Java Test Engineer Skill

Expert guidance for writing, reviewing, and fixing tests in Java/Spring Boot applications.

Core Principles

1. Test Pyramid Strategy

• Unit tests (70%): Fast, isolated, no Spring context
• Integration tests (20%): Spring context, real dependencies where practical
• E2E tests (10%): Full system, use sparingly

2. Testability Over Mocking

Design code to be testable rather than relying on heavy mocking:

• Prefer constructor injection over field injection
• Extract pure functions from side-effectful code
• Use interfaces at boundaries, concrete classes internally
• Small, focused classes with single responsibility

3. Test Behavior, Not Implementation

• Test public API, not internal methods
• Avoid testing private methods directly
• Tests should survive refactoring if behavior unchanged

4. Property Tests > Example Tests (When Applicable)

Prefer property-based tests over example-based tests when:

• Testing pure functions with clear invariants
• Validating parsers, serializers, converters (round-trip)
• Business rules with mathematical properties
• Input validation with large input spaces

---

Property-Based Testing (jqwik) - PREFERRED APPROACH

Property tests find edge cases you'd never think to write. Use them for any pure function or stateless logic.

When to Use Property Tests

| Use Property Tests | Use Example Tests | |-------------------|-------------------| | Pure functions | Side effects (DB, HTTP) | | Validation logic | Specific business scenarios | | Parsers/serializers | Integration flows | | Mathematical properties | UI interactions | | Large input spaces | Small, enumerable cases |

Maven Dependency

<dependency>
    <groupId>net.jqwik</groupId>
    <artifactId>jqwik</artifactId>
    <version>1.9.2</version>
    <scope>test</scope>
</dependency>

Common Property Patterns

1. Invariants - "This should always be true"

@Property
void priceShouldNeverBeNegative(@ForAll @Positive int qty,
                                 @ForAll @Positive int unitPrice,
                                 @ForAll @IntRange(min = 0, max = 100) int discount) {
    var total = calculator.calculate(qty, unitPrice, discount);
    assertThat(total).isGreaterThanOrEqualTo(0);
}

@Property
void cprShouldAlwaysHave10Digits(@ForAll("validCprs") String cpr) {
    assertThat(cpr.replaceAll("-", "")).hasSize(10);
}

2. Round-Trip / Symmetry - "Encode then decode = original"

@Property
void serializeDeserializeShouldBeIdentity(@ForAll("users") User user) {
    var json = mapper.writeValueAsString(user);
    var restored = mapper.readValue(json, User.class);
    assertThat(restored).isEqualTo(user);
}

@Property
void encryptDecryptShouldBeIdentity(@ForAll String plaintext,
                                     @ForAll("keys") SecretKey key) {
    var encrypted = crypto.encrypt(plaintext, key);
    var decrypted = crypto.decrypt(encrypted, key);
    assertThat(decrypted).isEqualTo(plaintext);
}

3. Idempotence - "Doing it twice = doing it once"

@Property
void normalizeShouldBeIdempotent(@ForAll String input) {
    var once = normalizer.normalize(input);
    var twice = normalizer.normalize(once);
    assertThat(twice).isEqualTo(once);
}

@Property
void sortingTwiceShouldNotChange(@ForAll List<Integer> list) {
    var sorted = sort(list);
    var sortedAgain = sort(sorted);
    assertThat(sortedAgain).isEqualTo(sorted);
}

4. Commutativity - "Order doesn't matter"

@Property
void additionShouldBeCommutative(@ForAll int a, @ForAll int b) {
    assertThat(calculator.add(a, b)).isEqualTo(calculator.add(b, a));
}

5. Test Oracle - "Compare with known-good implementation"

@Property
void customSortShouldMatchJavaSort(@ForAll List<Integer> list) {
    var expected = new ArrayList<>(list);
    Collections.sort(expected);

    var actual = customSort.sort(list);
    assertThat(actual).isEqualTo(expected);
}

6. Metamorphic - "Related inputs should have related outputs"

@Property
void doublingInputShouldDoubleOutput(@ForAll @Positive int qty,
                                      @ForAll @Positive int price) {
    var single = calculator.total(qty, price);
    var doubled = calculator.total(qty * 2, price);
    assertThat(doubled).isEqualTo(single * 2);
}

Custom Arbitraries

For detailed patterns on custom arbitraries, constraining generation, statistics, and combining with JUnit 5, see: references/jqwik-arbitraries.md

