Magic of Spring Boot testing: Data-driven unit tests

Sample application for the test

For my series of blog posts about Spring Boot testing, I’ve prepared a sample application.

One of the app functionality is formatting input values in the form of two sings after decimal points - XX.YY.

It has a UI part as well as a REST API part. UI part takes an input:

and returns a result:

For API part the same functionality is implemented for an “/format” endpoint:

The result object is:

Formatter service

The feature code for format operation is in FormatterService class.

It has one method “formatMoney()” which takes a String as an input and returns the value as a formatted String.

public String formatMoney(String inputMoney) {
        log.info("Received value for formatting: {}", inputMoney);
        if (inputMoney == null || inputMoney.isEmpty()) {
            return inputMoney;
        }
        inputMoney = inputMoney.replace(',', '.');
        DecimalFormatSymbols symbols = DecimalFormatSymbols.getInstance();
        symbols.setGroupingSeparator(' ');
        symbols.setDecimalSeparator('.');

        DecimalFormat formatter = new DecimalFormat("###,##0.00", symbols);
        String formatted = "";
        try {
            formatted = formatter.format(Double.parseDouble(inputMoney));
        } catch (NumberFormatException ex) {
            log.error(ex.getMessage());
        }
        log.info("Formatted result: {}", formatted);
        return formatted;
    }

In order to cover the test on the unit test level, we can write a lot of tests even for a single method, like:

  public class FormatterTest {
    private FormatterService formatterService;

    @Before
    public void beforeTest() {
        formatterService = new FormatterService();
    }

    @Test
    public void shouldFormatValue(){
        String input = "123.234";
        String result = "123.23";
        assertThat(formatterService.formatMoney(input)).isEqualTo(result);
    }

    @Test
    public void shouldFormatNegativeValue(){
        String input = "-123.236";
        String result = "-123.24";
        assertThat(formatterService.formatMoney(input)).isEqualTo(result);
    }

    // and so on ...
}

But in all cases, it will be testing the behavior of the method from an input data point of view.

Maybe it is a better way how to deal with data-driven unit tests?

Parameterized tests with JUnit

JUNit offers an ability to test method with multiple test data using Parameterized runner.

The steps for adding a parameterized test for formatter are the following:

1. Declare test data

   @Parameterized.Parameters
    public static Collection<String[]> data() {
        return Arrays.asList(new String[][]{
                {"2310000.159897", "2 310 000.16"},
                {"1600", "1 600.00"},
                {"0", "0.00"},
                {"-123456.456", "-123 456.46"},
                {"123234,456", "123 234.46"},
                {".", ""},
                {",", ""},
                {"111.", "111.00"},
                {".222", "0.22"},
                {null, null}
        });
    }

2. Declare input and output parameters

   @Parameterized.Parameter
   public String input;
   
   @Parameterized.Parameter(1)
   public String expected;

3. Initialize service and implement the test

   private FormatterService formatterService;
   
   @Before
   public void beforeTest() {
     formatterService = new FormatterService();
   }
   
   @Test
   public void shouldConvertValue() {
     assertThat(expected)
             .as("Invalid result of money conversion")
             .isEqualTo(formatterService.formatMoney(input));
   }

4. Mark test class to be executed with Parameterized.class from JUnit

   @RunWith(Parameterized.class)
   public class FormatterServiceTest {}

Full source code of the test available here

Benefits of the data-driven approach

Parameterized testing provides the following benefits:

• Increased focus on test data and corner cases

• Better code reusability and less code duplication

• Can be applicable at any test level: unit, component, integration, end-to-end

Conclusion

The data-driven approach in unit testing can potentially decrease the number of copy-paste code for unit tests and focus developer more on the test data and corner cases.

But as with any other testing approaches - it is not a silver bullet and should be used appropriately.

Service code, as well as the test code, can be found in the Boot-testing-examples repository.