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Generating test data with junit quickcheck

This post will demonstrate generating test data using junit-quickcheck. It will focus on practical examples, rather than the theory behind property based testing.

JUnit-quickcheck has its own JunitRunner, JUnitQuickcheck, which must be set on the class containing the test using an annotation, @RunWith(JUnitQuickcheck.class). With this annotation in place, the methods of the class annotated with @Property are executed. These @Property annotated methods are much like @Test annotated methods except for two things:

  1. The methods are called, by default, a 100 times
  2. @Property annotated methods can have parameters, which will be provided by junit-quickcheck using random values

Example system-under-testA lot of data

The system-under-test is a rest service, with one endpoint serving 3 books. The first few tests will target a domain model and a resource to be transformed into JSON. That resource, being part of a port in hexagonal architecture, has methods to transform from and to the domain model.

Build in data types

Junit-quickcheck can, without any further configuration or code, provide random values for a number of build-in datatypes.

In this example, values of type String and LocalDate (parameters title, author, publisher and publishedOn) are provided by junit-quickcheck. This test is run 100 times with random values.

User defined datatypes

Using junit-quickcheck we can also inject values of user defined types, but we have to specify how it creates them. We have the option to use setters, with the @From(Fields.class) annotation, or to use the constructor with @From(Ctor.class).

Here we let junit-quickcheck generate BookResource objects. If we were to any field to both Book and BookResource, and change the toBook and fromBook method accordingly, this test would still pass.

With @From(Ctor.class) we specify that junit-quickcheck should use the constructor. Unfortunately, we can only use constructors if all arguments are build-in types. In this case, BookResource has only String and a LocalDate as parameters. But we cannot generate a Book this way.

Custom generator

To let junit-quickcheck generate Book objects, which has parameters of user defined types, we must define and use a Generator.

In order to generate values of user defined types, with constructor parameters featuring even more user defined types, we must create our own Generator. Then, we reference this generator in @From annotation. This generator is, unfortunately, tightly coupled to the Book class. As a result, any new parameters in the constructor of Book will break this generator.

Mocking and collections

In the following tests target a Controller serving JSON documents. It depends on BookService to find Book objects, transforms those to BookResource objects en returns them.

Above, we set up mocking as usual.

Here we let junit-quickcheck generate lists of BookResource objects. Note the @From(Ctor.class) annotation binds to the BookResource, the type parameter of List and not List itself.


Using junit-quickcheck we can inject random values in our tests. This frees us from the burden of hard coding test data. Moreover, it generates more data than we would hard code. And, if we manage to avoid generators, our tests will not be tightly coupled to a data model.

Full source of the examples:

More information

Junit-quickcheck is a QuickCheck implementation for java, specifically targeting JUnit. QuickCheck is a property based testing library written in Haskell. It is used to check properties of functions, by applying those functions on generated input and checking that a property holds.

Generated values are by default completely random. However, using the @Seed annotation, the generated values become pseudo random. Thus, using a @Seed, our test become deterministic and repeatable once more. Please see for more details.

Junit-quickcheck can help find the smallest possible test case that fails: