authenticateOrRejectWithChallenge

final HttpChallenge challenge = HttpChallenge.create("MyAuth", "MyRealm");
 
// your custom authentication logic:
final Function<HttpCredentials, Boolean> auth = credentials -> true;
 
final Function<Optional<HttpCredentials>, CompletionStage<Either<HttpChallenge, String>>> myUserPassAuthenticator =
  opt -> {
    if (opt.isPresent() && auth.apply(opt.get())) {
      return CompletableFuture.completedFuture(Right.apply("some-user-name-from-creds"));
    } else {
      return CompletableFuture.completedFuture(Left.apply(challenge));
    }
  };
 
final Route route = path("secured", () ->
  authenticateOrRejectWithChallenge(myUserPassAuthenticator, userName ->
    complete("Authenticated!")
  )
).seal(system(), materializer());
 
// tests:
testRoute(route).run(HttpRequest.GET("/secured"))
  .assertStatusCode(StatusCodes.UNAUTHORIZED)
  .assertEntity("The resource requires authentication, which was not supplied with the request")
  .assertHeaderExists("WWW-Authenticate", "MyAuth realm=\"MyRealm\"");
 
final HttpCredentials validCredentials =
  BasicHttpCredentials.createBasicHttpCredentials("John", "p4ssw0rd");
testRoute(route).run(HttpRequest.GET("/secured").addCredentials(validCredentials))
  .assertStatusCode(StatusCodes.OK)
  .assertEntity("Authenticated!");