Add support for virtual threads in OtlpMetricRegistry configuration

See gh-42407

spring-boot-project/spring-boot-tools/spring-boot-test-support/src/main/java/org/springframework/boot/testsupport/assertj/ScheduledExecutorServiceAssert.java

@@ -0,0 +1,91 @@
+/*
+ * Copyright 2012-2024 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.boot.testsupport.assertj;
+
+import java.util.concurrent.ExecutionException;
+import java.util.concurrent.ScheduledExecutorService;
+import java.util.concurrent.ScheduledThreadPoolExecutor;
+import java.util.concurrent.TimeUnit;
+
+import org.assertj.core.api.AbstractAssert;
+import org.assertj.core.api.Assert;
+
+/**
+ * AssertJ {@link Assert} for {@link ScheduledThreadPoolExecutor}.
+ *
+ * @author Mike Turbe
+ * @author Moritz Halbritter
+ */
+public final class ScheduledExecutorServiceAssert
+		extends AbstractAssert<ScheduledExecutorServiceAssert, ScheduledExecutorService> {
+
+	private ScheduledExecutorServiceAssert(ScheduledExecutorService actual) {
+		super(actual, ScheduledExecutorServiceAssert.class);
+	}
+
+	/**
+	 * Verifies that the actual executor uses platform threads.
+	 * @return {@code this} assertion object
+	 * @throws AssertionError if the actual executor doesn't use platform threads
+	 */
+	public ScheduledExecutorServiceAssert usesPlatformThreads() {
+		isNotNull();
+		if (producesVirtualThreads()) {
+			failWithMessage("Expected executor to use platform threads, but it uses virtual threads");
+		}
+		return this;
+	}
+
+	/**
+	 * Verifies that the actual executor uses virtual threads.
+	 * @return {@code this} assertion object
+	 * @throws AssertionError if the actual executor doesn't use virtual threads
+	 */
+	public ScheduledExecutorServiceAssert usesVirtualThreads() {
+		isNotNull();
+		if (!producesVirtualThreads()) {
+			failWithMessage("Expected executor to use virtual threads, but it uses platform threads");
+		}
+		return this;
+	}
+
+	private boolean producesVirtualThreads() {
+		try {
+			return this.actual.schedule(() -> {
+				// https://openjdk.org/jeps/444
+				// jep 444 specifies that virtual threads will belong to
+				// a special thread group given the name "VirtualThreads"
+				ThreadGroup threadGroup = Thread.currentThread().getThreadGroup();
+				String threadGroupName = (threadGroup != null) ? threadGroup.getName() : "";
+				return threadGroupName.equalsIgnoreCase("VirtualThreads");
+			}, 0, TimeUnit.SECONDS).get();
+		}
+		catch (InterruptedException | ExecutionException ex) {
+			throw new AssertionError(ex);
+		}
+	}
+
+	/**
+	 * Creates a new assertion class with the given {@link ScheduledExecutorService}.
+	 * @param actual the {@link ScheduledExecutorService}
+	 * @return the assertion class
+	 */
+	public static ScheduledExecutorServiceAssert assertThat(ScheduledExecutorService actual) {
+		return new ScheduledExecutorServiceAssert(actual);
+	}
+
+}