Weld meets Vert.x

2016-4-11   vertx , integration   Martin Kouba

Vert.x defines itself as "a toolkit for building reactive applications on the JVM". Sounds cool and trendy. Weld, on the other hand, comes from the Java EE world, based on standards and traditional concepts. But wait, what if we try to combine the two worlds to get the best of them?

Vert.x makes use of a light-weight distributed messaging system to allow application components to communicate in a loosely coupled way. This should sound familiar to all CDI users where beans may produce and consume events as well. Weld team developed a working prototype of Weld/Vert.x integration that allows to automatically register certain observer methods as Vert.x message consumers. A simple echo message consumer could look like this:

import org.jboss.weld.vertx.VertxConsumer;
import org.jboss.weld.vertx.VertxEvent;

class Foo {
    public void echoConsumer(@Observes @VertxConsumer("test.echo.address") VertxEvent event) {
        event.setReply(event.getMessageBody());
    }
}

  • @VertxConsumer - a qualifier used to specify the address the consumer will be registered to: test.echo.address

  • VertxEvent - a wrapper of a Vert.x message

Since we’re working with a regular observer method, additional parameters may be declared (next to the event parameter). These parameters are injection points. So it’s easy to declare a message consumer dependencies:

public void consumerWithDependencies(@Observes @VertxConsumer("test.dependencies.address") VertxEvent event, CoolService coolService, StatsService statsService) {
    coolService.process(event.getMessageBody());
    statsService.log(event);
}
Note
 If you inject a dependent bean, it will be destroyed when the invocation completes.

Last but not least - an observer may also send/publish messages using the Vert.x event bus:

public void consumerStrikesBack(@Observes @VertxConsumer("test.publish.address") VertxEvent event) {
    event.messageTo("test.huhu.address").publish("huhu");
}

And how does it work under the hood? First of all, it’s necessary to deploy org.jboss.weld.vertx.WeldVerticle. This Verticle starts Weld SE container and automatically registers org.jboss.weld.vertx.VertxExtension to process all observer methods and detect observers which should become message consumers. Then a special handler is registered for each address to bridge the event bus to the CDI world. Handlers use Vertx.executeBlocking() since we expect the code to be blocking. Later on, whenever a new message is delivered to the handler, Event.fire() is used to notify all relevant observers.

The prototype is surely missing some features. Nevertheless, it shows the CDI programming model might be applicable to the "reactive" world even if not a first class citizen. Moreover, the prototype demonstrates the flexibility of the CDI extension mechanism.

If you want to try it out, you’ll have to clone the https://github.com/weld/weld-vertx repository and build it from source (i.e. run mvn clean install). And if you find it useful feel free to add comments to this blog post. Any feedback is appreciated!