The new Formatter API for Xtext 2.8 (Part 2)

In the first part of our Formatter series I explained how the new formatter API works and how you can use it for the formatting of a Table.

As TDD is an elemental concept in everyday software development, I now want to explain how the formatter can be tested with some simple JUnit tests.

Xtend Template Expressions

First of all I want to explain how Xtend Template Expressions work and how we can take advantage of this concept for our unit tests. Templates Expressions can be assigned to a variable and start with ''' and also end with it. Inside these marks you can write text which in the background converts to a CharSequence, but has all the formatting available you gave inside the marks. Lets take a look at an example:


Here we assigned an expected result to a variable called expectedResult which shows our table nicely formatted. This is how it should look like if the formatter works as expected. Below we have a variable called testInput, which shows the text unformatted.

Unit-Tests in Xtext

If you use the wizard for creating an Xtext project you can select the Testing Support. Once selected, there are two projects created for testing purposes, .tests-project and .ui.tests. Here we will concentrate on the .tests-project, which is used for runtime tests. Inside the package we will create a class called FormattingTest.xtend and annotate it with


These annotations enable the injector for this project, using the InjectorProvider of the main xtext project and let the class run with a special runner, which aims at testing purposes.

Now we are ready to go and write our first Unit-Test for the formatter.

Writing a first formatter test

Using the snippets you see in the picture above we can now create the variables. To make this test possible we will inject the serializer and the parser for our language using

@Inject extension ParseHelper<Document> 
@Inject extension ISerializer

The parseHelper has the first EntryRule of our grammar in brackets because we want to test the formatter for our whole grammar.

The ISerializer interface is injected to serialize our test input snippet, where the formatter is automatically applied to any text given.

To create a test case using the example variables from above, we can simply write the following:

def void testFormatter() {

This is all we need for a first test case.


Writing unit tests for the formatter is quite easy, once you have configured your test class. The serializer is used to write the formatted output for an Xtext model. For larger tests you can consider giving the test input and output from a file, because very large templates can get quite messy.

The sources can be found here.

Happy testing!

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About The Author

Dominik Jetzen works as a software engineer at itemis AG in Frankfurt, Germany. He works on projects using Xtext and is interested in model-driven software development.