verify() for interaction verification in JUnit - Test Execution Trace

Test Overview

This test checks that a method on a mock object is called exactly once during the test. It verifies interaction with a dependency using verify() from Mockito.

Test Code - JUnit with Mockito

JUnit

import static org.mockito.Mockito.*;
import org.junit.jupiter.api.Test;

class UserService {
    private final UserRepository repo;
    UserService(UserRepository repo) { this.repo = repo; }
    void addUser(String name) { repo.save(name); }
}

interface UserRepository {
    void save(String name);
}

public class UserServiceTest {
    @Test
    void testAddUserCallsSave() {
        UserRepository mockRepo = mock(UserRepository.class);
        UserService service = new UserService(mockRepo);

        service.addUser("Alice");

        verify(mockRepo, times(1)).save("Alice");
    }
}

Execution Trace - 6 Steps

Step	Action	System State	Assertion	Result
1	Test starts	JUnit test runner initializes	-	PASS
2	Creates mock UserRepository object	mockRepo is a mock instance with no real behavior	-	PASS
3	Creates UserService with mockRepo	UserService instance ready with mock dependency	-	PASS
4	Calls addUser("Alice") on UserService	Inside addUser, calls mockRepo.save("Alice")	-	PASS
5	Verifies mockRepo.save("Alice") was called once	Mockito checks recorded calls on mockRepo	verify(mockRepo, times(1)).save("Alice") confirms one call	PASS
6	Test ends successfully	Test passed with no errors	-	PASS

Failure Scenario

Failing Condition: mockRepo.save("Alice") is not called or called more than once

Execution Trace Quiz - 3 Questions

What does the verify() method check in this test?

A That addUser returns a value

B That UserService constructor runs without error

C That mockRepo.save("Alice") was called exactly once

D That mockRepo is not null

Key Result

Use verify() to check that your code interacts correctly with dependencies, ensuring expected methods are called the right number of times.