Handle when ThreadPerTaskExecutor.threadCount() is not available (#7045)

Closes gh-6726

Signed-off-by: Johnny Lim <izeye@naver.com>

Author: izeye

micrometer-core/src/main/java/io/micrometer/core/instrument/binder/jvm/ExecutorServiceMetrics.java

@@ -70,6 +70,8 @@ public class ExecutorServiceMetrics implements MeterBinder {
 
     private static final String CLASS_NAME_THREAD_PER_TASK_EXECUTOR = "java.util.concurrent.ThreadPerTaskExecutor";
 
+    private static final String METHOD_NAME_THREAD_PER_TASK_EXECUTOR_THREAD_COUNT = "threadCount";
+
     @Nullable
     private static final MethodHandle METHOD_HANDLE_THREAD_COUNT_FROM_THREAD_PER_TASK_EXECUTOR = getMethodHandleForThreadCountFromThreadPerTaskExecutor();
 
@@ -464,6 +466,12 @@ private void monitor(MeterRegistry registry, ForkJoinPool fj) {
     }
 
     private void monitorThreadPerTaskExecutor(MeterRegistry registry, ExecutorService executorService) {
+        if (METHOD_HANDLE_THREAD_COUNT_FROM_THREAD_PER_TASK_EXECUTOR == null) {
+            log.warn("{}.{}() is not available. Try '--add-opens java.base/java.util.concurrent=ALL-UNNAMED'.",
+                    CLASS_NAME_THREAD_PER_TASK_EXECUTOR, METHOD_NAME_THREAD_PER_TASK_EXECUTOR_THREAD_COUNT);
+            return;
+        }
+
         List<Meter> meters = asList(Gauge
             .builder(metricPrefix + "executor.active", executorService,
                     ExecutorServiceMetrics::getThreadCountFromThreadPerTaskExecutor)
@@ -487,7 +495,7 @@ private static long getThreadCountFromThreadPerTaskExecutor(ExecutorService exec
     private static MethodHandle getMethodHandleForThreadCountFromThreadPerTaskExecutor() {
         try {
             Class<?> clazz = Class.forName(CLASS_NAME_THREAD_PER_TASK_EXECUTOR);
-            Method method = clazz.getMethod("threadCount");
+            Method method = clazz.getMethod(METHOD_NAME_THREAD_PER_TASK_EXECUTOR_THREAD_COUNT);
             method.setAccessible(true);
             return MethodHandles.lookup().unreflect(method);
         }

micrometer-java21/src/test/java/io/micrometer/core/instrument/binder/jvm/ExecutorServiceMetricsTests.java

@@ -0,0 +1,46 @@
+/*
+ * Copyright 2026 VMware, Inc.
+ *
+ * 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 io.micrometer.core.instrument.binder.jvm;
+
+import io.micrometer.core.instrument.simple.SimpleMeterRegistry;
+import org.junit.jupiter.api.Test;
+
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * Tests for {@link ExecutorServiceMetrics}.
+ *
+ * @author Tommy Ludwig
+ * @author Johnny Lim
+ */
+class ExecutorServiceMetricsTests {
+
+    SimpleMeterRegistry registry = new SimpleMeterRegistry();
+
+    @Test
+    void monitorWithExecutorsNewVirtualThreadPerTaskExecutorWillBeDisabledWhenReflectionIsNotEnabled() {
+        ExecutorService unmonitored = Executors.newVirtualThreadPerTaskExecutor();
+        assertThat(unmonitored.getClass().getName()).isEqualTo("java.util.concurrent.ThreadPerTaskExecutor");
+        ExecutorService monitored = ExecutorServiceMetrics.monitor(registry, unmonitored, "test");
+        monitored.execute(() -> {
+        });
+        assertThat(registry.find("executor.active").gauge()).isNull();
+    }
+
+}