JUnit 5, the latest version of the popular Java testing framework, introduces several powerful features and improvements over its predecessors. Writing effective unit tests in JUnit 5 can significantly enhance the quality and maintainability of your codebase. This article explores the essential dos and don’ts of writing JUnit 5 unit tests, providing practical examples to illustrate best practices.

Dos

1. Use Meaningful Test Names

Do: Name your test methods clearly and descriptively to convey their purpose.

Example:

@Test
void shouldReturnTrueWhenInputIsPrime() {
    // Arrange
    PrimeNumberChecker checker = new PrimeNumberChecker();

    // Act
    boolean result = checker.isPrime(7);

    // Assert
    assertTrue(result);
}

2. Use Annotations Properly

Do: Utilize JUnit 5 annotations like @Test, @BeforeEach, @AfterEach, @BeforeAll, and @AfterAll appropriately to set up and tear down test environments.

Example:

class CalculatorTest {
    private Calculator calculator;

    @BeforeEach
    void setUp() {
        calculator = new Calculator();
    }

    @Test
    void shouldAddTwoNumbersCorrectly() {
        assertEquals(5, calculator.add(2, 3));
    }

    @AfterEach
    void tearDown() {
        calculator = null;
    }
}

3. Use Assertions Effectively

Do: Make use of the various assertions provided by JUnit 5 to verify test outcomes.

Example:

@Test
void shouldThrowExceptionWhenDividingByZero() {
    Calculator calculator = new Calculator();

    Exception exception = assertThrows(ArithmeticException.class, () -> calculator.divide(10, 0));

    assertEquals("/ by zero", exception.getMessage());
}

4. Test Edge Cases

Do: Write tests for boundary conditions and edge cases to ensure robustness.

Example:

@Test
void shouldReturnEmptyListWhenNoElements() {
    List<Integer> emptyList = Collections.emptyList();
    assertTrue(emptyList.isEmpty());
}

5. Use Parameterized Tests

Do: Use parameterized tests to run the same test with different inputs.

Example:

@ParameterizedTest
@ValueSource(ints = {1, 2, 3, 5, 8})
void shouldReturnTrueForOddNumbers(int number) {
    assertTrue(number % 2 != 0);
}

Don’ts

1. Don’t Write Tests That Depend on Each Other

Don’t: Avoid writing tests that rely on the outcome of other tests. Each test should be independent.

Example of what to avoid:

@Test
void testPartOne() {
    // Some test logic
}

@Test
void testPartTwo() {
    // Assumes testPartOne has run and succeeded
}

2. Don’t Ignore Exceptions

Don’t: Ensure that you handle exceptions properly in your tests.

Example of what to avoid:

@Test
void testShouldHandleException() {
    try {
        someMethodThatThrowsException();
    } catch (Exception e) {
        // Ignoring exception
    }
}

3. Don’t Use Static State

Don’t: Avoid using static variables that can retain state between tests.

Example of what to avoid:

class StaticStateTest {
    private static int counter = 0;

    @Test
    void testIncrementCounter() {
        counter++;
        assertEquals(1, counter);
    }

    @Test
    void testResetCounter() {
        counter = 0;
        assertEquals(0, counter);
    }
}

4. Don’t Overuse Mocks

Don’t: Use mocks judiciously. Over-mocking can lead to brittle tests that are hard to maintain.

Example of what to avoid:

@Test
void testWithTooManyMocks() {
    MyService service = mock(MyService.class);
    MyRepository repository = mock(MyRepository.class);
    MyHelper helper = mock(MyHelper.class);

    // Complex mock setup and verification
}

5. Don’t Test Implementation Details

Don’t: Focus on testing the behavior and outcomes rather than internal implementation details.

Example of what to avoid:

@Test
void testInternalState() {
    MyClass myClass = new MyClass();
    myClass.doSomething();

    // Accessing private field via reflection
    Field field = MyClass.class.getDeclaredField("internalState");
    field.setAccessible(true);
    assertEquals("expectedState", field.get(myClass));
}

Conclusion

Writing effective unit tests with JUnit 5 requires following best practices to ensure your tests are maintainable, reliable, and clear. Use meaningful names, proper annotations, and diverse assertions. Avoid test dependencies, static state, and over-mocking. By adhering to these dos and don’ts, you can create a robust suite of unit tests that help ensure your code functions as intended.