lang:java:vertx:promise

Table of Contents

About

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.

Example

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

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

Management

Creation

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

Documentation