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Karibu-Testing: The Vaadin Unit Testing

The Unit Testing library for Vaadin.

When building apps with Vaadin Flow, your app manipulates Vaadin components server-side, while the browser is just a mere "reflection" of the server-side state (The "reflection" part - state transfer from server to client - is handled by Vaadin Flow). To test your app, you want your tests to focus on the server-side where your logic resides, while skipping the "unimportant" browser part "distraction".

Karibu-Testing gives you the ability to call UI.getCurrent() straight from your JUnit test methods, and receive a meaningful result in the process. You can call UI.navigate() to navigate around in your app; you can instantiate your components and views directly from your JUnit test methods. In order to do so, Karibu-Testing mocks CurrentRequest, VaadinSession and others in your currently running JVM (in which your JUnit tests run).

Karibu-Testing only runs in JVM and only tests server-side code. There is no browser running and no JavaScript code running, which means that there is no communication between the browser and the server, which means that you don't even need to run the servlet container. You simply create new instance of your Vaadin form, modify the field values directly, then simulate a click to the "Save" button and see the binder validations running. You can even call UI.navigate() to navigate to a view; the navigation is handled completely server-side and adds components to UI.getCurrent(). You then assert on the contents of UI.getCurrent() to verify that your code ran properly.

In short, Karibu-Testing is here to perform:

  1. containerless testing: You don't need to launch the servlet container. Karibu-Testing creates Vaadin Session, the UI and other necessary Vaadin classes straight in the JVM which runs your JUnit tests.
  2. browserless testing: You look up components straight from UI.getCurrent(), bypassing browser and the JavaScript->Server bridge completely. You directly call methods on your server-side View classes and on the server-side Java Vaadin components.

If you like the library, please star it. The more stars, the more popularity and more maintainenance the library will receive.

Check out a 30 second video of live coding with Karibu-Testing to get a taste on how simple this library is.

The library supports Kotlin, Java and Groovy (Groovy support for Vaadin 14+ only).

News: UI Unit-Testing is being added to TestBench!

The Vaadin UI Unit-Testing idea, pioneered by Karibu-Testing, proved useful for many Vaadin users. Vaadin recognized the effort and is adding official support for UI unit-testing into TestBench. The Vaadin UI Unit-Testing is now available in alpha builds of TestBench for Vaadin 23.1.1; stable version will require at least Vaadin 23.2 (maybe higher).

At the moment the Vaadin API is being prototyped and is not finalized. The Vaadin API will most probably look a bit differently than Karibu-Testing API though:

  • Karibu-Testing is Kotlin-first and uses the extension function mechanism heavily. Java has no extension functions though; we chose the mechanism of static methods (LocatorJ and others) to make Karibu accessible in Java. While we do offer full support for Java (and a bit limited support for Groovy), we will stay with the static methods solution for Java.
  • TestBench is Java-first and static methods are not offered via auto-complete IDE mechanism, leading to a sub-par experience. Therefore, Vaadin team chose the way of creating wrapper classes. That offers better IDE support in Java but requires a lot of effort creating and managing those wrapper classes.

At the moment Karibu-Testing classes are included in the Vaadin UI Unit-Testing library (as an implementation detail). However, this can change at any time.

The development of both products will now happen independently. If the Vaadin guys have a lovely idea, please open a feature request for Karibu so that we can steal those ideas :-D (Did you know that greatest artists steal?)

Important: The development of Karibu-Testing will continue as usual; we will continue supporting both Vaadin 14 and 23, both Kotlin, Groovy and Java, and we will continue designing the API via the "extension functions" mechanism.

Getting Started / Main Documentation / Full Documentation

Vaadin 14+

>>>See The main Karibu-Testing documentation for Vaadin 14+ <<<. Karibu-Testing is compatible with any Vaadin 14+ version (14, 15, 16, 17, 18, 19, 20, 21, 22, 23 etc).

Vaadin 8

>>>See The main Karibu-Testing documentation for Vaadin 8 <<<. Karibu-Testing is compatible with any Vaadin 8.x version.

Why Unit-testing?

Advantages over the traditional testing with Selenium/TestBench:

  • Fast: Browserless tests are typically 100x faster than Selenium-based tests and run in 5-60 milliseconds, depending on their complexity.
  • Reliable: We don't need arbitrary sleeps since we're server-side and we can await until the request is fully processed. We don't use Selenium drivers which are known to fail randomly.
  • Headless: The tests run headless since there's no browser. There is no need to setup screen in your CI environment.
  • Simple: the test runs in the same JVM as the server. You start the server in your @BeforeClass and stop the server in your @AfterClass. There is no need to use Maven's Integration plugin to start the server in the background (and then remember to kill it afterwards, otherwise all future CI tests will fail to start the server since it's already running).
  • Robust: the test runs in the same JVM as the server. If the server fails to start and throws an exception, the test method too will fail with the same exception. No need to go hunting for exceptions in a log located somewhere on a CI server.
  • No need to write Page Object for every Vaadin component, form and view your app contains, effectively doubling your code-base. Remember - you're already on the server, and you have access to the actual Java components which already are providing you with high-level APIs, exactly as Page Objects do.
  • Full access to the Database. You're server-side - you can access the database from your tests the same way your business logic accesses the database. You can run a bunch of SQLs to restore the database to a known state before every test. Even better - you can run the test in a transaction then roll back after the test, to perform fast database revert to a known state.

With this technique you can run 600 UI tests in 7 seconds, as opposing to 1-2 hours with Selenium-based approach. Because of the speed, you can let the UI tests run after every commit by your continuous integration server.

