Testing with Avaje Inject

How to test beans and dependency injection with unit and component/integration tests.

Unit Testing

Unit tests manually construct beans and use mocks for dependencies. This is fast, isolated, and does not use the DI container.

Example:

@Test
void testService() {
  UserRepository mockRepo = mock(UserRepository.class);
  UserService service = new UserService(mockRepo);
  when(mockRepo.findById(1)).thenReturn(new User(1, "John"));
  User user = service.findById(1);
  assertEquals("John", user.name);
}

Best practices:

• Use for pure logic, no DI needed.
• Mock only direct dependencies.

Component/Integration Testing with @InjectTest

Component tests use the DI container to wire real beans. This is similar to Spring’s @SpringBootTest.

Example:

@InjectTest
class UserServiceTest {
  @Inject UserService userService;

  @Test
  void findsUser() {
    User user = userService.findById(1);
    assertNotNull(user);
  }
}

Best practices:

• Use for service/business logic.
• Avoid mocking unless necessary for external systems.

Providing Test Doubles (Mocks)

We can initialize @Inject fields with mocks to override real beans. This lets us control specific dependencies and give them test specific behavior while using the real DI graph.

Example:

@InjectTest
class ServiceWithMockTest {
  @Inject UserRepository userRepository = mock(UserRepository.class);
  @Inject UserService userService;

  @Test
  void testServiceWithMock() {
    when(userRepository.findById(1)).thenReturn(new User(1, "Jane"));
    User user = userService.findById(1);
    assertEquals("Jane", user.name);
  }
}

The DI container will use the initialized mock instead of creating a real UserRepository bean. This allows you to:

• Control external dependencies without manually constructing the service
• Leverage the full dependency graph while mocking specific beans
• Use mocks to invoke error conditions and simulate specific edge cases

Using Mockito Annotations

Use @Mock/@Spy for cleaner setup. Mocks/spies are auto-wired into the test DI container.

Example:

@InjectTest
class ServiceWithMockitoTest {
  @Mock UserRepository userRepository;
  @Spy Logger logger;
  @Inject UserService userService;

  @Test
  void testWithMockito() {
    when(userRepository.findById(1)).thenReturn(new User(1, "Alex"));
    User user = userService.findById(1);
    assertEquals("Alex", user.name);
    verify(userRepository).findById(1);
  }
}

Annotations explained:

• @Mock — Creates a complete mock that returns default values (null, empty collections, etc.)
• @Spy — Wraps a real object and allows selective method mocking while preserving real behavior

Best practices:

• Use @Mock for pure mocks, @Spy to partially mock real objects.

Troubleshooting & Tips

• If a bean isn’t injected, check for missing @Inject or @InjectTest.
• Use try-with-resources for manual TestScope (advanced).

More Examples

• See inject-test module for real-world tests.
• For advanced scenarios (e.g., Postgres/Ebean, LocalStack), see dedicated guides: testing-postgres-ebean.md, testing-localstack.md (coming soon).

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For more, see the full library reference and avaje.io/inject.