Quick example:

@Provide
Arbitrary<Order> orders() {
    return Combinators.combine(
        Arbitraries.longs().greaterOrEqual(1),
        Arbitraries.of(OrderStatus.class),
        Arbitraries.lists(orderItems()).ofMinSize(1).ofMaxSize(10)
    ).as(Order::new);
}

---

Unit Testing Patterns

JUnit 5 Structure

@ExtendWith(MockitoExtension.class)
class ServiceTest {
    @Mock Repository repo;
    @InjectMocks Service service;

    @Test
    void shouldDoSomething_whenCondition() {
        // given
        var input = createInput();
        when(repo.find(any())).thenReturn(Optional.of(entity));

        // when
        var result = service.process(input);

        // then
        assertThat(result).isEqualTo(expected);
        verify(repo).save(any());
    }
}

Naming Convention

methodName_shouldExpectedBehavior_whenCondition

Examples:

• calculateTotal_shouldApplyDiscount_whenCustomerIsPremium
• validateCpr_shouldThrowException_whenFormatInvalid

AssertJ Over JUnit Assertions

// Prefer
assertThat(result).hasSize(3).contains("a", "b");
assertThat(exception).isInstanceOf(ValidationException.class)
    .hasMessageContaining("invalid");

// Avoid
assertEquals(3, result.size());
assertTrue(result.contains("a"));

Testing Exceptions

@Test
void shouldThrowWhenInvalid() {
    assertThatThrownBy(() -> service.process(null))
        .isInstanceOf(IllegalArgumentException.class)
        .hasMessageContaining("must not be null");
}

// Or with JUnit 5
var ex = assertThrows(ValidationException.class,
    () -> service.validate(input));
assertThat(ex.getErrors()).hasSize(2);

Parameterized Tests

@ParameterizedTest
@CsvSource({
    "100, 10, 90",    // normal discount
    "50, 0, 50",      // no discount
    "200, 25, 150"    // max discount
})
void shouldCalculateDiscount(int price, int discount, int expected) {
    assertThat(calculator.apply(price, discount)).isEqualTo(expected);
}

@ParameterizedTest
@MethodSource("invalidInputs")
void shouldRejectInvalidInput(String input, String expectedError) {
    assertThatThrownBy(() -> validator.validate(input))
        .hasMessageContaining(expectedError);
}

static Stream<Arguments> invalidInputs() {
    return Stream.of(
        Arguments.of(null, "must not be null"),
        Arguments.of("", "must not be empty"),
        Arguments.of("abc", "must be numeric")
    );
}

Testing Either/Result Types (dk.oister.util.Either)

When testing code that returns Either<E, T>:

@Test
void shouldReturnRightOnSuccess() {
    Either<OrderError, Order> result = service.createOrder(validRequest);

    // Assert success case
    assertThat(result.isRight()).isTrue();
    assertThat(result.getRight()).satisfies(order -> {
        assertThat(order.status()).isEqualTo(PENDING);
        assertThat(order.total()).isEqualTo(Money.of(100));
    });
}

@Test
void shouldReturnLeftOnValidationFailure() {
    Either<OrderError, Order> result = service.createOrder(invalidRequest);

    // Assert failure case
    assertThat(result.isLeft()).isTrue();
    assertThat(result.getLeft()).isInstanceOf(ValidationError.class);
    assertThat(result.getLeft().message()).contains("invalid");
}

// Pattern matching with fold
@Test
void shouldHandleBothCases() {
    var result = service.processOrder(request);

    String message = result.fold(
        error -> "Failed: " + error.message(),
        order -> "Success: " + order.id()
    );

    assertThat(message).startsWith("Success:");
}

Property tests for Either composition:

@Property
void flatMapShouldPropagateLeft(@ForAll("validOrders") Either<OrderError, Order> first) {
    // Left should short-circuit
    Either<OrderError, Order> left = Either.left(new OrderError("error"));
    Either<OrderError, Order> result = left.flatMap(o -> first);

    assertThat(result.isLeft()).isTrue();
    assertThat(result.getLeft().message()).isEqualTo("error");
}

@Property
void mapShouldPreserveRight(@ForAll("orders") Order order) {
    Either<OrderError, Order> right = Either.right(order);
    Either<OrderError, Money> mapped = right.map(Order::total);

    assertThat(mapped.isRight()).isTrue();
    assertThat(mapped.getRight()).isEqualTo(order.total());
}

