Why Spy objects in JUnit? - Purpose & Use Cases

The Big Idea

What if you could watch your code's every move during tests without changing a thing?

The Scenario

Imagine testing a calculator app by manually writing down every button press and checking the result yourself each time.

You want to see if the calculator remembers previous inputs correctly, but you have no easy way to watch what happens inside the app.

The Problem

Manually tracking internal method calls or data changes is slow and error-prone.

You might miss important details or make mistakes when guessing what happened inside the code.

This makes tests unreliable and debugging frustrating.

The Solution

Spy objects let you watch real objects during tests.

You can see which methods were called, with what data, without changing the original behavior.

This helps you verify internal actions easily and confidently.

Before vs After

✗ Before

Calculator calc = new Calculator();
calc.add(5);
// Manually check if add was called and result is correct

✓ After

Calculator calc = spy(new Calculator());
calc.add(5);
verify(calc).add(5);

What It Enables

Spy objects enable precise checking of internal method calls while keeping real behavior intact.

Real Life Example

Testing a shopping cart where you want to confirm that adding an item calls the inventory check method exactly once.

Key Takeaways

Manual tracking of internal calls is slow and unreliable.

Spy objects watch real objects without changing them.

This makes tests clearer, faster, and more trustworthy.

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