@Spy for partial mocking in JUnit - Test Execution Trace

Test Overview

This test demonstrates how to use @Spy in JUnit with Mockito to partially mock an object. It verifies that a real method is called unless it is stubbed.

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.junit.jupiter.api.extension.ExtendWith;
import org.mockito.Spy;
import org.mockito.junit.jupiter.MockitoExtension;

@ExtendWith(MockitoExtension.class)
public class SpyPartialMockTest {

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

        public int multiply(int a, int b) {
            return a * b;
        }
    }

    @Spy
    Calculator calculatorSpy;

    @Test
    void testPartialMockingWithSpy() {
        // Stub multiply method to return fixed value
        doReturn(10).when(calculatorSpy).multiply(2, 3);

        // Call add method - real method should be called
        int sum = calculatorSpy.add(2, 3);

        // Call multiply method - stubbed method should be called
        int product = calculatorSpy.multiply(2, 3);

        assertEquals(5, sum, "add method should call real implementation");
        assertEquals(10, product, "multiply method should return stubbed value");
    }
}

Execution Trace - 7 Steps

Step	Action	System State	Assertion	Result
1	Test starts	JUnit test runner initialized with MockitoExtension	-	PASS
2	Mockito creates a spy instance of Calculator	Calculator spy object ready with real methods available	-	PASS
3	Stub multiply(2, 3) method to return 10	multiply method on spy is stubbed for inputs (2,3)	-	PASS
4	Call add(2, 3) on spy	Real add method executes and returns 5	Verify add returns 5	PASS
5	Call multiply(2, 3) on spy	Stubbed multiply method returns 10	Verify multiply returns 10	PASS
6	Assertions check add == 5 and multiply == 10	Assertions pass successfully	assertEquals(5, sum) and assertEquals(10, product)	PASS
7	Test ends	Test completed with all assertions passing	-	PASS

Failure Scenario

Failing Condition: If the spy is not created properly or multiply method is not stubbed correctly

Execution Trace Quiz - 3 Questions

What does the @Spy annotation do in this test?

A Creates a full mock with no real method calls

B Creates a partial mock that calls real methods unless stubbed

C Injects dependencies automatically

D Runs the test in a separate thread

Key Result

Using @Spy allows you to test real method behavior while selectively overriding specific methods, which helps in testing complex objects without fully mocking them.