Modern Java development demands a strong foundation in testing. It’s not enough to just write a few unit tests — real-world applications require flexible, maintainable, and thorough test coverage. With JUnit 5 and Mockito, Java developers can craft tests that are expressive, isolated, and powerful.

In this guide, we’ll explore advanced testing strategies including mocking behavior, parameterized testing, lifecycle management, and integration testing. Whether you’re building microservices or a complex enterprise system, these techniques will improve your code confidence and reliability.

Getting Started with JUnit 5 and Mockito

Add the necessary dependencies in your pom.xml:

<dependency>
<groupId>org.junit.jupiter</groupId>
<artifactId>junit-jupiter</artifactId>
<version>5.9.3</version>
<scope>test</scope>
</dependency>
<dependency>
<groupId>org.mockito</groupId>
<artifactId>mockito-core</artifactId>
<version>5.2.0</version>
<scope>test</scope>
</dependency>

Optional for advanced mocking:

<dependency>
<groupId>org.mockito</groupId>
<artifactId>mockito-inline</artifactId>
<version>5.2.0</version>
</dependency>

JUnit 5: The Modern Testing Framework

JUnit 5 is more than just an update — it’s a modular, extensible platform that supports modern testing needs.

  • Use @Test, @BeforeEach, @AfterEach, @BeforeAll, @AfterAll
  • Supports nested tests, tagging, and conditional execution
  • Extensions replace old runners for customizing test behavior
@BeforeEach
void setUp() {
service = new UserService(userRepository);
}

You can create nested contexts using @Nested to structure test suites logically.

@Nested
class WhenUserExists {
@Test
void shouldReturnUser() {
// test logic
}
}

Mocking with Mockito

Mockito allows you to isolate units of code by replacing dependencies with mocks and defining expected behavior.

@Mock
private UserRepository userRepository;

@InjectMocks
private UserService userService;

Define mock behavior and verify interactions:

when(userRepository.findById(1L)).thenReturn(Optional.of(new User("Alice")));
User user = userService.findUser(1L);
verify(userRepository).findById(1L);

Use argument matchers like any() or eq() for flexible test logic.

Parameterized Tests

Write one test method and run it with many inputs:

@ParameterizedTest
@CsvSource({
"2, 3, 5",
"10, 20, 30"
})
void testAdd(int a, int b, int expected) {
assertEquals(expected, calculator.add(a, b));
}

This reduces duplication and improves test coverage for edge cases.

Mocking Static and Final Methods

With mockito-inline, you can mock static or final methods:

try (MockedStatic<Utility> utilMock = mockStatic(Utility.class)) {
utilMock.when(() -> Utility.isValid(anyString())).thenReturn(true);
assertTrue(Utility.isValid("test"));
}

This is helpful for legacy code that isn’t easy to refactor.

Lifecycle and Reusability with @TestInstance

JUnit 5 introduces @TestInstance to control test instance lifecycle:

@TestInstance(TestInstance.Lifecycle.PER_CLASS)
class MyTestSuite {
@BeforeAll
void initOnce() {
// run before all tests
}
}

This is particularly useful when setup is expensive or reused across tests.

Integration Testing and TestContainers

Unit tests are not enough — verify real-world scenarios using integration testing:

@SpringBootTest
@AutoConfigureMockMvc
class ApiIntegrationTest {
@Autowired
private MockMvc mockMvc;

    @Test
    void testEndpoint() throws Exception {
        mockMvc.perform(get("/api/data"))
               .andExpect(status().isOk());
    }
}

For containerized dependencies like PostgreSQL or Kafka, use TestContainers:

@Container
static PostgreSQLContainer<?> postgres = new PostgreSQLContainer<>("postgres:13");

Best Practices for Java Testing

  • Isolate unit tests completely from databases or networks.
  • Use meaningful test names: shouldReturnUser_whenValidIdIsGiven().
  • Avoid mocking what you don’t own — prefer testing real behavior.
  • Keep test logic as simple and readable as possible.
  • Run tests in CI pipelines with clear logging on failure.

Conclusion

Testing is a critical part of modern software development. With JUnit 5 and Mockito, Java developers can write expressive, modular, and reliable tests that stand the test of time. From unit to integration testing, these tools empower you to catch bugs early, refactor safely, and ship confidently.

Incorporating advanced testing practices ensures your team delivers high-quality, maintainable, and bug-free applications.