@InjectMocks annotation in JUnit - Test Execution Trace

Test Overview

This test uses the @InjectMocks annotation to create an instance of the class under test and automatically inject its mocked dependencies. It verifies that the method returns the expected result using the injected mocks.

Test Code - JUnit 5 with Mockito

JUnit

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

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)
public class CalculatorServiceTest {

    @Mock
    private Calculator calculator;

    @InjectMocks
    private CalculatorService calculatorService;

    @Test
    public void testAdd() {
        when(calculator.add(2, 3)).thenReturn(5);
        int result = calculatorService.add(2, 3);
        assertEquals(5, result);
    }
}

class Calculator {
    public int add(int a, int b) {
        return a + b;
    }
}

class CalculatorService {
    private Calculator calculator;

    public int add(int a, int b) {
        return calculator.add(a, b);
    }
}

Execution Trace - 6 Steps

Step	Action	System State	Assertion	Result
1	Test framework initializes and processes @ExtendWith(MockitoExtension.class) to enable Mockito support	MockitoExtension is active, ready to process mocks and inject mocks	-	PASS
2	Mockito creates a mock instance of Calculator due to @Mock annotation	Calculator mock object is created and ready	-	PASS
3	Mockito creates an instance of CalculatorService and injects the Calculator mock into it because of @InjectMocks	CalculatorService instance has its calculator field set to the mock Calculator	-	PASS
4	Test method 'testAdd' runs: stub calculator.add(2, 3) to return 5	Mock behavior defined: calculator.add(2, 3) returns 5	-	PASS
5	Call calculatorService.add(2, 3), which internally calls mocked calculator.add(2, 3)	calculatorService calls calculator mock, which returns 5	Verify that calculatorService.add(2, 3) returns 5	PASS
6	Assert that the returned result equals 5 using assertEquals	Assertion compares expected 5 with actual 5	assertEquals(5, result) passes	PASS

Failure Scenario

Failing Condition: If @InjectMocks fails to inject the mock Calculator into CalculatorService, the calculator field is null

Execution Trace Quiz - 3 Questions

What does the @InjectMocks annotation do in this test?

A Verifies that the mock was called

B Creates a mock object of the class under test

C Creates an instance of the class under test and injects mocked dependencies

D Stubs the method behavior of the mock

Key Result

Use @InjectMocks to automatically create the class under test and inject its mocked dependencies, simplifying setup and avoiding null references.