junit

# JUnit 5 JUnit is the programmer-friendly testing framework for Java and the JVM. JUnit 5 is the current major version, composed of the JUnit Platform, JUnit Jupiter, and JUnit Vintage. ## When to Use - **Java & JVM Ecosystem Testing**: The gold standard testing framework (JUnit 5 / Jupiter) for Java, Kotlin, and Spring Boot applications. - **Parameterized & Dynamic Testing**: Executing tests across CSV sources, method sources, and enum arguments. - **Lifecycle Extensions**: Customizing test execution with JUnit 5 Extension model (`@ExtendWith`). - **Spring Boot Integration Testing**: Pairing with `@SpringBootTest` and MockMvc to test enterprise web applications. ## Quick Start ```java import static org.junit.jupiter.api.Assertions.assertEquals; import org.junit.jupiter.api.Test; class CalculatorTest { @Test void addition() { assertEquals(2, 1 + 1, "Optional failure message"); } } ``` ## Core Concepts ### JUnit 5 Architecture (Platform + Jupiter + Vintage) JUnit 5 separates the test execution engine from the developer API: ```text [ JUnit Platform (Launcher, IDE & Maven/Gradle Execution) ] ├── [ JUnit Jupiter (Modern API, Annotations, Extensions) ] └── [ JUnit Vintage (Backward compatibility with JUnit 3/4) ] ``` ### Parameterized Tests (`@ParameterizedTest`) Executes a test method repeatedly with differing input sets: ```java import org.junit.jupiter.params.ParameterizedTest; import org.junit.jupiter.params.provider.CsvSource; import static org.junit.jupiter.api.Assertions.*; class StringUtilityTest { @ParameterizedTest(name = "reverse({0}) should be {1}") @CsvSource({ "radar, radar", "hello, olleh", "'', ''" }) void testReversal(String input, String expected) { assertEquals(expected, StringUtility.reverse(input)); } } ``` ### Lifecycle Callbacks & Nested Test Contexts Organizes tests hierarchically with `@Nested` and handles setup/teardown cleanly: ```java import org.junit.jupiter.api.*; class OrderServiceTest { @BeforeEach void setUp() { /* Common setup */ } @Nested @DisplayName("When order has no discount") class NoDiscount { @Test void calculatesFullPrice() { /* ... */ } } } ``` ## Common Patterns ### Parameterized Tests with CSV or Method Source **Problem**: Writing multiple `@Test` methods for different inputs leads to verbose and brittle test classes. **Solution**: Use `@ParameterizedTest` with `@CsvSource`: ```java import org.junit.jupiter.params.ParameterizedTest; import org.junit.jupiter.params.provider.CsvSource; import static org.junit.jupiter.api.Assertions.assertEquals; class MathUtilsTest { @ParameterizedTest(name = "{0} + {1} should equal {2}") @CsvSource({ "1, 2, 3", "10, 20, 30", "-5, 5, 0" }) void testAddition(int a, int b, int expected) { assertEquals(expected, MathUtils.add(a, b)); } } ``` ## Best Practices **Do**: - Standardize on JUnit Jupiter (JUnit 5): Remove the legacy `junit-vintage-engine` dependency from modern projects. - Use AssertJ for Fluent Assertions: Pair JUnit with `assertThat(result).isNotNull().hasSize(3)` for clear assertion failures. - Leverage `@TempDir` for File Tests: Inject temporary directories automatically cleaned up after test completion. - Use Display Names: Provide descriptive `@DisplayName("Should reject expired payment tokens")` annotations. **Don't**: - Import JUnit 4 annotations: Avoid importing `org.junit.Test`; always use `org.junit.jupiter.api.Test`. - Make test classes or methods `public` in JUnit 5: Jupiter classes and methods can and should be package-private. - Ignore test execution order: Tests should be completely independent; avoid relying on execution sequences. ## Troubleshooting | Error | Cause | Solution | | :--------------------------------------------------------- | :------------------------------------------------------------------- | :-------------------------------------------------------------------------------------- | | `No tests found matching selector` | Missing JUnit 5 Jupiter engine dependency in pom.xml / build.gradle. | Add `org.junit.jupiter:junit-jupiter-engine` and enable `useJUnitPlatform()` in Gradle. | | `org.opentest4j.AssertionFailedError` | Expected value does not match actual output. | Inspect assertion message: parameter ordering is `assertEquals(expected, actual)`. | | `IllegalStateException: Failed to load ApplicationContext` | Spring Boot configuration error during test setup. | Check `@ContextConfiguration` or mock missing beans using `@MockBean`. | ## References - [JUnit 5 User Guide](https://junit.org/junit5/docs/current/user-guide/)