[Http-Client] Use virtual thread executor by default (#442)

* use virtual thread executor by default

* only try if > 21

* [Http client] #442 extract helper method for newVirtualThreadPerTaskExecutor

---------

Co-authored-by: Rob Bygrave <[email protected]>

Author: SentryMan

http-client/src/main/java/io/avaje/http/client/DHttpClientBuilder.java

@@ -16,10 +16,15 @@
 import java.util.List;
 import java.util.Optional;
 import java.util.concurrent.Executor;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
 import java.util.function.Function;
 
 import static java.util.Objects.requireNonNull;
 
+import java.lang.invoke.MethodHandles;
+import java.lang.invoke.MethodType;
+
 final class DHttpClientBuilder implements HttpClient.Builder, HttpClient.Builder.State {
 
   private java.net.http.HttpClient client;
@@ -99,6 +104,8 @@ private java.net.http.HttpClient defaultClient() {
     }
     if (executor != null) {
       builder.executor(executor);
+    } else if (Integer.getInteger("java.specification.version") >= 21) {
+      builder.executor(virtualThreadExecutor());
     }
     if (proxy != null) {
       builder.proxy(proxy);
@@ -118,6 +125,17 @@ private java.net.http.HttpClient defaultClient() {
     return builder.build();
   }
 
+  private static ExecutorService virtualThreadExecutor() {
+    try {
+      return (ExecutorService)
+        MethodHandles.lookup()
+          .findStatic(Executors.class, "newVirtualThreadPerTaskExecutor", MethodType.methodType(ExecutorService.class))
+          .invokeExact();
+    } catch (Throwable e) {
+      return null;
+    }
+  }
+
   /**
    * Create a reasonable default BodyAdapter if avaje-jsonb or Jackson are present.
    */