Scratch.java

import java.util.List;

class Scratch {

  public static void main(String[] args) {
    // in Intelij scratch couldn't get static imports to work - probably needs to be in a package
    ServerInstance server = Javalin.create(
      Routing.path("/api",
        Routing.get("/users", UserController::getAll),
        Routing.post("/users", UserController::create),
        Routing.path("/users",
          Routing.get("/:user-id", UserController::getOne),
          Routing.patch("/:user-id", UserController::update),
          Routing.delete("/:user-id", UserController::delete)
        )
      )
    );
  }

}

class Javalin {

  public static ServerInstance create(ServerInstanceMutator routing) { // potentially vararg
    ServerInstance server = new ServerInstance();
    routing.applyTo(server); // could also be server.resolve(routing);
    return server;
  }

}

class Routing implements ServerInstanceMutator {

  public static ServerInstanceMutator path(String path, ServerInstanceMutator... subRoutings) {
    return new CompositeRouting(path, List.of(subRoutings));
  }

  public static ServerInstanceMutator get(String path, Runnable mapping) {
    return new Routing(path, mapping);
  }

  public static ServerInstanceMutator post(String path, Runnable mapping) {
    return new Routing(path, mapping);
  }

  public static ServerInstanceMutator patch(String path, Runnable mapping) {
    return new Routing(path, mapping);
  }

  public static ServerInstanceMutator delete(String path, Runnable mapping) {
    return new Routing(path, mapping);
  }

  private final String path;
  private final Runnable mapping;

  public Routing(String path, Runnable mapping) {
    this.path = path;
    this.mapping = mapping;
  }

  // should probably be somehow encapsulated - like maybe not *public*
  public void applyTo(ServerInstance context) {
    context.resolve(path, mapping);
  }

}

class CompositeRouting implements ServerInstanceMutator {

  private final String path;
  private final List<ServerInstanceMutator> subRoutings;

  public CompositeRouting(String path, List<ServerInstanceMutator> subRoutings) {
    this.path = path;
    this.subRoutings = subRoutings;
  }

  public void applyTo(ServerInstance context) {
    for (ServerInstanceMutator route : subRoutings) {
      route.applyTo(context.subContext(path));
    }
  }

}

interface ServerInstanceMutator {

  void applyTo(ServerInstance context);

}

class ServerInstance {

  public ServerInstance subContext(String path) {
    throw new RuntimeException("TODO");
  }

  public void resolve(String path, Runnable mapping) {
    throw new RuntimeException("TODO");
  }

}

class UserController {

  public static void getAll() {}
  public static void create() {}
  public static void getOne() {}
  public static void update() {}
  public static void delete() {}

}