Minor style improvements.

Author: peter-hofer

substratevm/src/com.oracle.svm.core/src/com/oracle/svm/core/thread/ContinuationsFeature.java

@@ -54,13 +54,13 @@ public void afterRegistration(AfterRegistrationAccess access) {
             VMError.guarantee(JavaVersionUtil.JAVA_SPEC >= 19);
             ImageSingletons.add(VirtualThreads.class, new LoomVirtualThreads());
         } else if (JavaVersionUtil.JAVA_SPEC == 17) {
+            ImageSingletons.add(VirtualThreads.class, new SubstrateVirtualThreads());
+        } else {
             /*
-             * GR-37518: ForkJoinPool on 11 syncs on a String which doesn't have its own monitor
+             * GR-37518: on 11, ForkJoinPool syncs on a String which doesn't have its own monitor
              * field, and unparking a virtual thread in additionalMonitorsLock.unlock causes a
              * deadlock between carrier thread and virtual thread. 17 uses a ReentrantLock.
              */
-            ImageSingletons.add(VirtualThreads.class, new SubstrateVirtualThreads());
-        } else {
             throw UserError.abort("Continuations are currently supported only on JDK 17 with option %s, or on JDK 19 with preview features enabled (-J--enable-preview).",
                             SubstrateOptionsParser.commandArgument(SubstrateOptions.SupportContinuations, "+"));
         }

substratevm/src/com.oracle.svm.core/src/com/oracle/svm/core/thread/LoomVirtualThreads.java

@@ -42,10 +42,11 @@
 import com.oracle.svm.core.util.VMError;
 
 /**
- * In a Project Loom JDK, code specific to virtual threads is part of the {@link Thread} methods,
- * e.g. {@code yield} or {@code sleep}, and the implementation for platform threads is generally in
- * methods named {@code yield0} or {@code sleep0}, so we only substitute that platform thread code
- * in {@link Target_java_lang_Thread}, and generally do not expect these methods to be reachable.
+ * In a JDK that supports Project Loom virtual threads (JEP 425, starting with JDK 19 as preview
+ * feature), code specific to virtual threads is part of {@link Thread} methods, e.g. {@code yield}
+ * or {@code sleep}, and the implementation for platform threads is generally in methods named
+ * {@code yield0} or {@code sleep0}, so we only substitute that platform thread code in
+ * {@link Target_java_lang_Thread}, and do not expect several methods here to be reachable.
  *
  * @see <a href="https://openjdk.java.net/projects/loom/">Project Loom (Wiki, code, etc.)</a>
  */
@@ -200,7 +201,7 @@ private static StackTraceElement[] asyncGetStackTrace(Target_java_lang_VirtualTh
         return stackTrace;
     }
 
-    private static StackTraceElement[] asyncMountedGetStackTrace(@SuppressWarnings("unused") Target_java_lang_VirtualThread thread) {
+    private static StackTraceElement[] asyncMountedGetStackTrace(Target_java_lang_VirtualThread thread) {
         return StackTraceUtils.asyncGetStackTrace(Thread.class.cast(thread));
     }
 

substratevm/src/com.oracle.svm.core/src/com/oracle/svm/core/thread/PlatformThreads.java

@@ -105,10 +105,12 @@
  * @see JavaThreads
  */
 public abstract class PlatformThreads {
-    private static final Field FIELDHOLDER_STATUS_FIELD = (!ImageInfo.inImageCode() || JavaVersionUtil.JAVA_SPEC < 19) ? null
-                    : ReflectionUtil.lookupField(Target_java_lang_Thread_FieldHolder.class, "threadStatus");
-    private static final Field THREAD_STATUS_FIELD = (!ImageInfo.inImageCode() || JavaVersionUtil.JAVA_SPEC >= 19) ? null
-                    : ReflectionUtil.lookupField(Target_java_lang_Thread.class, "threadStatus");
+    private static final Field FIELDHOLDER_STATUS_FIELD = (JavaVersionUtil.JAVA_SPEC >= 19 && ImageInfo.inImageCode())
+                    ? ReflectionUtil.lookupField(Target_java_lang_Thread_FieldHolder.class, "threadStatus")
+                    : null;
+    private static final Field THREAD_STATUS_FIELD = (JavaVersionUtil.JAVA_SPEC < 19 && ImageInfo.inImageCode())
+                    ? ReflectionUtil.lookupField(Target_java_lang_Thread.class, "threadStatus")
+                    : null;
 
     @Fold
     public static PlatformThreads singleton() {