code: public Javalin addHandler(@NotNull HandlerType handlerType, @NotNull String path, @NotNull Handler handler, @NotNull Set<Role> roles) {
          servlet.addHandler(handlerType, path, handler, roles);
          eventManager.fireHandlerAddedEvent(new HandlerMetaInfo(handlerType, Util.prefixContextPath(servlet.getConfig().contextPath, path), handler, roles));
          return this;
      }
docs: Adds a request handler for the specified handlerType and path to the instance.
  Requires an access manager to be set on the instance.
  This is the method that all the verb-methods (get/post/put/etc) call.
  
  @see AccessManager
  @see <a href="https://javalin.io/documentation#handlers">Handlers in docs</a>