For writing tests in general assertions are a fundamental element. They represent the expectation of the tester regarding the System Under Test (SUT). To understand how tapir treats assertions we have to split them into two blocks: implicit and explicit assertions.

Implicit assertions

Implicit assertions are assertions you do not write down in your test code. They are derived from the statements you choose for interacting with the SUT. Implicit assertions wait for prerequisites to be fulfilled before executing subsequent statements. If the assertion fails constantly within a certain time (configurable via the implicit wait time property) the test fails. There are several layers where you use tapir’s implicit assertions.

Interaction with an UI element

Whenever you interact with an ui component implicit assertions are checked by the framework. Let’s explain this by example: We have a simple button we would like to click. In order to be able to click on a button, the following prerequisites have to be fulfilled:

  1. The button must be displayed
  2. The button must be enabled (clickable)

When you call these prerequisites are asserted automatically. You do not have to check them explicitly, because tapir derives them from your intent to click the button.

Interaction with an UI element on a page

Imagine you have two pages which both have a button which is located via the name submit. Clicking the button on Page1 redirects you to Page2. We want to automate this flow:

  1. Click on submit-button on Page1
  2. Click on submit-button on Page2

While this seems to be trivial on the first sight, a major problem arises when taking a deeper look: the submit-button of both pages is addressed by the same locator (name=submit). After clicking the submit-button on Page1, the very same submit-button might be clicked again, because the SUT didn’t switch to Page 2 fast enough. Even worse, it might lead to flakiness as the SUT sometimes switches to Page 2 fast enough. You could explicitly assert that Page 2 is active, but that information can be derived from your intent as well.

In tapir UI elements are organized within Page Objects. You inject these pages into your test code in order to interact with the contained UI elements. If your page implements PageActiveCheck (which is highly recommended!) tapir derives additional implicit assertions.

This is the corresponding test code:

You define your intent by telling the framework on which page you would like to click the submit-button. This causes tapir to implicitly wait for Page 2 to be active as it otherwise wouldn’t be possible to click on a button on that page. There is no need to declare this explicitly.

Explicit assertions

Explicit assertions can by used rarely because tapir asserts a lot implicitly. Just by using a couple of UI interaction statements you can test, if your SUT’s screenflow works as expected. You do not need any explicit assertions for that. Anyway, there are two main use-cases where a explicit assertion is necessary and reasonable:

  1. Assert the value of UI elements
  2. Wait for a long-running task to be completed

Asserting the value of UI elements

Your tests gain additional value by asserting the states or values of your UI elements. You might want to check, if the text of a label is equal to your expectation. tapir reuses AssertJ which provides a great fluent API for this purpose. By default the CoreAssertions class is injected as an extension in all your test classes and actions. The CoreAssertions provide a fluent API for all common Java types like String, Integer, BigDecimal, etc..


Unfortunately asserting values is error-prone as it is likely that in the moment you check the assertion the actual value does not match the expected one. Therefore tapir again waits implicitly for the value to match the expected one. This is an extension to the core AssertJ library which checks the condition once and fails on violation. The implicit waiting makes your tests much more stable and reliable.

In order to use CoreAssertions you might add this dependency:

In order to set the description of an assertion, AssertJ provides you with the method as. However, as is also a reserved keyword in Xtend, which may confuse Eclipse and the Xtend compiler. To solve this, escape the method name with the circumflex character and write ˆas instead of as.

Waiting for long-running tasks

All implicit assertions wait until the condition is fulfilled or the implicit wait time is exceeded. There might be situation where you have to wait for a long-running task performed by your SUT. The implicit wait time’s purpose is to handle short delays of your SUT and shall not be configured to exceed 10 seconds. Therefore we need another concept for waiting for long-running tasks.

The following statements expects the name-label to be “Peter” within 60 seconds (In the example import static java.util.concurrent.TimeUnit.* is omitted).

assertThatWithin(60, SECONDS)[page1.nameLabel.text].isEqualTo("Peter")

Adding custom assertions

If the assertions provided by CoreAssertions are not sufficient for you, you have two options:

  1. Reuse assertions provided by AssertJ
  2. Write assertions for your custom types

You need this dependency:


Reuse assertions provided by AssertJ

tapir’s CoreAssertions are a subset of AssertJ’s Assertions class. If your would like to have a tapir API for a type which is already handled by an assertThat method of Assertions, proceed with the section Generating waiting assertions from AssertJ Assertions class.

Write assertions for your custom types

In case you want to add assertions for your custom types, AssertJ describes how to do that: Custom assertions.

We recommend to follow this guide. You end-up with a *Assertions class which contains one or more static methods called assertThat. This is the equivalent to the AssertJ Assertions class. In order to use it with tapir, proceed with the chapter Generating waiting assertions from AssertJ Assertions class.

Generating waiting assertions from AssertJ Assertions class

tapir provides a generator which transforms an AssertJ Assertions class containing static assertThat-methods into assertThat methods which expect a lambda expression and provide the functionality to wait for the assertion to pass.

You just have to create a class and annotate it with @AssertJAssertionsWrapper. You have to define the class to be wrapped by using the annotation’s value attribute.

In our example we would like to provide assertion for As this is already provided by org.assertj.core.api.Assertions, we do not have to implement this by ourselves.

With includes and excludes you select/deselect the assertThat-method by the type of their first parameter. So for our File assertion this is all we need to so.

import de.bmiag.tapir.assertion.annotation.AssertJAssertionsWrapper
import org.assertj.core.api.Assertions

@AssertJAssertionsWrapper(value=Assertions, includes=#[File])
class MyAssertions {


In order to use MyAssertions in our test code we can inject it as an extension:

class MyTest {
    def step() {
        val file = new File(<somePath>)