4 minute read

Photo on Unsplash by Juan Gomez

Hello there,

TestNG needs no introduction, It is one of the more popular test frameworks available in Java/JVM ecosystem and offers many advanced features like running tests in parallel, flexible annotations to do setups and teardowns at different levels and group tests and run them selectively.

One of the more useful features in TestNG are Data Providers which can be used to pass multiple variations of data to the same test method and have TestNG automatically generate tests

This saves the test author so much of time while avoiding him/her the effort of writing duplicated code with just the data being different.

When I switched to using Kotlin to write my tests in, I decided to use TestNG as the framework of choice. However, the implementation of data providers was a tricky problem in the beginning. Hopefully this post saves you that time.

A simple problem with data providers:

Let’s assume that you want to write tests for a simple method add()in a Calculator class which takes 2 nos, adds them up and verifies the result is equal to an expected value.

Below code represents the Calculator class.

Let’s see a basic test for this class:

The above test works and passes. What if we want to test if our calculator is able to perform addition operation for multiple different sets of data. Well that’s where a data provider comes in picture.

As per TestNG docs:

@DataProvider : Marks a method as supplying data for a test method. The annotated method must return an Object[][] where each Object[] can be assigned the parameter list of the test method. The @Test method that wants to receive data from this DataProvider needs to use a dataProvider name equals to the name of this annotation.


Couple of things to note:

  1. Kotlin Any is equivalent to Object
  2. We need to pass an array nested inside another array such that each nested array acts as a row of data for the test method
  3. Thus for above example, we create a variable testData of type ArrayList<Array<Int>> to hold this required data. Note: Int could be any primitive or complex data type (for instance even objects of a required type)
  4. We add the required rows of data as arrays of Int and keep on adding it to testData arrayList
  5. Finally, It’s important to return a MutableIterator of these arrays in order for this to work. We can get this by applying .iterator() method on the arrayList.

Now when we run this test, we observe below tests are generated in IntelliJ.

Test runner

Awesome. The same pattern can be repeated for any required data provider.

Abstracting data providers

So what if we have a set of data providers which are used by many test classes and we want to abstract this detail away from the Test class.

Below are some rules which are used for discovering data providers by TestNG:

By default, the data provider will be looked for in the current test class or one of its base classes.

TestNG docs

Cool. Let’s see an example of this.

Let’s assume we want to use the same data provider for a new sub() method of calculator class which subtracts the two nos.

We can define a new CalculatorBaseTest class (remember to mark it with open keyword to allow it to be extended) and move this method there and ensure that our CalculatorTests inherits from this class as below:

class CalculatorTests : CalculatorBaseTest()

Here is the complete base class.

Here is the updated Calculator class with the sub() method

And the updated Test class:

Moving data providers in a dedicated class

What if we don’t want to create an inheritance hierarchy and want all the related data providers to be in a single file. Turns out this can also be achieved easily in Kotlin.

As per docs:

If you want to put your data provider in a different class, it needs to be a static method or a class with a non-arg constructor, and you specify the class where it can be found in the dataProviderClass attribute:

In Kotlin, static methods are defined using companion object inside classes. However if we try with just putting the data provider method inside and try, it does not work and the Tests are ignored. After a bit of Googling, I figured out that you also need to annotate the method with @JvmStatic to ensure this works.

Here is the standalone file with the data provider in it:

And the test method which picks up the values from the above specified data provider, Note: The @Test annotation needs a argument dataProviderClass to be passed with a reference of KClass and thus it needs to be <NameOfTheClass>::class

With this, we can organize our data providers in desired files.

Sequential is so boring. Let’s parallelize:

With all the above examples, TestNG would execute all the generated test methods in a single thread and depending on how much time each method takes, this can greatly increase your test execution time.

Turns out this is so trivial to setup that you would greatly appreciate how wonderful TestNG is as a framework.

We need to do below steps:

  1. Add parallel = true in the @DataProvider annotated method
  2. Optionally add threadPoolSize to specify the no of threads that should be used to execute the tests in @Test annotation.

Here is the updated BaseTest method:

And the updated Test:

This small change can greatly add parallel super powers to your tests. However do ensure you use this carefully. Multithreading and concurrency are niche topics in itself and it helps to understand how the nuts and bolts work to ensure you avoid yourself some debugging pain. More on this in a future post.

And that’s it for this post folks. If you found this useful, Do share it with your friends and colleagues and I would like to hear in the comments in case you have suggestions.