---

Mocking Best Practices

When to Mock

• External services (HTTP clients, message queues)
• Time-dependent code (Clock)
• Randomness (Random, UUID generators)
• Database (only in unit tests)

When NOT to Mock

• Value objects and DTOs
• Pure functions
• Your own code in integration tests
• Things you can use real instances of

Mock Injection Patterns

// Constructor injection - preferred
class Service {
    private final Repository repo;
    private final Clock clock;

    Service(Repository repo, Clock clock) {
        this.repo = repo;
        this.clock = clock;
    }
}

// In test
var clock = Clock.fixed(Instant.parse("2024-01-15T10:00:00Z"), ZoneId.UTC);
var service = new Service(mockRepo, clock);

Verify Sparingly

// Good: verify critical interactions
verify(emailService).send(any(Email.class));

// Bad: over-verification couples tests to implementation
verify(repo).findById(1L);
verify(mapper).toDto(any());
verify(validator).validate(any());
// These are implementation details

Argument Captors

@Captor ArgumentCaptor<Email> emailCaptor;

@Test
void shouldSendWelcomeEmail() {
    service.registerUser(user);

    verify(emailService).send(emailCaptor.capture());
    var email = emailCaptor.getValue();
    assertThat(email.getTo()).isEqualTo(user.getEmail());
    assertThat(email.getSubject()).contains("Welcome");
}

---

Spring Boot Integration Tests

For detailed Spring Boot test configurations, MockMvc patterns, repository tests with Testcontainers, and security testing, see: references/spring-testing.md

Quick patterns:

// Base test configuration (Spring Boot 3.4+)
@SpringBootTest
@ActiveProfiles("test")
@Transactional
abstract class BaseIntegrationTest {
    @Autowired protected MockMvc mockMvc;
    @MockitoBean protected ExternalService externalService;  // @MockBean deprecated in 3.4+
}

// Controller test with JWT
@WebMvcTest(UserController.class)
class UserControllerTest {
    @Test
    void shouldReturnUser() throws Exception {
        mockMvc.perform(get("/api/users/1")
                .with(jwt().authorities(new SimpleGrantedAuthority("ROLE_USER"))))
            .andExpect(status().isOk());
    }
}

---

Async Testing Patterns

CompletableFuture Testing

@Test
void shouldCompleteWithinTimeout() {
    CompletableFuture<Order> future = service.processAsync(request);

    // Use assertj's completablefuture support
    assertThat(future)
        .succeedsWithin(Duration.ofSeconds(5))
        .satisfies(order -> assertThat(order.status()).isEqualTo(COMPLETED));
}

@Test
void shouldHandleAsyncFailure() {
    CompletableFuture<Order> future = service.processAsync(invalidRequest);

    assertThat(future)
        .failsWithin(Duration.ofSeconds(5))
        .withThrowableOfType(ExecutionException.class)
        .havingCause()
        .isInstanceOf(ValidationException.class);
}

Awaitility for Polling Assertions

// Maven: org.awaitility:awaitility:4.2.0

@Test
void shouldEventuallyUpdateStatus() {
    service.startAsyncProcess(orderId);

    await().atMost(Duration.ofSeconds(10))
        .pollInterval(Duration.ofMillis(500))
        .untilAsserted(() -> {
            var order = repo.findById(orderId).orElseThrow();
            assertThat(order.status()).isEqualTo(COMPLETED);
        });
}

@Test
void shouldPublishEventWithinTimeout() {
    service.processOrder(request);

    await().atMost(Duration.ofSeconds(5))
        .until(() -> eventCaptor.getEvents(), hasSize(1));

    assertThat(eventCaptor.getEvents().get(0))
        .isInstanceOf(OrderCreatedEvent.class);
}

WebFlux Testing with WebTestClient

@SpringBootTest(webEnvironment = WebEnvironment.RANDOM_PORT)
class ReactiveControllerTest {
    @Autowired WebTestClient webClient;

    @Test
    void shouldStreamOrders() {
        webClient.get().uri("/api/orders/stream")
            .accept(MediaType.TEXT_EVENT_STREAM)
            .exchange()
            .expectStatus().isOk()
            .expectBodyList(Order.class)
            .hasSize(3);
    }

