Spy objects in JUnit - Build an Automation Script

Verify spy object records method calls correctly

Preconditions (2)

Step 1: Create a spy object of the Calculator class

Step 2: Call the add method on the spy object with parameters 5 and 3

Step 3: Verify that the add method was called exactly once with parameters 5 and 3

Step 4: Assert that the returned result is 8

✅ Expected Result: The spy object records the method call correctly, the verification passes, and the assertion confirms the result is 8

Automation Requirements - JUnit 5 with Mockito

Assertions Needed:

Verify method add(int, int) was called once with arguments 5 and 3

Assert the returned value equals 8

Best Practices:

Use Mockito.spy() to create spy objects

Use verify() to check method calls

Use assertEquals() from JUnit for assertions

Avoid spying on null or uninitialized objects

Automated Solution

JUnit

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

import org.junit.jupiter.api.Test;

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

public class CalculatorSpyTest {

    @Test
    void testAddMethodCalledOnceWithCorrectParameters() {
        // Create a real Calculator instance
        Calculator realCalculator = new Calculator();
        // Create a spy of the real Calculator
        Calculator spyCalculator = spy(realCalculator);

        // Call add method on spy
        int result = spyCalculator.add(5, 3);

        // Verify add was called once with 5 and 3
        verify(spyCalculator).add(5, 3);

        // Assert the result is 8
        assertEquals(8, result, "The add method should return 8");
    }
}

This test creates a spy object of the Calculator class using Mockito.spy(). The spy wraps a real Calculator instance, so the actual add method runs.

We call add(5, 3) on the spy, then use verify() to check that the method was called exactly once with those parameters.

Finally, we assert the returned result is 8 using JUnit's assertEquals(). This confirms both the spy recorded the call and the method behaves correctly.

This approach helps test interactions with real objects while still verifying method calls.

Common Mistakes - 3 Pitfalls

Using mock() instead of spy() when you want to call real methods

Verifying method calls on the real object instead of the spy

Not initializing the real object before spying

Bonus Challenge

Now add data-driven testing with 3 different input pairs for the add method using JUnit 5 parameterized tests and spy objects.

Show Hint

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