Disable Loom support on JVMs that support the virtual threads API but don't really implement it. (#11856)

 Disable Loom support on JVMs that support the virtual threads API but don't really implement it.

This avoids a crash when a thread is cast to VirtualThread but is in fact a ThreadBuilder.BoundVirtualThread that uses a platform thread instead.

Espresso is an example of such a JVM.

* remove unused imports

---------

Co-authored-by: Sergio del Amo <[email protected]>

Author: mikehearn

context/src/main/java/io/micronaut/runtime/context/scope/refresh/RefreshScope.java

@@ -19,7 +19,6 @@
 import io.micronaut.context.BeanContext;
 import io.micronaut.context.BeanRegistration;
 import io.micronaut.context.LifeCycle;
-import io.micronaut.context.annotation.ConfigurationProperties;
 import io.micronaut.context.annotation.ConfigurationReader;
 import io.micronaut.context.annotation.Requires;
 import io.micronaut.context.event.ApplicationEventListener;

context/src/main/java/io/micronaut/scheduling/LoomSupport.java

@@ -71,10 +71,18 @@ public final class LoomSupport {
             isVirtual = MethodHandles.lookup()
                 .findVirtual(Thread.class, "isVirtual", MethodType.methodType(boolean.class));
 
-            // invoke, this will throw an UnsupportedOperationException if we don't have --enable-preview
-            ofVirtual.invoke();
-
-            sup = true;
+            // This will throw if this Java doesn't support Loom, or if it does but only with
+            // --enable-preview.
+            Thread probe = (Thread) unstarted.invoke(ofVirtual.invoke(), (Runnable) () -> { });
+
+            // This checks if the JVM actually creates real virtual threads, or if it uses
+            // 'bound threads' which are just platform threads. As of June 2025 the Espresso JVM
+            // falls into this category. Checking here voids casting issues later in code that
+            // makes assumptions about the internals.
+            sup = Class.forName("java.lang.VirtualThread").isInstance(probe);
+            if (!sup) {
+                failure = new Exception("This JVM doesn't fully implement virtual threads and produces regular platform threads instead.");
+            }
         } catch (Throwable e) {
             newThreadPerTaskExecutor = null;
             ofVirtual = null;