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. Includes TDD patterns and test coverage strategies. When to Use This Skill Use when you need to: Write unit tests with JUnit 5 Create mocks with Mockito Build integration tests with Testcontainers Implement TDD/BDD practices Improve test coverage 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 // 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 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

< 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 □ Run single test in isolation □ Check mock setup matches invocation □ Verify @BeforeEach setup □ Review @Transactional boundaries □ Check for shared mutable state Usage Skill("java-testing")