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Vert.x Application Configuration

Previously in ‘Introduction to Vert.x’

In this post, we developed a very simple Vert.x 3 application, and saw how this application can be tested, packaged and executed. That was nice, isn’t it ? Well, ok, that was only the beginning. In this post, we are going to enhance our application to support external configuration.

So just to remind you, we have an application starting a HTTP server on the port 8080 and replying a polite “Hello” message to all HTTP requests. The previous code is available here. The code developed in this post is in the post-2 branch.

So, why do we need configuration?

That’s a good question. The application works right now, but well, let’s say you want to deploy it on a machine where the port 8080 is already taken. We would need to change the port in the application code and in the test, just for this machine. That would be sad. Fortunately, Vert.x applications are configurable.

Vert.x configurations are using the JSON format, so don’t expect anything complicated. They can be passed to verticle either from the command line, or using an API. Let’s have a look.

No ‘8080’ anymore

The first step is to modify the class to not bind to the port 8080, but to read it from the configuration:

public void start(Future<Void> fut) {
      .requestHandler(r -> {
        r.response().end("<h1>Hello from my first " +
            "Vert.x 3 application</h1>");
          // Retrieve the port from the configuration,
          // default to 8080.
          config().getInteger("http.port", 8080),
          result -> {
            if (result.succeeded()) {
            } else {

So, the only difference with the previous version is config().getInteger("http.port", 8080). Here, our code is now requesting the configuration and check whether the http.port property is set. If not, the port 8080 is used as fall-back. The retrieved configuration is a JsonObject.

As we are using the port 8080 by default, you can still package our application and run it as before:

mvn clean package
java -jar target/my-first-app-1.0-SNAPSHOT-fat.jar

Simple right ?

API-based configuration - Random port for the tests

Now that the application is configurable, let’s try to provide a configuration. In our test, we are going to configure our application to use the port 8081. So, previously we were deploying our verticle with:

vertx.deployVerticle(MyFirstVerticle.class.getName(), context.asyncAssertSuccess());

Let’s now pass some deployment options:

port = 8081;
DeploymentOptions options = new DeploymentOptions()
    .setConfig(new JsonObject().put("http.port", port)
vertx.deployVerticle(MyFirstVerticle.class.getName(), options, context.asyncAssertSuccess());

The DeploymentOptions object lets us customize various parameters. In particular, it lets us inject the JsonObject retrieved by the verticle when using the config() method.

Obviously, the test connecting to the server needs to be slightly modified to use the right port (port is a field):

vertx.createHttpClient().getNow(port, "localhost", "/", response -> {
  response.handler(body -> {

Ok, well, this does not really fix our issue. What happens when the port 8081 is used too. Let’s now pick a random port:

ServerSocket socket = new ServerSocket(0);
port = socket.getLocalPort();

DeploymentOptions options = new DeploymentOptions()
    .setConfig(new JsonObject().put("http.port", port)

vertx.deployVerticle(MyFirstVerticle.class.getName(), options, context.asyncAssertSuccess());

So, the idea is very simple. We open a server socket that would pick a random port (that’s why we put 0 as parameter). We retrieve the used port and close the socket. Be aware that this method is not perfect and may fail if the picked port becomes used between the close method and the start of our HTTP server. However, it would work fine in the very high majority of the case.

With this in place, our test is now using a random port. Execute them with:

mvn clean test

External configuration - Let’s run on another port

Ok, well random port is not what we want in production. Could you imagine the face of your production team if you tell them that your application is picking a random port. It can actually be funny, but we should never mess with the production team.

So for the actual execution of your application, let’s pass the configuration in an external file. The configuration is stored in a json file.

Create the src/main/conf/my-application-conf.json with the following content:

  "http.port" : 8082

And now, to use this configuration just launch your application with:

java -jar target/my-first-app-1.0-SNAPSHOT-fat.jar -conf src/main/conf/my-application-conf.json

Open a browser on http://localhost:8082, here it is !

How does that work ? Remember, our fat jar is using the Starter class (provided by Vert.x) to launch our application. This class is reading the -conf parameter and create the corresponding deployment options when deploying our verticle.


After having developed your first Vert.x application, we have seen how this application is configurable, and this without adding any complexity to our application. In the next post, we are going to see how we can use vertx-web to develop a small application serving static pages and a REST API. A bit more fancy, but still very simple.

Happy Coding and & Stay Tuned!