Why we should use Property testing?

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Why we should use Property testing?

How you can have a top coverage with fewer unit tests


6 min read

Featured on Hashnode

When we are talking about code quality we always land to the code coverage check: we need to be sure to test all the code lines based on the provided inputs.

What is the best code coverage?

We are mostly talking about 80% of code coverage. This seems a reasonable value to have something good without losing a lot of time... but is that enough? How we can say we have "enough coverage" to be sure we can put in production, without fear, just after each change?

A good and simple rule can be:

if we change something in the code: an if a loop, a value, variable init, ... a test should fail somewhere.

This is because changing something in the code should modify the way your application is working: same input different output. What about if the output is not changing? That's a good question, but in a basic way we could say: never mind that code, we are still having the expected result.

Let now check the following function

public ResponseEntity<List<Book>> getAllBooks(@RequestParam(required = false) String title) {
    try {
        List<Book> books = Optional.ofNullable(title)
               .filter(t -> !t.isEmpty())
        if (books.isEmpty()) {
           return new ResponseEntity<>(HttpStatus.NO_CONTENT);

        return new ResponseEntity<>(books, HttpStatus.OK);
    } catch (Exception e) {
        return new ResponseEntity<>(null, HttpStatus.INTERNAL_SERVER_ERROR);

It will generate a JSON response containing the list of books retrieved from the DataBase. If we provide the title parameter it will retrieve books with the provided word(s) within the title, if the parameter is empty or null it will retrieve all the database books. Then it will create a response based on the retrieved value.

What are the tests we should create to validate this simple code? I can try to list them here:

  • with null title, check if thefindAll method is invoked
  • with an empty title, check if thefindAll method is invoked
  • with a valid title, check if findByTitleContaining with the title parameter
  • if the List<Book> books is empty, the answer should be HttpStatus.NO_CONTENT
  • if the List<Book> books is not empty, the answer should be HttpStatus.OK with the list of retrieved books
  • if an error is created somewhere, the answer must be HttpStatus.INTERNAL_SERVER_ERROR

Then to go further, what about the values provided to title? Can it be any possible char, number, different alphabet, ... ? How many tests do we have to write to be sure we have a good code coverage?

Property Testing

The side problem with what we have just seen is the maintainability of your code. Imagine we wrote only 6 tests (but with 6, we are using a single title possible value!!) if we change anything in the method we maybe have to change the 6 tests at once. This means each change will require a 6 times greater effort than without any test.

But we have a proper coverage and in all other cases, I can be sure any other change around can't break this code.

A simple solution can be to use PBT, Property-Based Test. We will write a single test that triggers hundred/thousand tests at once with the same code. The following example is using the jqwik library:

public void testReadAllBooksEmpty(@WithNull @ForAll String title) {
    ResponseEntity<List<Book>> response = cut.getAllBooks(title);
    if (title != null && !title.isEmpty()) {
        verify(bookRepository, never()).findAll();
    } else {
        verify(bookRepository, never()).findByTitleContaining(title);
    assertEquals("Unexpected HTTP Status Code", response.getStatusCode(), HttpStatus.NO_CONTENT);

NOTE: as we have an if in the test, I know it should be 2 different tests instead. I just wanted to keep it "extreme" to show how simple can be. I didn't want to be a Unit Test Purist ๐Ÿ˜…

The @Property annotation is specifying that the method is a PBT. Then the @ForAll annotation over a parameter is a way to say we want to inject different values for all the tests and the @WithNull is testing with a null title; there are several other parameters and different ways to control how you want to manage the values injected into the title parameter.

With this basic configuration, it is running by default 1000 tests with 1000 different values (with the empty one too):

timestamp = 2021-12-31T17:40:22.380933, BookControllerTest:testReadAllBooks = 
tries = 1000                  | # of calls to property
checks = 1000                 | # of not rejected calls
generation = RANDOMIZED       | parameters are randomly generated
after-failure = PREVIOUS_SEED | use the previous seed
when-fixed-seed = ALLOW       | fixing the random seed is allowed
edge-cases#mode = MIXIN       | edge cases are mixed in
edge-cases#total = 3          | # of all combined edge cases
edge-cases#tried = 3          | # of edge cases tried in current run
seed = -8434577657060517927   | random seed to reproduce generated values

and we are validating that we are calling the correct bookRepository method based on the title parameter and the response is empty with NO_CONTENT status code. We know it is always an empty response because the bookRepository is mocked and we didn't initialize it.

Getting the first list we wrote, with this single test method we tested:

  • with null title check if thefindAll method is invoked
  • with an empty title check if thefindAll method is invoked
  • with a valid title check if findByTitleContaining with the title parameter
  • if the List<Book> books is empty the answer should be HttpStatus.NO_CONTENT

We can then create a second one, for example, to test the information are correctly returned when the repository is giving valid book objects.

Code changes

As we said at the beginning, good code coverage does not allow to change stuff without failing tests. For example:

List<Book> books = Optional.ofNullable(title)
                    //.filter(t -> !t.isEmpty())

removing the empty filter, if we have a good test, should fail.

timestamp = 2021-12-31T17:57:31.304953, BookControllerTest:testReadAllBooksEmpty = 
    Wanted but not invoked:
    -> at net.mornati.springnativepoc.controller.BookControllerTest.testReadAllBooksEmpty(BookControllerTest.java:36)
    However, there was exactly 1 interaction with this mock:
    -> at java.base/java.util.Optional.map(Optional.java:260)


But using PBT we have also some added tests that we didn't plan. Imagine for example we want (or an error in the code, so we don't want) to filter title longer than 10 chars. Code can be something like

List<Book> books = Optional.ofNullable(title)
                    .filter(t -> !t.isEmpty())
                    .filter(t -> t.length() <= 10)

As in the 1000 automatic tests we have a lot of different titles with different sizes, when we run the test without changing anything else, the code is not working as expected

    Wanted but not invoked:
    -> at net.mornati.springnativepoc.controller.BookControllerTest.testReadAllBooksEmpty(BookControllerTest.java:33)
    However, there was exactly 1 interaction with this mock:
    -> at java.base/java.util.Optional.orElseGet(Optional.java:364)

we are calling the findAll method instead of the findByTitleContaining.

With manual tests, this case can be also covered by chance: we have to use a title longer than 10 chars within our test. So automatically we have better coverage without changing everything and, as I said at the beginning better maintainability.


I tried to show you how powerful can be a PBT and why we should use them. It is for sure a very simple example. In real-life PBT are much more complex: several parameters, custom object parameters, ... the JQwik framework I used is allowing all of these possibilities. How to know if it should be good to write a PBT instead of a simple Unit Test? Basically, we can say, any time you are calling a method with "static" parameters, you should write property or parametrized test instead.

var result = myMethod("xxx");

var result2 = myMethod2(2, new Car("Peugeot"));

In my opinion, these are tests you should try to rewrite to have a better and automatic code coverage.