8338890: Add monitoring/management interface for the virtual thread scheduler

Reviewed-by: kevinw

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

@@ -65,6 +65,7 @@ import java.util.PropertyPermission;
 import java.util.ResourceBundle;
 import java.util.Set;
 import java.util.WeakHashMap;
+import java.util.concurrent.Executor;
 import java.util.function.Supplier;
 import java.util.concurrent.ConcurrentHashMap;
 import java.util.stream.Stream;
@@ -2766,6 +2767,10 @@ public final class System {
                 }
             }
 
+            public Executor virtualThreadDefaultScheduler() {
+                return VirtualThread.defaultScheduler();
+            }
+
             public StackWalker newStackWalkerInstance(Set<StackWalker.Option> options,
                                                       ContinuationScope contScope,
                                                       Continuation continuation) {

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

@@ -142,6 +142,14 @@ final class VirtualThread extends BaseVirtualThread {
     // termination object when joining, created lazily if needed
     private volatile CountDownLatch termination;
 
+
+    /**
+     * Returns the default scheduler.
+     */
+    static Executor defaultScheduler() {
+        return DEFAULT_SCHEDULER;
+    }
+
     /**
      * Returns the continuation scope used for virtual threads.
      */

src/java.base/share/classes/jdk/internal/access/JavaLangAccess.java

@@ -44,6 +44,7 @@ import java.util.List;
 import java.util.Map;
 import java.util.Set;
 import java.util.concurrent.ConcurrentHashMap;
+import java.util.concurrent.Executor;
 import java.util.concurrent.RejectedExecutionException;
 import java.util.stream.Stream;
 
@@ -601,6 +602,11 @@ public interface JavaLangAccess {
      */
     void unparkVirtualThread(Thread thread);
 
+    /**
+     * Returns the virtual thread default scheduler.
+     */
+    Executor virtualThreadDefaultScheduler();
+
     /**
      * Creates a new StackWalker
      */

src/java.base/share/classes/module-info.java

@@ -172,6 +172,7 @@ module java.base {
         jdk.jartool,
         jdk.jlink,
         jdk.jfr,
+        jdk.management,
         jdk.net,
         jdk.sctp,
         jdk.crypto.cryptoki;