Java Testing Skill

Write comprehensive tests for Java applications with modern testing practices.

Overview

This skill covers Java testing with JUnit 5, Mockito, AssertJ, and integration testing with Spring Boot Test and Testcontainers. Targets Java 25 and Spring Boot 4.0.x test slices. Includes TDD patterns and test coverage strategies.

When to Use This Skill

Use when you need to:

•Write unit tests with JUnit 5 (Java 25)
•Create mocks with Mockito
•Build integration tests with Testcontainers
•Implement TDD/BDD practices
•Test Spring Boot 4.0.x applications (@SpringBootTest, @WebMvcTest)
•Improve test coverage

Project context: Java 25, Spring Boot 4.0.2, spring-boot-starter-*-test, Maven

Topics Covered

JUnit 5

•@Test, @Nested, @DisplayName
•@ParameterizedTest with sources
•Lifecycle annotations
•Extensions and custom annotations

Mockito

•@Mock, @InjectMocks, @Spy
•Stubbing (when/thenReturn)
•Verification (verify, times)
•BDD style (given/willReturn)

AssertJ

•Fluent assertions
•Collection assertions
•Exception assertions
•Custom assertions

Integration Testing

•@SpringBootTest slices
•Testcontainers setup
•MockMvc for APIs
•Database testing

Quick Reference

java

// Unit Test with Mockito
@ExtendWith(MockitoExtension.class)
class UserServiceTest {

    @Mock
    private UserRepository userRepository;

    @InjectMocks
    private UserService userService;

    @Test
    @DisplayName("Should find user by ID")
    void shouldFindUserById() {
        // Given
        User user = new User(1L, "John");
        given(userRepository.findById(1L)).willReturn(Optional.of(user));

        // When
        Optional<User> result = userService.findById(1L);

        // Then
        assertThat(result)
            .isPresent()
            .hasValueSatisfying(u ->
                assertThat(u.getName()).isEqualTo("John"));
        then(userRepository).should().findById(1L);
    }
}

// Parameterized Test
@ParameterizedTest
@CsvSource({
    "valid@email.com, true",
    "invalid-email, false",
    "'', false"
})
void shouldValidateEmail(String email, boolean expected) {
    assertThat(validator.isValid(email)).isEqualTo(expected);
}

// Integration Test with Testcontainers
@Testcontainers
@SpringBootTest
class OrderRepositoryIT {

    @Container
    static PostgreSQLContainer<?> postgres =
        new PostgreSQLContainer<>("postgres:15");

    @DynamicPropertySource
    static void configure(DynamicPropertyRegistry registry) {
        registry.add("spring.datasource.url", postgres::getJdbcUrl);
        registry.add("spring.datasource.username", postgres::getUsername);
        registry.add("spring.datasource.password", postgres::getPassword);
    }

    @Autowired
    private OrderRepository repository;

    @Test
    void shouldPersistOrder() {
        Order saved = repository.save(new Order("item", 100.0));
        assertThat(saved.getId()).isNotNull();
    }
}

// API Test with MockMvc
@WebMvcTest(UserController.class)
class UserControllerTest {

    @Autowired
    private MockMvc mockMvc;

    @MockBean
    private UserService userService;

    @Test
    void shouldReturnUser() throws Exception {
        given(userService.findById(1L))
            .willReturn(Optional.of(new User(1L, "John")));

        mockMvc.perform(get("/api/users/1"))
            .andExpect(status().isOk())
            .andExpect(jsonPath("$.name").value("John"));
    }
}

Test Data Builders

java

public class UserTestBuilder {
    private Long id = 1L;
    private String name = "John Doe";
    private String email = "john@example.com";
    private boolean active = true;

    public static UserTestBuilder aUser() {
        return new UserTestBuilder();
    }

    public UserTestBuilder withName(String name) {
        this.name = name;
        return this;
    }

    public UserTestBuilder inactive() {
        this.active = false;
        return this;
    }

    public User build() {
        return new User(id, name, email, active);
    }
}

// Usage
User user = aUser().withName("Jane").inactive().build();

Coverage Goals

xml

<!-- JaCoCo configuration -->
<configuration>
    <rules>
        <rule>
            <element>BUNDLE</element>
            <limits>
                <limit>
                    <counter>LINE</counter>
                    <value>COVEREDRATIO</value>
                    <minimum>0.80</minimum>
                </limit>
            </limits>
        </rule>
    </rules>
</configuration>

Troubleshooting

Common Issues

Problem	Cause	Solution
Mock not working	Missing @ExtendWith	Add MockitoExtension
NPE in test	Mock not initialized	Check @InjectMocks
Flaky test	Shared state	Isolate test data
Context fails	Missing bean	Use @MockBean

Debug Checklist

code

□ Run single test in isolation
□ Check mock setup matches invocation
□ Verify @BeforeEach setup
□ Review @Transactional boundaries
□ Check for shared mutable state

Usage

code

Skill("java-testing")

Related Skills

•java-testing-advanced - Advanced patterns
•java-spring-boot - Spring test slices