Since we're bypassing the browser and talking to Vaadin server API directly, you don't even need to start the servlet container - you can just add the server jars onto testing classpath and call Vaadin server API which will in turn invoke your server logic.

A 15-minute video explains everything behind the browserless testing technique.

Kotlin, Java and Groovy support

The Karibu-Testing library is Standalone and Fully Supports Java and Groovy.

Even though the Karibu-Testing is endorsed by Vaadin-on-Kotlin, the Karibu-Testing does not really depend on any other technology than Vaadin. You don't have to use Vaadin-on-Kotlin nor Karibu-DSL to use Karibu-Testing. You don't even need to write your app nor your tests in Kotlin, since the library provides native API for Kotlin, Java and Groovy.

You can therefore plug this library in into your Java+Vaadin-based project as a test dependency, and write the test code in Java, Kotlin or Groovy, whichever suits you more.

Karibu-Testing is published on Maven Central, so it's very easy to add as a Maven dependency.

Project Status

The library is mature and stable since 2020. It has been extensively tested and employed in numerous real-world projects. Small features are being added infrequently, and compatibility with all latest Vaadin versions is checked on every commit.

Example Projects

A list of a very simple example projects that employ Karibu Testing:


Karibu-Testing is designed to bypass browser and the servlet container. It's a double-edged sword: while this provides insane speed, it also sets limits what you can do with Karibu-Testing:

  • There is no browser: It's not possible to test nor call JavaScript code. That also limits testability of your Polymer Templates; see Testing with Vaadin for more details. If you need to test JavaScript code, you need to use Selenium or TestBench in addition to Karibu-Testing.
  • Since your JUnit methods access views which in turn access your business logic beans and your database directly, You must be able to "start" your app in the same JVM which runs the tests..

Here are a few tips on how to "start" your app in the JUnit JVM:

  • For simple apps with just the UI and no database that's very easy: simply call MockVaadin.setup { MyUI() } before every test, and MockVaadin.tearDown() after every test. That will instantiate your UI along with all necessary Vaadin environment.
  • For more complex apps employing database access (for example Vaadin-on-Kotlin apps) you need to bootstrap Vaadin-on-Kotlin. Luckily that's very easy, simply configure your database in VaadinOnKotlin.dataSourceConfig and then init vok: VaadinOnKotlin.init() before all tests. Or even better, since you typically do this in a ServletContextListener such as the Bootstrap class, simply call that: Bootstrap().contextInitialized(null). Then, when the app is bootstrapped, you can proceed to setting up your UI by calling MockVaadin.setup { MyUI() } before every test, and MockVaadin.tearDown() after every test.
  • For more complex apps not using Vaadin-on-Kotlin, just use the same approach of simply calling all ServletContextListener which you have in your app, before all tests are executed. Then, when the app is bootstrapped, you can proceed to setting up your UI by calling MockVaadin.setup { MyUI() } before every test, and MockVaadin.tearDown() after every test.
  • For Spring-based apps it's best to use Spring testing capabilities to bootstrap your app with Spring Test. Then, after that's done, use Spring injector to obtain the instance of your UI: call MockVaadin.setup { beanFactory!!.getBean( } before every test, and MockVaadin.tearDown() after every test. Please see the karibu-testing-spring example project for more details.
  • For JavaEE-based apps you need to figure out how to launch your app in some kind of embedded JavaEE container in JUnit's JVM.

More Resources

This project also offers a mock/fake servlet API implementation, see mock-servlet-environment for more details.


Q: I'm getting java.lang.IllegalStateException: UI.getCurrent() must not be null or no VaadinSession bound to current thread.

A: You probably forgot to call MockVaadin.setup() before the test. Just call MockVaadin.setup() e.g. from your @Before-annotated method if you're using JUnit.

Alternatively it could be that Spring is instantiating Vaadin component eagerly when the ApplicationContext is constructed. One workaround is to mark Vaadin components with @Lazy so that they are instantiated lazily.

Q: I'm getting RouteNotFoundError/NotFoundException instead of my views (Vaadin 14+)

A: The @Route-annotated view classes have not been discovered and registered. Please discover the routes via new Routes().autoDiscoverViews("com.example.yourpackage") (make sure to use the correct Java package where all of your views are placed) then call MockVaadin.setup(routes).

Also when migrating from Karibu-Testing 1.1 to 1.2.x and you're using a custom VaadinServlet, please make sure to call routes.register(service.context as VaadinServletContext) from the createServletService() method, as shown in Issue #60.

Q: Can I integrate CDI (e.g. weld?)

A: You can get inspiration here.

Q: Performance speed-up tips?

  1. (Vaadin 14+): the view auto-discovery is rather slow: you can discover the routes only once (for example in your @BeforeClass-annotated method), then store the Routes instance into a static field and reuse it for every call to MockVaadin.setup(routes).
  2. (Vaadin 14+): new Routes().autoDiscoverViews("") is slower than new Routes().autoDiscoverViews("com.example.yourpackage")
  3. (Vaadin 14+): PWA icon computation is horribly slow (2 seconds per test); make sure it's off (the Routes.skipPwaInit should be true which is also the default value).
  4. Instead of logging in by filing the login form before every test, you can simply login by placing User instance into your session directly (this of course pretty much depends on how security is handled in your app).
  5. The first test is usually slower since all Vaadin-related classes need to be loaded (could take 1-2 seconds to run); however any subsequent tests should be much faster (~5-60 milliseconds).


Licensed under Apache 2.0

Copyright 2017-2018 Martin Vysny

Licensed under the Apache License, Version 2.0 (the "License"); you may not use this software except in compliance with the License. You may obtain a copy of the License at

Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.

Contributing / Developing

See Contributing.