jooby/src/main/java/org/jooby/Deferred.java
public void handler(final Request req, final Handler handler) throws Exception {
    this.handler = requireNonNull(handler, "Handler is required.");
    if (initializer != null) {
      initializer.run(req, this);
    }
  }
public void handler(final Request req, final Handler handler) throws Exception {
    this.handler = requireNonNull(handler, "Handler is required.");
    if (initializer != null) {
      initializer.run(req, this);
    }
  }
Setup a handler for this deferred. Application code should never call this method: INTERNAL USE
ONLY.

@param req Current request.
@param handler A response handler.
@throws Exception If initializer fails to start.