Route all VT creation through scheduler.newThread() for custom schedulers

src/java.base/share/classes/java/lang/Thread.java

@@ -968,27 +968,42 @@ public interface VirtualThreadScheduler {
          * @implSpec The default implementation creates a new virtual thread. It should
          * be rare to override this method.
          *
-         * @param builder the virtual thread builder
-         * @param preferredCarrier the preferred carrirer, can be {@code null}
+         * @param name the thread name, can be {@code null}
+         * @param characteristics the thread characteristics
+         * @param preferredCarrier the preferred carrier, can be {@code null}
          * @param task the object to run when the thread executes
          * @return the {@code VirtualThreadTask} that scheduler executes
          * @throws UnsupportedOperationException if this is the built-in default scheduler
-         *
-         * @see <a href="Thread.html#inheritance">Inheritance when creating threads</a>
          */
-        default VirtualThreadTask newThread(Builder.OfVirtual builder,
+        default VirtualThreadTask newThread(String name, int characteristics,
                                             Thread preferredCarrier,
                                             Runnable task) {
-            Objects.requireNonNull(builder);
             Objects.requireNonNull(task);
             if (this == VirtualThread.builtinScheduler(false)) {
                 throw new UnsupportedOperationException();
             }
-            var vbuilder = (ThreadBuilders.VirtualThreadBuilder) builder;
-            var vthread = (VirtualThread) vbuilder.unstarted(task, preferredCarrier);
+            var vthread = new VirtualThread(this, preferredCarrier, name, characteristics, task);
             return vthread.virtualThreadTask();
         }
 
+        /**
+         * Creates a new virtual thread using the given builder's configuration.
+         * Delegates to {@link #newThread(String, int, Thread, Runnable)}.
+         *
+         * @param builder the virtual thread builder
+         * @param preferredCarrier the preferred carrier, can be {@code null}
+         * @param task the object to run when the thread executes
+         * @return the {@code VirtualThreadTask} that scheduler executes
+         */
+        default VirtualThreadTask newThread(Builder.OfVirtual builder,
+                                            Thread preferredCarrier,
+                                            Runnable task) {
+            Objects.requireNonNull(builder);
+            var vbuilder = (ThreadBuilders.VirtualThreadBuilder) builder;
+            return newThread(vbuilder.nextThreadName(), vbuilder.characteristics(),
+                    preferredCarrier, task);
+        }
+
         /**
          * Schedules a task that becomes enabled for execution after the given delay.
          *

src/java.base/share/classes/java/lang/ThreadBuilders.java

@@ -405,12 +405,23 @@ public Thread newThread(Runnable task) {
     /**
      * Creates a new virtual thread to run the given task.
      */
+    // true when a custom scheduler is installed via implClass, JIT constant-folds
+    static final boolean CUSTOM_SCHEDULER =
+            VirtualThread.defaultScheduler() != VirtualThread.builtinScheduler(true);
+
     private static Thread newVirtualThread(Thread.VirtualThreadScheduler scheduler,
                                            Thread preferredCarrier,
                                            String name,
                                            int characteristics,
                                            Runnable task) {
         if (ContinuationSupport.isSupported()) {
+            // Route through the custom scheduler's newThread() when implClass is set.
+            // scheduler is non-null only with the legacy per-builder scheduler field,
+            // superseded by the global custom scheduler API (implClass).
+            if (scheduler == null && CUSTOM_SCHEDULER) {
+                return VirtualThread.defaultScheduler()
+                        .newThread(name, characteristics, preferredCarrier, task).thread();
+            }
             return new VirtualThread(scheduler, preferredCarrier, name, characteristics, task);
         } else {
             if (scheduler != null)