Java Programmers: Discover How to Mock Spring in 4 Steps!

Writing tests is not easy even when you're mixing some framework.

Because of this, this article will help you to decide which kind of mock to use for each coverage type.

In the picture below, we can see the model, a simple model, but enough to understand the mocks.

Class diagram representing the domain

1. Mock

Use org.mockito.Mock when unit testing your business logic (only using JUnit and Mockito).

1.1. Scope

Test scope for Mock

1.2. Code

CustomerServiceMockTest class

1.3. Benefits

Isolate the scope of the test, ie, we're not starting the application to test the method, the scope is just called the method and verify

1.4. Drawbacks

Calling the service creates an instance manually :(

2. InjectMocks

The use of org.mockito.InjectMocks, enable us to mock any call for the subject, ie, looking at line 39, any call to the CustomerDAO.findAllCustomers will return a list with one element.

2.1. Scope

The test scope is marked as the black rectangle on the diagram below

2.2. Code

2.3. Benefits

You're testing a component that's great! Besides you can initialize the component (Bean) using the annotation ExtendsWith or using the MockitoAnnotations.openMocks(this)

2.4. Drawbacks

Since this is a UT, I cannot see any drawbacks.

3. MockBean

Use @org.springframework.boot.test.mock.mockito.MockBean when you write a test that is backed by a Spring Test Context and you want to add or replace a bean with a mocked version of it.

3.1. Scope

3.2. Code

3.3. Benefits

This kind of test using SpringBootTest enables us to run the Spring Boot application, but pay attention to external resources, ie, Database, Queue, and External Services. Those services should be mocked and properly configured.

Since is out of the scope of this article, those resources will not be covered.

3.4. Drawbacks

Additional work to make your whole application works, if you have resources.

4. No Mocks

This is a bonus section.

Since this is a small application without any Controller or Resource, is a piece of cake to avoid mocks, but this is not the rule is the exception.

4.1. Scope

4.2. Code

CustomerServiceNoMocksSpringBootTest class

4.3. Benefits

This kind of test could be classified as E2E (End To End) test since there are no resources.

4.4. Drawback

If you don't have any resources I can't see any drawback.

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5. Stack

• Java 11
• Spring Boot 2.7.11
• Gradle
• JUnit 5
• Gradle 7.4
• IntelliJ Ultimate

6. References

16-bits-spring/mocks at main · luizgustavocosta/16-bits-spring

MockBean (Spring Boot 3.0.6 API)

SpringBootTest (Spring Boot 3.0.6 API)

@Mock vs. @MockBean When Testing Spring Boot Applications - rieckpil

Mocking with (and without) Spring Boot