor-service' configuration parameter to \
-				'WORKER_THREAD_POOL' and report any issues to the JUnit team. Alternatively, set the configuration \
-				parameter to 'FORK_JOIN_POOL' to hide this message and keep using the original implementation."""));
-	}
-
 	private void assertDefaultConfigParam(@Nullable String configValue, Lifecycle expected) {
 		var lifecycle = getDefaultTestInstanceLifecycleConfigParam(configValue);
 		assertThat(lifecycle).isEqualTo(expected);