How do you implement mocks and spies in Spring Boot testing?

Table of Contents

• Introduction
  • 1. What is the Difference Between Mocks and Spies?
  • 2. Using @Mock in Spring Boot Testing
    • Example: Using @Mock in a Test Class
  • 3. Using @Spy in Spring Boot Testing
    • Example: Using @Spy in a Test Class
  • 4. Combining Mocks and Spies in the Same Test
    • Example: Combining Mocks and Spies
  • 5. Verifying Mock Interactions
    • Example of Verifying Interactions:
• 6. Conclusion

Introduction

In Spring Boot testing, using mocks and spies is essential for simulating the behavior of dependencies in unit tests. By mocking or spying on beans, you can isolate the unit of work under test, without needing the real implementations of services or repositories.

Mocks and spies are provided by Mockito, a popular mocking framework integrated into Spring Boot's testing environment. Understanding how to use mocks and spies effectively helps you test specific behaviors and verify interactions in your application, making your tests more reliable and efficient.

In this guide, we'll explore how to use Mockito’s @Mock and @Spy annotations to implement mocks and spies in Spring Boot tests.

1. What is the Difference Between Mocks and Spies?

Before diving into examples, it’s important to understand the distinction between mocks and spies in the context of testing:

• Mock: A mock is a simulated object that mimics the behavior of real objects, but you control its behavior. Mocks return predefined values when methods are called, and interactions can be verified.
• Spy: A spy is a real object that is partially mocked. Unlike a mock, it uses the real implementation but allows you to override specific methods to monitor or manipulate certain behavior. It’s useful when you want to test a real object’s behavior but also need to verify certain interactions or stub out specific methods.

2. Using @Mock in Spring Boot Testing

The **@Mock** annotation is used to create mock objects for dependencies in your tests. It allows you to define specific behaviors for methods in the mock object, such as returning values when certain methods are called.

Example: Using @Mock in a Test Class

Consider a service that depends on a repository to fetch data. You can mock the repository to isolate the service’s logic from the actual database.

In this example:

• **@Mock** creates a mock instance of MyRepository.
• **@InjectMocks** injects the mock into the MyService class.
• The **when()** and **thenReturn()** methods are used to define the behavior of the mock.
• The **verify()** method ensures that the mocked method was called.

3. Using @Spy in Spring Boot Testing

The **@Spy** annotation is used to create a spy of an object. A spy is a partial mock: you can spy on a real object while still allowing its methods to be invoked as usual, unless specified otherwise.

Example: Using @Spy in a Test Class

Let's say you have a service with a method that performs multiple operations. You can spy on the service to monitor the method calls but still use the real behavior for most methods.

In this example:

• **@Spy** creates a spy instance of MyRepository.
• The **when()** method is used to mock the behavior of getData() in the spy.
• The real method implementations will be used unless overridden.

4. Combining Mocks and Spies in the Same Test

It is possible to use both mocks and spies in the same test class. This allows you to mock certain behaviors while spying on others.

Example: Combining Mocks and Spies

Here:

• We use **@Mock** to mock MyRepository.
• We use **@Spy** to spy on SomeOtherService, and we can still stub its methods.
• The **doReturn()** method is used to stub methods in the spy, as it allows us to override the behavior of specific methods.

5. Verifying Mock Interactions

When testing, you often need to verify that the mocked or spied methods were called, and with the correct parameters. This is essential to ensure that your application is interacting with its dependencies as expected.

Example of Verifying Interactions:

6. Conclusion

Using mocks and spies in Spring Boot testing is crucial for isolating the unit under test, controlling dependencies, and verifying interactions. By leveraging Mockito's @Mock and @Spy annotations, you can simulate the behavior of services, repositories, and other beans, allowing you to focus on testing the core logic of your application.

• Mocks are useful when you want to simulate the behavior of dependencies and control their responses.
• Spies are useful when you need to test a real object but still want to monitor or modify specific method calls.

These techniques allow you to create effective and isolated tests, improving the maintainability and reliability of your Spring Boot application.