How do I use Mockito to mock a complex object graph

1. Creating Mock Objects**

First, you need to create mock objects for each class in the object graph. Mockito provides several ways to do this:

- Using `@Mock` Annotation: This is typically used with JUnit tests. You annotate fields with `@Mock`, and Mockito initializes them automatically when you use `MockitoAnnotations.initMocks(this)` or with JUnit 5's `@ExtendWith(MockitoExtension.class)`.

java
  @Mock
  private Database database;
  

- Using `mock()` Method: You can manually create mock objects using the `mock()` method.

java
  Database database = Mockito.mock(Database.class);
  

2. Defining Mock Behavior**

After creating mock objects, you need to define their behavior. This is done using `when()` and `thenReturn()` methods for simple cases, or `doAnswer()` for more complex scenarios.

- Simple Mocking: Use `when()` followed by `thenReturn()` to specify return values for methods.

java
  when(database.isAvailable()).thenReturn(true);
  

- Complex Mocking: Use `doAnswer()` to handle complex logic, such as returning different values based on input parameters.

java
  doAnswer(invocation -> {
      // Custom logic here
      Object[] args = invocation.getArguments();
      // Handle args as needed
      return "Mocked result";
  }).when(database).isAvailable();
  

3. Mocking Complex Object Graphs**

For complex graphs, you'll need to mock each object and define how they interact with each other.

- Example: Suppose you have a `Service` class that depends on a `Database` and a `NetworkClient`. You would mock both `Database` and `NetworkClient`, then define their behaviors.

java
  @Mock
  private Database database;
  
  @Mock
  private NetworkClient networkClient;
  
  @InjectMocks
  private Service service;
  
  // Define behaviors
  when(database.isAvailable()).thenReturn(true);
  when(networkClient.sendRequest(anyString())).thenReturn("Success");
  

4. Using Custom Answers**

Custom answers allow you to dynamically calculate return values based on method arguments. This is useful for mocking methods that should behave differently based on input.

- Example: Mocking a method that returns the first argument.

java
  when(list.add(anyString())).thenAnswer(returnsFirstArg());
  

5. Testing the Complex Object Graph**

After setting up your mocks, you can test the behavior of your complex object graph by calling methods on the object under test and verifying that the expected interactions occur.

- Example: Testing if `service.query()` behaves correctly based on mock behaviors.

java
  // Assuming service.query() uses database and networkClient
  String result = service.query("some query");
  assertEquals("Expected result", result);
  
  // Verify interactions
  verify(database).isAvailable();
  verify(networkClient).sendRequest(anyString());
  

6. Best Practices**

- Keep Mocks Simple: Avoid overly complex mock logic. If mocks become too complicated, consider using a test double or a fake implementation instead.
- Use Argument Matchers: Use `any()`, `eq()`, etc., to match method arguments flexibly.
- Verify Interactions: Use `verify()` to ensure that the expected methods were called on mocks.

By following these steps and best practices, you can effectively use Mockito to mock complex object graphs in your unit tests.