when().thenThrow() for exceptions in JUnit - Test Execution Trace

Test Overview

This test uses when().thenThrow() to simulate an exception from a mocked service method. It verifies that the exception is correctly thrown and handled.

Test Code - JUnit 5 with Mockito

JUnit

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

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

class UserService {
    UserRepository repo;
    UserService(UserRepository repo) {
        this.repo = repo;
    }
    public String getUserName(int id) {
        return repo.findNameById(id);
    }
}

interface UserRepository {
    String findNameById(int id);
}

public class UserServiceTest {

    @Test
    void testGetUserNameThrowsException() {
        UserRepository mockRepo = Mockito.mock(UserRepository.class);
        when(mockRepo.findNameById(1)).thenThrow(new RuntimeException("User not found"));

        UserService service = new UserService(mockRepo);

        RuntimeException thrown = assertThrows(RuntimeException.class, () -> {
            service.getUserName(1);
        });

        assertEquals("User not found", thrown.getMessage());
    }
}

Execution Trace - 5 Steps

Step	Action	System State	Assertion	Result
1	Create mock of UserRepository	mockRepo is a mock object with no behavior yet	-	PASS
2	Stub mockRepo.findNameById(1) to throw RuntimeException("User not found")	mockRepo configured to throw exception when findNameById(1) is called	-	PASS
3	Create UserService instance with mockRepo	service uses mockRepo internally	-	PASS
4	Call service.getUserName(1) inside assertThrows	service calls mockRepo.findNameById(1), which throws RuntimeException	assertThrows verifies RuntimeException is thrown	PASS
5	Check exception message equals "User not found"	exception caught with message "User not found"	assertEquals verifies exception message	PASS

Failure Scenario

Failing Condition: Mock not configured to throw exception or method called with wrong argument

Execution Trace Quiz - 3 Questions

What does when().thenThrow() do in this test?

A It verifies that an exception was thrown.

B It makes the mock method throw an exception when called with specific arguments.

C It catches exceptions thrown by the real method.

D It prevents the method from being called.

Key Result

Use when().thenThrow() to simulate exceptions from dependencies and assertThrows to verify your code handles them properly.