junit5-tests

Overview

Per junit.org/junit5/docs/current/user-guide:

Three components (released 2017, replacing JUnit 4):

• JUnit Jupiter - the new programming + extension model
• JUnit Vintage - backward-compat for JUnit 3/4 tests
• JUnit Platform - runner foundation (used by Jupiter, other test frameworks)

This skill targets JUnit Jupiter (the modern API). For Kotlin-native tests with similar power but DSL-style, see kotest-tests.

How to use

1. Add the junit-jupiter dependency and enable useJUnitPlatform() (Step 1).
2. Write a @Test method with an assertEquals assertion; run mvn test / ./gradlew test (Step 2).
3. Add lifecycle hooks (@BeforeEach / @AfterEach) for shared setup and teardown (Step 3).
4. Collapse repeated cases into a @ParameterizedTest with a source provider (Step 4).
5. Pull in collaborators via @ExtendWith (e.g. MockitoExtension) and annotate display names + conditions (Steps 5-6).
6. Enable parallel execution in junit-platform.properties once tests are independent (Step 7).
7. Wire JaCoCo coverage and CI reporting (Step 8).

Step 1 - Install (Maven / Gradle)

Maven pom.xml:

<dependency>
    <groupId>org.junit.jupiter</groupId>
    <artifactId>junit-jupiter</artifactId>
    <version>5.11.0</version>
    <scope>test</scope>
</dependency>

Gradle build.gradle.kts:

dependencies {
    testImplementation("org.junit.jupiter:junit-jupiter:5.11.0")
}

tasks.test {
    useJUnitPlatform()
}

Step 2 - First test

import org.junit.jupiter.api.Test;
import static org.junit.jupiter.api.Assertions.*;

class CalculatorTest {
    @Test
    void addsTwoNumbers() {
        assertEquals(3, Calculator.add(1, 2));
    }
}

Run: mvn test or ./gradlew test.

Step 3 - Lifecycle annotations

Per j5-ug:

class UserServiceTest {
    @BeforeAll
    static void initAll() { /* once before all */ }

    @AfterAll
    static void tearDownAll() { /* once after all */ }

    @BeforeEach
    void init() { /* before each test */ }

    @AfterEach
    void tearDown() { /* after each test */ }

    @Test
    void test1() { ... }

    @Test
    void test2() { ... }
}

Step 4 - Parameterized tests

import org.junit.jupiter.params.ParameterizedTest;
import org.junit.jupiter.params.provider.*;

class ParametrizedTest {
    @ParameterizedTest
    @ValueSource(ints = {1, 2, 3, 5, 8})
    void numbersInFibonacci(int n) {
        assertTrue(isFibonacci(n));
    }

    @ParameterizedTest
    @CsvSource({
        "1, 2, 3",
        "0, 0, 0",
        "-1, 1, 0",
    })
    void addCases(int a, int b, int expected) {
        assertEquals(expected, Calculator.add(a, b));
    }

    @ParameterizedTest
    @MethodSource("addProvider")
    void addsViaMethodSource(int a, int b, int expected) {
        assertEquals(expected, Calculator.add(a, b));
    }

    static Stream<Arguments> addProvider() {
        return Stream.of(
            Arguments.of(1, 2, 3),
            Arguments.of(0, 0, 0)
        );
    }
}

Source providers: @ValueSource, @CsvSource, @CsvFileSource, @MethodSource, @EnumSource, @ArgumentsSource.

Step 5 - Extensions (@ExtendWith)

JUnit 5's extension model (replaces JUnit 4's @Rule / @RunWith):

@ExtendWith(MockitoExtension.class)
class UserServiceTest {
    @Mock
    private UserRepository repo;

    @InjectMocks
    private UserService service;

    @Test
    void createsUser() {
        when(repo.save(any())).thenReturn(new User(1, "Alice"));
        User u = service.create("Alice");
        assertEquals(1, u.getId());
    }
}

Common extensions: MockitoExtension, SpringExtension (Spring), SystemStubsExtension (env vars / system properties), TempDirectory.

Step 6 - Display names + conditional execution

@DisplayName("User service")
class UserServiceTest {
    @Test
    @DisplayName("creates a user with email lowercased")
    void createsUserWithLowercaseEmail() { ... }

    @Test
    @EnabledOnOs(OS.LINUX)
    void linuxOnlyTest() { ... }

    @Test
    @EnabledIfEnvironmentVariable(named = "INTEGRATION", matches = "true")
    void integrationOnly() { ... }

    @Test
    @Disabled("Re-enable after fixing JIRA-1234")
    void temporarilyDisabled() { ... }
}

Step 7 - Parallel execution

Enable in junit-platform.properties:

junit.jupiter.execution.parallel.enabled = true
junit.jupiter.execution.parallel.mode.default = concurrent
junit.jupiter.execution.parallel.config.strategy = dynamic

Per-test override:

@Execution(ExecutionMode.SAME_THREAD)
class TestNotParallelizable { ... }

Step 8 - Coverage and CI

Wire JaCoCo coverage (Maven / Gradle) and CI reporting (GitHub Actions + Codecov; Surefire emits JUnit XML for junit-xml-analysis in the qa-test-reporting plugin) per references/coverage-and-ci.md.

Worked example

Testing Calculator.add across many inputs:

1. Add org.junit.jupiter:junit-jupiter:5.11.0 (test scope) and enable useJUnitPlatform().
2. Replace three near-identical @Test methods with one @ParameterizedTest + @CsvSource({"1, 2, 3", "0, 0, 0", "-1, 1, 0"}) taking (int a, int b, int expected).
3. Assert assertEquals(expected, Calculator.add(a, b)) - each row reports as its own test.
4. Run ./gradlew test: three cases execute; the -1, 1, 0 row fails, isolating the sign bug while the others stay green.
5. Add jacocoTestReport (Step 8) to confirm the add branch is now covered.

Anti-patterns

Limitations

• JUnit 5 requires Java 8+ minimum.
• Migration from JUnit 4 isn't 100% automatic; rule replacement via extensions is non-trivial.
• Parallel execution requires test independence; shared mutable state breaks it.

References

• j5-ug - official user guide
• junit.org/junit5 - landing
• maven.apache.org/surefire - Maven Surefire (test runner)
• jacoco.org - coverage tool
• references/coverage-and-ci.md - JaCoCo config + CI reporting
• kotest-tests, spock-tests, testng-tests, scalatest - sister tools
• test-code-conventions - test code hygiene