Skip to main content

My first Vert.x 3 Application

Let’s say, you heard someone saying that Vert.x is awesome. Ok great, but you may want to try it by yourself. Well, the next natural question is “where do I start ?”. This post is a good starting point. It shows how is built a very simple vert.x application (nothing fancy), how it is tested and how it is packaged and executed. So, everything you need to know before building your own groundbreaking application.

The code developed in this post is available on github. This post is part of the Introduction to Vert.x series. The code of this post in in the post-1 branch.

Let’s start !

First, let’s create a project. In this post, we use Apache Maven, but you can use Gradle or the build process tool you prefer. You could use the Maven jar archetype to create the structure, but basically, you just need a directory with:

  1. a src/main/java directory
  2. a src/test/java directory
  3. a pom.xml file

So, you would get something like:

├── pom.xml
├── src
│   ├── main
│   │   └── java
│   └── test
│       └── java

Let’s create the pom.xml file with the following content:

<project xmlns=""




This pom.xml file is pretty straightforward:

  • it declares a dependency on vertx-core
  • it configures the maven-compiler-plugin to use Java 8.

This second point is important, Vert.x applications require Java 8.

Let’s code !

Ok, now we have made the pom.xml file. Let’s do some real coding… Create the src/main/java/io/vertx/blog/first/ file with the following content:


import io.vertx.core.AbstractVerticle;
import io.vertx.core.Future;

public class MyFirstVerticle extends AbstractVerticle {

  public void start(Future<Void> fut) {
        .requestHandler(r -> {
          r.response().end("<h1>Hello from my first " +
              "Vert.x 3 application</h1>");
        .listen(8080, result -> {
          if (result.succeeded()) {
          } else {

This is actually our not fancy application. The class extends AbstractVerticle. In the Vert.x world, a verticle is a component. By extending AbstractVerticle, our class gets access to the vertx field.

The start method is called when the verticle is deployed. We could also implement a stop method, but in this case Vert.x takes care of the garbage for us. The start method receives a Future object that will let us inform Vert.x when our start sequence is completed or report an error. One of the particularity of Vert.x is its asynchronous / non-blocking aspect. When our verticle is going to be deployed it won’t wait until the start method has been completed. So, the Future parameter is important to notify of the completion.

The start method creates a HTTP server and attaches a request handler to it. The request handler is a lambda, passed in the requestHandler method, called every time the server receives a request. Here, we just reply Hello ... (nothing fancy I told you). Finally, the server is bound to the 8080 port. As this may fails (because the port may already be used), we pass another lambda expression checking whether or not the connection has succeeded. As mentioned above it calls either fut.complete in case of success or to report an error.

Let’s try to compile the application using:

mvn clean compile

Fortunately, it should succeed.

That’s all for the application.

Let’s test

Well, that’s good to have developed an application, but we can never be too careful, so let’s test it. The test uses JUnit and vertx-unit - a framework delivered with vert.x to make the testing of vert.x application more natural.

Open the pom.xml file to add the two following dependencies:


Now create the src/test/java/io/vertx/blog/first/ with the following content:


import io.vertx.core.Vertx;
import io.vertx.ext.unit.Async;
import io.vertx.ext.unit.TestContext;
import io.vertx.ext.unit.junit.VertxUnitRunner;
import org.junit.After;
import org.junit.Before;
import org.junit.Test;
import org.junit.runner.RunWith;

public class MyFirstVerticleTest {

  private Vertx vertx;

  public void setUp(TestContext context) {
    vertx = Vertx.vertx();

  public void tearDown(TestContext context) {

  public void testMyApplication(TestContext context) {
    final Async async = context.async();

    vertx.createHttpClient().getNow(8080, "localhost", "/",
     response -> {
      response.handler(body -> {

This is a JUnit test for our verticle. The test uses vertx-unit, so we use a custom runner. vert.x-unit makes easy to test asynchronous interactions, which are the basis of vert.x applications.

In the setUp method, we creates an instance of Vertx and deploy our verticle. You may have noticed that unlike the traditional JUnit @Before method, it receives a TestContext. This object lets us control the asynchronous aspect of our test. For instance, when we deploy our verticle, it starts asynchronously, as most Vert.x interactions. We cannot check anything until it gets started correctly. So, as second argument of the deployVerticle method, we pass a result handler: context.asyncAssertSuccess(). It fails the test if the verticle does not start correctly. In addition it waits until the verticle has completed its start sequence. Remember, in our verticle, we call fut.complete(). So it waits until this method is called, and in the case of a failures, fails the test.

Well, the tearDown method is straightforward, and just terminates the vertx instance we created.

Let’s now have a look to the test of our application: the testMyApplication method. The test emits a request to our application and checks the result. Emitting the request and receiving the response is asynchronous. So we need a way to control this. As the setUp and tearDown methods, the test method receives a TestContext. From this object we creates an async handle (async) that lets us notify the test framework when the test has completed (using async.complete()).

So, once the async handle is created, we create a HTTP client and emits a HTTP request handled by our application with the getNow() method (getNow is just a shortcut for get(...).end()). The response is handled by a lambda. In this lambda we retrieves the response body by passing another lambda to the handler method. The body argument is the response body (as a buffer object). We check that the body contains the "Hello" String and declare the test complete.

Let’s take a minute to mention the assertions. Unlike in traditional JUnit tests, it uses context.assert.... Indeed, if the assertion fails, it will interrupt the test immediately. So it’s pretty important to always uses these assertion methods because of the asynchronous aspect of the Vert.x application and so tests.

Our test can be run from an IDE, or using Maven:

mvn clean test


So, let’s sum up. We have an application and a test. Well, let’s now package the application. In this post we package the application in a fat jar. A fat jar is a standalone executable Jar file containing all the dependencies required to run the application. This is a very convenient way to package Vert.x applications as it’s only one file. It also make them easy to execute.

To create a fat jar, edit the pom.xml file and add the following snippet just before </plugins>:


It uses the maven-shade-plugin to create the fat jar. In the manifestEntries it indicates the name of our verticle. You may wonder from where comes the Starter class. It’s actually a class from vert.x, that is going to create the vertx instance and deploy our verticle.

So, with this plugin configured, let’s launch:

mvn clean package

This is going to create target/my-first-app-1.0-SNAPSHOT-fat.jar embedding our application along with all the dependencies (including vert.x itself).

Executing our application

Well, it’s nice to have a fat jar, but we want to see our application running! As said above, thanks to the fat jar packaging, running Vert.x application is easy as:

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

Then, open a browser to http://localhost:8080.

To stop the application, hit CTRL+C.


This Vert.x 3 crash class has presented how you can develop a simple application using Vert.x 3, how to test it, package it and run it. So, you now know everything you need to build amazing system on top of Vert.x 3. Next time we will see how to configure our application.

Happy coding & Stay tuned !