Replace Method with MethodHandle for ThreadPerTaskExecutor.threadCount()

Signed-off-by: Johnny Lim <[email protected]>

Author: izeye

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

@@ -28,6 +28,8 @@
 import io.micrometer.core.instrument.internal.TimedExecutorService;
 import io.micrometer.core.instrument.internal.TimedScheduledExecutorService;
 
+import java.lang.invoke.MethodHandle;
+import java.lang.invoke.MethodHandles;
 import java.lang.reflect.Field;
 import java.lang.reflect.Method;
 import java.util.List;
@@ -60,7 +62,7 @@ public class ExecutorServiceMetrics implements MeterBinder {
     private static final String CLASS_NAME_THREAD_PER_TASK_EXECUTOR = "java.util.concurrent.ThreadPerTaskExecutor";
 
     @Nullable
-    private static final Method METHOD_THREAD_COUNT_FROM_THREAD_PER_TASK_EXECUTOR = getMethodForThreadCountFromThreadPerTaskExecutor();
+    private static final MethodHandle METHOD_HANDLE_THREAD_COUNT_FROM_THREAD_PER_TASK_EXECUTOR = getMethodHandleForThreadCountFromThreadPerTaskExecutor();
 
     private static boolean allowIllegalReflectiveAccess = true;
 
@@ -461,20 +463,20 @@ private void monitorThreadPerTaskExecutor(MeterRegistry registry, ExecutorServic
 
     private static long getThreadCountFromThreadPerTaskExecutor(ExecutorService executorService) {
         try {
-            return (long) METHOD_THREAD_COUNT_FROM_THREAD_PER_TASK_EXECUTOR.invoke(executorService);
+            return (long) METHOD_HANDLE_THREAD_COUNT_FROM_THREAD_PER_TASK_EXECUTOR.invoke(executorService);
         }
         catch (Throwable e) {
             throw new RuntimeException(e);
         }
     }
 
     @Nullable
-    private static Method getMethodForThreadCountFromThreadPerTaskExecutor() {
+    private static MethodHandle getMethodHandleForThreadCountFromThreadPerTaskExecutor() {
         try {
             Class<?> clazz = Class.forName(CLASS_NAME_THREAD_PER_TASK_EXECUTOR);
             Method method = clazz.getMethod("threadCount");
             method.setAccessible(true);
-            return method;
+            return MethodHandles.lookup().unreflect(method);
         }
         catch (Throwable e) {
             return null;