Spy objects in JUnit - Test Execution Trace

Test Overview

This test uses a spy object to verify that a method is called with the correct argument and that the method returns the expected value.

Test Code - JUnit with Mockito

JUnit

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

import org.junit.jupiter.api.Test;
import java.util.ArrayList;
import java.util.List;

public class SpyTest {

    @Test
    public void testSpyOnList() {
        List<String> list = spy(new ArrayList<>());

        doReturn("hello").when(list).get(0);

        String result = list.get(0);

        verify(list).get(0);
        assertEquals("hello", result);
    }
}

Execution Trace - 7 Steps

Step	Action	System State	Assertion	Result
1	Test starts	JUnit test runner initialized	—	PASS
2	Create spy object of List interface	Spy object wrapping List.class created	—	PASS
3	Stub get(0) method to return "hello"	Spy configured to return "hello" when get(0) is called	—	PASS
4	Call get(0) on spy object	Method get(0) invoked on spy	Return value is "hello"	PASS
5	Verify get(0) was called on spy	Mockito verifies method invocation	get(0) was called exactly once	PASS
6	Assert returned value equals "hello"	JUnit assertion checks equality	result equals "hello"	PASS
7	Test ends	Test completes successfully	—	PASS

Failure Scenario

Failing Condition: The spy's get(0) method is not stubbed or returns a different value

Execution Trace Quiz - 3 Questions

What does the spy object do in this test?

A It wraps the real List and allows method call verification

B It creates a completely fake List with no real behavior

C It automatically returns null for all methods

D It prevents any method calls on the List

Key Result

Using spy objects lets you check if methods are called and control their return values, which helps test interactions without losing real behavior.

Practice

1. What is the main purpose of using a spy object in JUnit testing?

easy

A. To generate random data for testing

B. To completely replace the real object with a fake one

C. To observe real object behavior and verify method calls

D. To speed up test execution by skipping method calls

Solution

1. Step 1: Understand spy object behavior

   A spy wraps a real object, allowing it to behave normally while tracking interactions.

2. Step 2: Identify spy's main use

   Spies let you verify if certain methods were called without changing the real behavior.

3. Final Answer:

   To observe real object behavior and verify method calls -> Option C

4. Quick Check:

   Spy = observe real + verify calls [OK]

Hint: Spy watches real object calls without changing behavior [OK]

Common Mistakes:

• Confusing spy with mock that replaces behavior
• Thinking spy speeds up tests by skipping calls
• Believing spy generates test data

2. Which of the following is the correct way to create a spy object using Mockito in JUnit?

easy

A. MyClass spy = Mockito.spy(new MyClass());

B. MyClass spy = Mockito.mock(MyClass.class);

C. MyClass spy = new MyClass();

D. MyClass spy = Mockito.verify(new MyClass());

Solution

1. Step 1: Recall Mockito spy syntax

   Mockito creates a spy by wrapping a real instance: Mockito.spy(realObject).

2. Step 2: Match options to syntax

   MyClass spy = Mockito.spy(new MyClass()); correctly uses Mockito.spy(new MyClass()) to create a spy.

3. Final Answer:

   MyClass spy = Mockito.spy(new MyClass()); -> Option A

4. Quick Check:

   Spy creation = Mockito.spy(real instance) [OK]

Hint: Spy wraps real instance with Mockito.spy() [OK]

Common Mistakes:

• Using Mockito.mock() instead of spy()
• Trying to spy without a real instance
• Confusing verify() with spy()

3. Given the code below, what will be the output of the test?

List<String> list = new ArrayList<>();
List<String> spyList = Mockito.spy(list);
spyList.add("apple");
spyList.add("banana");
Mockito.verify(spyList).add("apple");
System.out.println(spyList.size());

medium

A. 0

B. Throws an exception

C. 1

D. 2

Solution

1. Step 1: Understand spy behavior on add()

   Spy calls real methods, so add("apple") and add("banana") add elements to the list.

2. Step 2: Verify method call and size

   Verification confirms add("apple") was called. The list now contains two elements, so size() returns 2.

3. Final Answer:

   2 -> Option D

4. Quick Check:

   Spy calls real methods, size = 2 [OK]

Hint: Spy calls real methods; size reflects added elements [OK]

Common Mistakes:

• Assuming spy does not call real methods
• Expecting size to be 0 after add calls
• Confusing verify with changing behavior

4. Identify the error in the following test code using a spy:

List<String> list = new ArrayList<>();
List<String> spyList = Mockito.spy(list);
Mockito.when(spyList.size()).thenReturn(5);
spyList.add("orange");
System.out.println(spyList.size());

medium

A. Spy must be created with Mockito.mock()

B. Cannot stub spy methods with Mockito.when() directly

C. List cannot be spied because it's an interface

D. No error; output will be 1

Solution

1. Step 1: Recognize stubbing on spy

   Stubbing spy methods with Mockito.when() can cause real method calls and errors.

2. Step 2: Correct stubbing approach

   Use doReturn(5).when(spyList).size() to stub spy methods safely.

3. Final Answer:

   Cannot stub spy methods with Mockito.when() directly -> Option B

4. Quick Check:

   Spy stubbing = use doReturn() not when() [OK]

Hint: Stub spy methods with doReturn(), not when() [OK]

Common Mistakes:

• Using Mockito.when() on spy causing real calls
• Confusing spy creation with mock creation
• Thinking interfaces can't be spied

5. You want to test a service method that calls a repository's save() method. You want to keep the real repository behavior but verify save() was called exactly once. Which is the best approach using spy objects?

hard

A. Create a spy of the repository, call the service method, then verify save() was called once

B. Mock the repository and stub save() to do nothing, then verify call count

C. Use a real repository without spy or mock and check database manually

D. Create a mock repository and verify save() was never called

Solution

1. Step 1: Understand spy use case

   Spy lets real methods run while tracking calls, perfect for verifying save() without changing behavior.

2. Step 2: Apply spy to repository and verify

   Create spy of repository, call service method, then verify save() was called once.

3. Final Answer:

   Create a spy of the repository, call the service method, then verify save() was called once -> Option A

4. Quick Check:

   Spy = real behavior + verify calls [OK]

Hint: Spy real repo to verify calls without changing behavior [OK]

Common Mistakes:

• Mocking instead of spying loses real behavior
• Checking database manually is slow and unreliable
• Verifying save was never called contradicts test goal