Spy objects in JUnit

Introduction

Spy objects help you watch how real objects behave during tests. They let you check what methods were called and with what data, without changing the real object's behavior.

You want to verify that a method was called on an object during a test.

You need to check the arguments passed to a method without mocking the whole object.

You want to keep the original behavior of an object but still track its interactions.

You are testing a class that depends on another class and want to observe how it uses that class.

You want to debug or understand the flow of method calls in your code during testing.

Syntax

JUnit

MyClass realObject = new MyClass();
MyClass spyObject = Mockito.spy(realObject);

// Use spyObject in your test
Mockito.verify(spyObject).methodName(expectedArgument);

Spy wraps a real object, so the original methods run unless stubbed.

Use Mockito.verify() to check method calls on the spy.

Examples

This example spies on a real list. It adds an item and verifies the add method was called with "apple".

JUnit

List<String> list = new ArrayList<>();
List<String> spyList = Mockito.spy(list);

spyList.add("apple");
Mockito.verify(spyList).add("apple");

Here, the spy is used to override the getData() method to return a mocked result, while still tracking calls.

JUnit

MyService service = new MyService();
MyService spyService = Mockito.spy(service);

Mockito.doReturn("mocked result").when(spyService).getData();

String result = spyService.getData();
Mockito.verify(spyService).getData();

Sample Program

This test creates a spy of a real list. It adds two fruits, then verifies the add method was called with each fruit. It also checks the list size is 2, showing the real methods ran.

JUnit

import static org.mockito.Mockito.*;
import org.junit.jupiter.api.Test;
import java.util.ArrayList;
import java.util.List;

public class SpyTest {

    @Test
    public void testSpyObject() {
        List<String> realList = new ArrayList<>();
        List<String> spyList = spy(realList);

        spyList.add("banana");
        spyList.add("orange");

        verify(spyList).add("banana");
        verify(spyList).add("orange");

        assert(spyList.size() == 2);
    }
}

Important Notes

Spies call real methods by default, so be careful with side effects.

You can stub spy methods to change behavior using doReturn() or when().

Use spies when you want to partially mock an object but keep most behavior real.

Summary

Spy objects let you watch real objects during tests without changing their behavior by default.

They help verify method calls and arguments while keeping original logic intact.

Use spies to combine real behavior with verification in your unit tests.

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