Choosing the right test double in JUnit - Test Execution Trace

Test Overview

This test verifies that a service correctly uses a mock dependency to return expected data. It checks that the mock is called and the service returns the mocked value.

Test Code - JUnit 5 with Mockito

JUnit

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

import org.junit.jupiter.api.BeforeEach;
import org.junit.jupiter.api.Test;

interface DataFetcher {
    String fetchData();
}

class DataService {
    private final DataFetcher fetcher;

    DataService(DataFetcher fetcher) {
        this.fetcher = fetcher;
    }

    String getData() {
        return fetcher.fetchData();
    }
}

public class DataServiceTest {
    private DataFetcher fetcherMock;
    private DataService service;

    @BeforeEach
    void setup() {
        fetcherMock = mock(DataFetcher.class);
        service = new DataService(fetcherMock);
    }

    @Test
    void testGetDataReturnsMockedValue() {
        when(fetcherMock.fetchData()).thenReturn("mocked data");

        String result = service.getData();

        verify(fetcherMock).fetchData();
        assertEquals("mocked data", result);
    }
}

Execution Trace - 8 Steps

Step	Action	System State	Assertion	Result
1	Test starts	JUnit test runner initializes test class	-	PASS
2	Creates mock of DataFetcher interface using Mockito.mock()	fetcherMock is a mock object that can simulate DataFetcher	-	PASS
3	Creates DataService instance with fetcherMock injected	service uses fetcherMock as its dependency	-	PASS
4	Sets up mock behavior: fetcherMock.fetchData() returns 'mocked data'	fetcherMock is programmed to return 'mocked data' when fetchData() is called	-	PASS
5	Calls service.getData() which calls fetcherMock.fetchData()	fetcherMock.fetchData() is invoked	-	PASS
6	Verifies fetcherMock.fetchData() was called exactly once	Mockito verifies interaction with mock	fetcherMock.fetchData() was called once	PASS
7	Asserts that service.getData() returned 'mocked data'	Result string is 'mocked data'	assertEquals('mocked data', result)	PASS
8	Test ends successfully	All assertions passed, no exceptions thrown	-	PASS

Failure Scenario

Failing Condition: The mock was not set up to return the expected value or fetchData() was not called

Execution Trace Quiz - 3 Questions

What is the purpose of using a mock in this test?

A To replace the DataService with a fake

B To simulate the DataFetcher dependency and control its behavior

C To test the real DataFetcher implementation

D To check the database connection

Key Result

Use mocks to isolate the unit under test by simulating dependencies. Always verify that mocks are called as expected to ensure correct interaction.