    @Test
    void shouldHandleReactiveError() {
        webClient.get().uri("/api/orders/999")
            .exchange()
            .expectStatus().isNotFound()
            .expectBody()
            .jsonPath("$.error").isEqualTo("Order not found");
    }
}

---

Test Data Builders

For complete builder and mother patterns, see: references/test-data-builders.md

Quick example:

class UserBuilder {
    static UserBuilder aUser() { return new UserBuilder(); }
    UserBuilder asAdmin() { return withRoles(Role.ADMIN); }
    User build() { return new User(email, name, roles); }
}

// Usage
var admin = aUser().withEmail("admin@test.com").asAdmin().build();

---

Test Anti-Patterns to Avoid

1. Test Interdependence

// BAD: tests depend on execution order
static User sharedUser;

@Test void test1_createUser() { sharedUser = service.create(...); }
@Test void test2_updateUser() { service.update(sharedUser, ...); }

// GOOD: each test is independent
@Test void shouldCreateUser() { var user = service.create(...); }
@Test void shouldUpdateUser() {
    var user = service.create(...);
    service.update(user, ...);
}

2. Over-Mocking

// BAD: mocking everything
@Mock Mapper mapper;
@Mock Validator validator;
@Mock Logger logger;

// GOOD: use real implementations where practical
var mapper = new UserMapper();  // stateless, fast
var validator = new UserValidator();

3. Testing Implementation Details

// BAD: breaks when you refactor
verify(repo, times(1)).findById(any());
verify(cache).get(any());
verify(mapper).toEntity(any());

// GOOD: test observable behavior
assertThat(result.getName()).isEqualTo("expected");

4. Ignoring Edge Cases

// Always test:
// - null inputs
// - empty collections
// - boundary values
// - error conditions
@ParameterizedTest
@NullAndEmptySource
@ValueSource(strings = {" ", "   "})
void shouldRejectInvalidInput(String input) {
    assertThatThrownBy(() -> service.process(input))
        .isInstanceOf(IllegalArgumentException.class);
}

5. Slow Tests

// BAD: unnecessary Spring context
@SpringBootTest
class SimpleCalculatorTest { ... }

// GOOD: plain unit test
class SimpleCalculatorTest {
    Calculator calc = new Calculator();
    ...
}

---

Debugging Test Failures

Common Issues

1. Flaky tests: Usually caused by:

   • Time-dependent code (use Clock)
   • Shared mutable state
   • Race conditions in async code
   • Random data without seed

2. Spring context failures:

   • Check @ActiveProfiles("test")
   • Verify @MockBean for external dependencies
   • Check for duplicate bean definitions

3. Database test issues:

   • Verify @Transactional for rollback
   • Check isolation level
   • Use @DirtiesContext sparingly (slow)

Test Isolation Checklist

• [ ] No shared mutable state between tests
• [ ] Database rolled back after each test
• [ ] Mocks reset with @BeforeEach or MockitoExtension
• [ ] No file system side effects
• [ ] Fixed time/random seeds where needed

---

Test Coverage Guidelines

What to Cover

• All public methods of services
• All controller endpoints (happy + error paths)
• Business logic edge cases
• Security boundaries
• Data validation

What NOT to Obsess Over

• Getters/setters/constructors
• Configuration classes
• Framework code
• Trivial delegation methods

Coverage Targets

• Line coverage: 70-80% (not a hard rule)
• Branch coverage: Focus on complex conditionals
• Mutation testing: Better metric than line coverage

---

Test Execution Commands

# All tests
mvn test

# Single class
mvn test -Dtest=UserServiceTest

# Single method
mvn test -Dtest=UserServiceTest#shouldCreateUser

# By tag
mvn test -Dgroups=integration

# With coverage
mvn test jacoco:report

# Parallel execution
mvn test -DforkCount=2 -DreuseForks=true

---

Reference Files

For detailed patterns, see:

• references/jqwik-arbitraries.md: Custom arbitraries, constraining, statistics, shrinking
• references/spring-testing.md: Spring Boot integration, MockMvc, security testing
• references/test-data-builders.md: Builder and Mother patterns for test data

---

Checklist for New Tests

• [ ] Test name describes behavior, not method
• [ ] Follows given/when/then structure
• [ ] Uses AssertJ for assertions
• [ ] Independent of other tests
• [ ] Fast (unit < 100ms, integration < 5s)
• [ ] Covers happy path + key error cases
• [ ] No unnecessary mocking
• [ ] Cleans up resources (files, connections)