Is it possible to assert that a real method is called via spying in Go like Java Mockito?

ONLINECAST

2 min read

·

06-10-2024

5

0

Share

Mocking Methods in Go: Can We Spy Like Mockito?

Mocking is a crucial technique in testing, allowing us to isolate and verify the behavior of specific parts of our code. While Java developers have the powerful Mockito framework for mocking and spying, Go's approach is a bit different. This article explores whether we can achieve a similar level of control over real method calls in Go, particularly when it comes to "spying".

Understanding the Challenge

In Java Mockito, we can create a spy object that wraps a real instance of a class. This allows us to intercept method calls, verify if they were called, and even modify their return values. However, Go's philosophy leans towards interface-based programming and doesn't inherently support the same level of dynamic interception and modification as Java.

A Code Example

Let's illustrate this with a simple example. Imagine we have a UserRepository interface:

type UserRepository interface {
	GetUser(id int) (*User, error)
}

And a service using this interface:

type UserService struct {
	repo UserRepository
}

func (s *UserService) GetUserDetails(id int) (*User, error) {
	user, err := s.repo.GetUser(id)
	if err != nil {
		return nil, err
	}
	// ... further processing ...
	return user, nil
}

In Java Mockito, we could spy on the repo instance, verifying that GetUser is called when GetUserDetails is invoked. In Go, we can use mockgen to generate mocks, but we cannot directly spy on a real UserRepository instance.

Workarounds and Alternatives

While Go doesn't offer a direct equivalent to Mockito's spying capabilities, we can achieve similar functionality through a combination of techniques:

1. Interface-Based Testing: We can create a test implementation of the UserRepository interface that records the calls to its methods. This allows us to assert if the expected methods were called.

   type MockUserRepository struct {
     GetUserCalled bool
     UserId int
   }
   
   func (m *MockUserRepository) GetUser(id int) (*User, error) {
     m.GetUserCalled = true
     m.UserId = id
     return &User{Id: id}, nil
   }
   

2. Dependency Injection: Injecting this mock implementation into our UserService instance enables us to verify the calls made through the interface.

   func TestGetUserDetails(t *testing.T) {
     mockRepo := &MockUserRepository{}
     service := &UserService{repo: mockRepo}
     _, err := service.GetUserDetails(123)
     assert.NoError(t, err)
     assert.True(t, mockRepo.GetUserCalled)
     assert.Equal(t, 123, mockRepo.UserId)
   }
   

3. Testable Interfaces: Designing our code with testable interfaces is essential. This ensures we can easily mock or stub out dependencies for testing purposes.

Conclusion

While Go doesn't offer a direct "spying" mechanism like Java Mockito, we can still achieve similar results by leveraging techniques like interface-based testing, dependency injection, and well-designed interfaces. This allows us to effectively test our code and verify the behavior of real methods without relying on external libraries.

Additional Resources

• Mockgen Documentation: Generate mocks for Go interfaces.
• Go Testing: Official Go documentation for testing.
• Interface-Based Programming in Go: Understanding interfaces in Go.

By embracing these best practices, we can write robust and reliable Go code while maintaining a strong focus on testability.