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Module 2: Mockito (The Stunt Double)

📚 Module 2: Mockito (The Stunt Double)

Focus: Moving from “Real dependencies” to “Fake partners.”

What if you want to test a Service, but that service needs to talk to a Database that hasn’t been built yet? Or an Email Server that costs money every time you send a test email? We use Mockito to create fakes.

🏗️ Step 1: The Problem (The “Expensive” Dependency)

Imagine you are filming a movie.

  • You have a scene where the main actor jumps off a 50-story building.
  • You don’t want to use the real actor (The real Database) because if it fails, the actor is destroyed!

🏗️ Step 2: Mocks (The “Stunt Double”)

Mockito allows you to create a Stunt Double (a Mock) for your objects.

🧩 The Analogy: The Dummy

  • Instead of using a real bank account with real money, you use a Plastic Credit Card.
  • You tell the plastic card: “When someone asks for your balance, say ‘$1,000,000’.”
  • The card doesn’t have a real brain; it just follows your instructions.

In Code:

@Test
public void testProductService() {
    // 1. Create a "Stunt Double" for the database
    ProductRepository mockRepo = mock(ProductRepository.class);
    
    // 2. Tell the double how to behave
    when(mockRepo.findById(1L)).thenReturn(Optional.of(new Product("Mock Laptop")));
    
    // 3. Test your service using the double
    ProductService service = new ProductService(mockRepo);
    String name = service.getProductName(1L);
    
    assertEquals("Mock Laptop", name);
}

🏗️ Step 3: Why use Mockito?

  1. Isolation: You are testing YOUR code, not the database’s code.
  2. Reliability: Mocks never fail because of a slow internet connection.
  3. Speed: Mocks live in memory and are 1,000,000x faster than a real database.

🥅 Module 2 Review

  1. Mock: A fake object that simulates the behavior of a real one.
  2. Stubbing: Telling the mock how to answer (when(...).thenReturn(...)).
  3. Verification: Checking if the mock was actually called (verify(...)).
  4. Mockito: The most popular mocking library in the Java world.