Chapter 148: Mockito in Java

Mockito builds test doubles: mocks record interactions, stubs answer calls with canned values. You cut dependencies on databases, HTTP, and clocks so one class’s logic fails alone when wrong.

1. Topic title

Mockito: replace collaborators with programmable fakes; assert both results and calls

2. What it means

when(repo.findById(1L)).thenReturn(Optional.of(entity)) stubs behavior.

verify(repo).save(any()) asserts side effects happened—use sparingly on true collaborators, not every private helper.

@ExtendWith(MockitoExtension.class) wires Jupiter + Mockito annotations (@Mock, @InjectMocks).

3. Why it is used

Fast tests without Docker for every save.

Edge cases (repository throws, timeout) hard to stage in integration environments.

4. Mental sketch

Mocks are stunt doubles on a film set—they wear the costume of PaymentGateway but never charge a real card.

5. Java code example

import static org.junit.jupiter.api.Assertions.assertEquals;
import static org.mockito.Mockito.verify;
import static org.mockito.Mockito.when;
 
import java.util.Optional;
 
import org.junit.jupiter.api.Test;
import org.junit.jupiter.api.extension.ExtendWith;
import org.mockito.InjectMocks;
import org.mockito.Mock;
import org.mockito.junit.jupiter.MockitoExtension;
 
@ExtendWith(MockitoExtension.class)
class ServiceWithMockTest {
    @Mock
    ItemRepository repo;
 
    @InjectMocks
    ItemServiceImpl service;
 
    @Test
    void loads() {
        when(repo.findById(1L)).thenReturn(Optional.of(new ItemEntity(1L, "pen")));
        assertEquals("pen", service.find(1L).orElseThrow().name());
        verify(repo).findById(1L);
    }
}

ItemEntity / ItemServiceImpl names assume Chapter 121/123 shapes—swap for your types.

6. Explanation of code

@InjectMocks picks constructor with most matching @Mock types—ambiguous constructors need manual wiring.

7. Common mistakes

Over-mocking value objects—prefer real immutable DTOs.

Unnecessary verify on every call—noise couples tests to implementation details.

8. Best practices

ArgumentCaptor for complex arguments when equals is hard.

Strict stubbing defaults—use lenient() only when branched code skips some stubs.

9. Small practice task

Stub repo.save to throw DataIntegrityViolationException and assert your service wraps it in a domain exception.

Beginner tip

If UnnecessaryStubbingException appears, either remove the when or mark lenient()—Mockito helps find dead setup.