Mocking dependencies for Java testing with Mockito.
Jul 08, 2025 am 02:25 AM
Mockito is a commonly used mocking framework in Java unit testing, used to simulate dependency behavior to avoid side effects caused by real calls. 1. Mock is to create a "fake" object instead of real dependencies, which facilitates control of return values, verify calls, and avoid external influences; 2. Use Mockito to create Mock objects through annotations or manually; 3. The core functions include when(...).thenReturn(...) to define behavior and verify(...) to verify calls; 4. Precautions include avoiding excessive mock, requiring additional tools to handle static methods, ensuring correct initialization, and verifying the number of calls. Mastering these core content can effectively improve testing efficiency and reliability.
In Java unit testing, Mockito is one of the most commonly used mocking frameworks. It can help you simulate the behavior of dependent objects and avoid side effects caused by real calls, such as database access, network requests, etc. This way you can focus on the logic testing of the current class.
What is a Mock? Why Mock dependencies?
Mock is to create a "fake" object to replace real dependencies.
For example: You are testing a UserService which relies on a UserRepository . If you don't use mock, you may really check the database every time you test, which is inefficient and prone to errors.
The advantage of using Mockito is that you can:
- Control the data returned by dependencies
- Verify that a method has been called
- Avoid external systems affecting test results
How to create a Mock object using Mockito?
Using Mockito is simple, add dependencies (such as Gradle):
testImplementation 'org.mockito:mockito-core:5.0.0'
Then you can use annotations or manually create mocks in your tests:
@Mock
private UserRepository userRepository;
@BeforeEach
void setUp() {
MockitoAnnotations.openMocks(this);
}Or write directly:
UserRepository userRepository = Mockito.mock(UserRepository.class);
Both methods are OK, depending on which one you are used to.
How to define Mock behavior and verification calls?
This is the most core feature of Mockito. Common methods are:
-
when(...).thenReturn(...): Define what a method call returns -
verify(...): Verify whether a method has been called
For example:
when(userRepository.findById(1L)).thenReturn(Optional.of(new User("Alice")));
User user = userService.getUserById(1L);
assertEquals("Alice", user.getName());
verify(userRepository).findById(1L);The above code means:
- When
userRepository.findById(1L)is called, an Optional containing Alice is returned - Then call
userService.getUserById()method - Finally, verify whether this method really calls
findById
This approach is great for testing business logic without relying on real data sources.
Frequently Asked Questions and Notes
- Don't overmock : If a class does not have complex external dependencies, there is no need to force mocking.
- Note static methods and constructors : Mockito does not support mock static methods by default, and it needs to be used with PowerMock or using a newer version of Mockito Inline.
- Don't forget to initialize mocks : be sure to call
MockitoAnnotations.openMocks(this);before using@Mockannotation, otherwise the pointer will be thrown. - Number of verification times : You can use
verify(x, times(2))to confirm that the method has been called several times, or you can useatLeastOnce()and other more flexible methods.
Basically that's all. Mockito is quick to use and powerful, and is an indispensable tool in Java unit testing. As long as you master a few core APIs, you can write clear and reliable test code.
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