Vert.x - Promise/Future

Java Conceptuel Diagram


promise and future in Vert.x

Do not confuse futures with promises.

  • futures represent the “read-side” of an asynchronous result,
  • promises represent the “write-side”.

They allow you to defer the action of providing a result. In most cases, you don’t need to create promises yourself in a Vert.x application.

Vert.x futures are not JDK futures, they can be

  • composed
  • and queried in a non-blocking fashion.

They shall be used for simple coordination of asynchronous tasks, and especially those of:

  • deploying verticles
  • and checking if they were successfully deployed or not.


with the start method of a verticle, prepare the database, then start the webserver

public void start(Promise<Void> promise) throws Exception {
	Future<Void> steps =
	  .compose(v -> startHttpServer());



Promise<Void> promise = Promise.promise();


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