feat: Add LoadMetrics support for virtual thread executor.

actor/src/main/scala/org/apache/pekko/dispatch/AbstractDispatcher.scala

@@ -426,11 +426,18 @@ final class VirtualThreadExecutorConfigurator(config: Config, prerequisites: Dis
             vt
           }
         }
-      case _ => VirtualThreadSupport.newVirtualThreadFactory(prerequisites.settings.name + "-" + id);
+      case _ => newVirtualThreadFactory(prerequisites.settings.name + "-" + id);
     }
     new ExecutorServiceFactory {
-      import VirtualThreadSupport._
-      override def createExecutorService: ExecutorService = newThreadPerTaskExecutor(tf)
+      override def createExecutorService: ExecutorService with LoadMetrics = {
+        val pool = getVirtualThreadDefaultScheduler // the default scheduler of virtual thread
+        new VirtualizedExecutorService(
+          tf,
+          pool, // the default scheduler of virtual thread
+          loadMetricsProvider = (_: Executor) => pool.getActiveThreadCount >= pool.getParallelism,
+          cascadeShutdown = false // we don't want to cascade shutdown the default virtual thread scheduler
+        )
+      }
     }
   }
 }

actor/src/main/scala/org/apache/pekko/dispatch/VirtualThreadSupport.scala

@@ -21,7 +21,7 @@ import org.apache.pekko.annotation.InternalApi
 import org.apache.pekko.util.JavaVersion
 
 import java.lang.invoke.{ MethodHandles, MethodType }
-import java.util.concurrent.{ ExecutorService, ThreadFactory }
+import java.util.concurrent.{ ExecutorService, ForkJoinPool, ThreadFactory }
 import scala.util.control.NonFatal
 
 @InternalApi
@@ -73,4 +73,21 @@ private[dispatch] object VirtualThreadSupport {
     }
   }
 
+  /**
+   * Try to get the default scheduler of virtual thread.
+   */
+  def getVirtualThreadDefaultScheduler: ForkJoinPool =
+    try {
+      require(isSupported, "Virtual thread is not supported.")
+      val clazz = Class.forName("java.lang.VirtualThread")
+      val fieldName = "DEFAULT_SCHEDULER"
+      val field = clazz.getDeclaredField(fieldName)
+      field.setAccessible(true)
+      field.get(null).asInstanceOf[ForkJoinPool]
+    } catch {
+      case NonFatal(e) =>
+        // --add-opens java.base/java.lang=ALL-UNNAMED
+        throw new UnsupportedOperationException("Failed to create newThreadPerTaskExecutor.", e)
+    }
+
 }

actor/src/main/scala/org/apache/pekko/dispatch/VirtualizedExecutorService.scala

@@ -0,0 +1,125 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You 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
+ *
+ *    http://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.apache.pekko.dispatch
+
+import org.apache.pekko.annotation.InternalApi
+
+import java.util
+import java.util.concurrent.{ Callable, Executor, ExecutorService, Future, ThreadFactory, TimeUnit }
+
+/**
+ * A virtualized executor service that creates a new virtual thread for each task.
+ * Will shut down the underlying executor service when this executor is being shutdown.
+ *
+ * INTERNAL API
+ */
+@InternalApi
+final class VirtualizedExecutorService(
+    vtFactory: ThreadFactory,
+    underlying: ExecutorService,
+    loadMetricsProvider: Executor => Boolean,
+    cascadeShutdown: Boolean)
+    extends ExecutorService with LoadMetrics {
+  require(VirtualThreadSupport.isSupported, "Virtual thread is not supported.")
+  require(vtFactory != null, "Virtual thread factory must not be null")
+  require(underlying != null, "Underlying executor service must not be null")
+  require(loadMetricsProvider != null, "Load metrics provider must not be null")
+
+  def this(prefix: String,
+      underlying: ExecutorService,
+      loadMetricsProvider: Executor => Boolean,
+      cascadeShutdown: Boolean) = {
+    this(VirtualThreadSupport.newVirtualThreadFactory(prefix), underlying, loadMetricsProvider, cascadeShutdown)
+  }
+
+  private val executor = VirtualThreadSupport.newThreadPerTaskExecutor(vtFactory)
+
+  override def atFullThrottle(): Boolean = loadMetricsProvider(this)
+
+  override def shutdown(): Unit = {
+    executor.shutdown()
+    if (cascadeShutdown) {
+      underlying.shutdown()
+    }
+  }
+
+  override def shutdownNow(): util.List[Runnable] = {
+    val r = executor.shutdownNow()
+    if (cascadeShutdown) {
+      underlying.shutdownNow()
+    }
+    r
+  }
+
+  override def isShutdown: Boolean = {
+    if (cascadeShutdown) {
+      executor.isShutdown && underlying.isShutdown
+    } else {
+      executor.isShutdown
+    }
+  }
+
+  override def isTerminated: Boolean = {
+    if (cascadeShutdown) {
+      executor.isTerminated && underlying.isTerminated
+    } else {
+      executor.isTerminated
+    }
+  }
+
+  override def awaitTermination(timeout: Long, unit: TimeUnit): Boolean = {
+    if (cascadeShutdown) {
+      executor.awaitTermination(timeout, unit) && underlying.awaitTermination(timeout, unit)
+    } else {
+      executor.awaitTermination(timeout, unit)
+    }
+  }
+
+  override def submit[T](task: Callable[T]): Future[T] = {
+    executor.submit(task)
+  }
+
+  override def submit[T](task: Runnable, result: T): Future[T] = {
+    executor.submit(task, result)
+  }
+
+  override def submit(task: Runnable): Future[_] = {
+    executor.submit(task)
+  }
+
+  override def invokeAll[T](tasks: util.Collection[_ <: Callable[T]]): util.List[Future[T]] = {
+    executor.invokeAll(tasks)
+  }
+
+  override def invokeAll[T](
+      tasks: util.Collection[_ <: Callable[T]], timeout: Long, unit: TimeUnit): util.List[Future[T]] = {
+    executor.invokeAll(tasks, timeout, unit)
+  }
+
+  override def invokeAny[T](tasks: util.Collection[_ <: Callable[T]]): T = {
+    executor.invokeAny(tasks)
+  }
+
+  override def invokeAny[T](tasks: util.Collection[_ <: Callable[T]], timeout: Long, unit: TimeUnit): T = {
+    executor.invokeAny(tasks, timeout, unit)
+  }
+
+  override def execute(command: Runnable): Unit = {
+    executor.execute(command)
+  }
+}

project/JdkOptions.scala

@@ -49,6 +49,8 @@ object JdkOptions extends AutoPlugin {
 
   lazy val versionSpecificJavaOptions =
     if (isJdk17orHigher) {
+      // for virtual threads
+      "--add-opens=java.base/java.lang=ALL-UNNAMED" ::
       // for aeron
       "--add-opens=java.base/sun.nio.ch=ALL-UNNAMED" ::
       // for LevelDB