Friday Repost: Groovy Test Data Builder Pattern

The Friday Repost series are copies from my earlier writings. Since I don’t want to loose them, and they might prove useful to others, I’m reposting them on this blog

Most of us value the quality of our code highly. I do, and I expect you, as a reader, also to care for the quality of your code. Because of that, we write tests. Test which usually contain of 3 parts: the setup, the execution and the verification. And sometimes some cleanup. In this blog, I’d like to focus on the setup part, and try to get rid of some boilerplate code which can easily clutter our tests!

As I mentioned before, we usually write some setup code which serves as input for our test. This code can look like this:

Writing this code can be time consuming, plus the code doesn’t reveal the intention of the test. What are the significant bits in this setup code? Is it the age? Is it my name? Or a combination?

To solve the creation of testdata, there are multiple patterns. On of those patterns is the Test Data Builder pattern. The idea behind the Test Data Builder pattern is that test data should be easily to construct, which is usually accomplished by chaining method calls (also called a Fluent Interface, or a Builder).

I’ve taken the concept to Groovy and made an TestDataBuilder Category. A Groovy Category adds functionality to your code within a limited scope (defined by the ‘use’ keyword). This enables the object which is used in the setup of the test to ‘borrow’ the method of the TestDataBuilder Groovy class, which allows it to construct itself with testdata. This is best demonstrated by the use of an example:

By placing the creation of the data in the scope of the Category, the .build() method becomes available in for the Person class. Calling the build method will instantiate the Person class and populate all the fields with values which all equal true. So numbers will become 1, booleans true, Strings will get the name of the variable as a value, etc.

To use this in tests, and make the code more intention revealing, we can choose to populate some fields with our own values, and have the rest be generated.

The benefits of this is that you save a lot of setup code, and make the code a lot more clear and maintainable. If you want to use this, the code can be found on (This class was previously called the ObjectMother, an error on my side. The change in name isn’t reflected yet in GitHub). Yes, there are some limitations, like it doesn’t handly all standard datatypes yet (like dates), nor does it populate associations (list with other objects) yet. Have fun, and any feedback on the code would be highly valued!

1 reply
  1. jesuspesca
    jesuspesca says:

    The post and class are really nice; thanks a lot. I’ve noticed that replacing the ‘default’ in switch statement by:
    default: return null
    default: return

    it builds the complete object graph.

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