+htp #20705 allows javadsl creating custom rejections

akka-docs/rst/java/code/docs/http/javadsl/server/directives/BasicDirectivesExamplesTest.java

@@ -544,7 +544,7 @@ public class BasicDirectivesExamplesTest extends JUnitRouteTest {
     //#mapRouteResult
   }
 
-  public static final class MyCustomRejection implements akka.http.scaladsl.server.Rejection {}
+  public static final class MyCustomRejection implements CustomRejection {}
 
   @Test
   public void testMapRouteResultPF() {

akka-http/src/main/scala/akka/http/javadsl/server/Rejections.scala

@@ -27,9 +27,14 @@ import scala.collection.JavaConverters._
  * A rejection encapsulates a specific reason why a Route was not able to handle a request. Rejections are gathered
  * up over the course of a Route evaluation and finally converted to [[akka.http.scaladsl.model.HttpResponse]]s by the
  * `handleRejections` directive, if there was no way for the request to be completed.
+ *
+ * If providing custom rejections, extend [[CustomRejection]] instead.
  */
 trait Rejection
 
+/** To be extended by user-provided custom rejections, such that they may be consumed in either Java or Scala DSLs. */
+trait CustomRejection extends akka.http.scaladsl.server.Rejection
+
 /**
  * Rejection created by method filters.
  * Signals that the request was rejected because the HTTP method is unsupported.