Why @Spy for partial mocking in JUnit? - Purpose & Use Cases

The Big Idea

Discover how to speed up tests without losing real behavior accuracy!

The Scenario

Imagine testing a complex calculator app where you want to check only the addition method but the subtraction method calls a slow external service.

The Problem

Manually running tests means you either wait for the slow service every time or skip testing parts, risking bugs. It's slow, frustrating, and error-prone.

The Solution

@Spy lets you keep the real behavior for most methods but replace just the slow ones with fake responses. This way, tests run fast and focus only on what matters.

Before vs After

✗ Before

Calculator calc = new Calculator();
int result = calc.add(2, 3); // calls real add
int slow = calc.subtract(5, 2); // calls slow external service

✓ After

@Spy Calculator calc = new Calculator();
doReturn(3).when(calc).subtract(5, 2); // fake subtract
int result = calc.add(2, 3); // real add used

What It Enables

It enables fast, focused tests by mixing real and fake behaviors seamlessly.

Real Life Example

Testing a payment system where you want to verify fee calculation but avoid calling the real bank API every time.

Key Takeaways

@Spy helps test parts of code while faking others.

It saves time by avoiding slow or unreliable calls.

It keeps tests accurate and easier to maintain.

Practice

1. What does the @Spy annotation do in JUnit testing?

easy

A. Automatically runs all test methods in a class.

B. Creates a completely fake object with no real method calls.

C. Creates a real object but allows overriding some methods for partial mocking.

D. Mocks static methods of a class.

Solution

1. Step 1: Understand @Spy purpose

   @Spy creates a real instance of the class but allows selective method overriding.

2. Step 2: Compare with other mocking types

   Unlike full mocks, spies call real methods unless stubbed.

3. Final Answer:

   Creates a real object but allows overriding some methods for partial mocking. -> Option C

4. Quick Check:

   @Spy = Partial real object with override [OK]

Hint: Remember: @Spy means real object with some fake methods [OK]

Common Mistakes:

• Confusing @Spy with @Mock which creates full fake objects
• Thinking @Spy mocks static methods
• Assuming @Spy runs tests automatically

2. Which of the following is the correct way to initialize a spy object in a JUnit 5 test class using Mockito?

easy

A. @Spy MyClass mySpy; MockitoAnnotations.openMocks(this);

B. @Spy MyClass mySpy = new MyClass();

C. @Spy MyClass mySpy = Mockito.mock(MyClass.class);

D. @Spy MyClass mySpy = Mockito.spy(null);

Solution

1. Step 1: Recognize proper spy initialization

   Using @Spy requires calling MockitoAnnotations.openMocks(this) to initialize the spy fields.

2. Step 2: Check other options

   @Spy MyClass mySpy = new MyClass(); misses initialization call; C uses mock instead of spy; D tries spying on null which is invalid.

3. Final Answer:

   @Spy MyClass mySpy; MockitoAnnotations.openMocks(this); -> Option A

4. Quick Check:

   Initialize spies with openMocks(this) [OK]

Hint: Always call MockitoAnnotations.openMocks(this) to init @Spy [OK]

Common Mistakes:

• Not calling MockitoAnnotations.openMocks(this)
• Confusing spy creation with mock creation
• Trying to spy on null or uninitialized objects

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

@Spy
List<String> spyList = new ArrayList<>();

@Test
void testSpyList() {
    spyList.add("one");
    spyList.add("two");
    Mockito.doReturn(100).when(spyList).size();
    System.out.println(spyList.size());
}

medium

A. 2

B. 100

C. 0

D. Throws NullPointerException

Solution

1. Step 1: Understand spy behavior with doReturn

   The spy calls real methods unless stubbed. Here, size() is stubbed to return 100.

2. Step 2: Analyze output of spyList.size()

   Despite adding two elements, size() returns 100 due to stubbing.

3. Final Answer:

   100 -> Option B

4. Quick Check:

   Stubbed spy method returns stub value [OK]

Hint: Stub spy methods with doReturn to override real calls [OK]

Common Mistakes:

• Expecting real method size() to return actual list size
• Using when(spyList.size()) instead of doReturn for spies
• Forgetting spy initialization causing NullPointerException

4. Identify the error in the following test code using @Spy:

@Spy
MyService service;

@Test
void testService() {
    service = new MyService();
    Mockito.doReturn("mocked").when(service).getData();
    assertEquals("mocked", service.getData());
}

medium

A. The spy object is not initialized with MockitoAnnotations.openMocks(this).

B. You cannot assign a new instance to a @Spy annotated field.

C. doReturn cannot be used with spy objects.

D. The assertEquals method is used incorrectly.

Solution

1. Step 1: Check spy initialization

   The @Spy field must be initialized by calling MockitoAnnotations.openMocks(this) before use.

2. Step 2: Analyze assignment and usage

   Assigning new instance manually is allowed but without initialization, spy won't work properly.

3. Final Answer:

   The spy object is not initialized with MockitoAnnotations.openMocks(this). -> Option A

4. Quick Check:

   Always init @Spy with openMocks(this) [OK]

Hint: Always initialize @Spy fields before test runs [OK]

Common Mistakes:

• Forgetting to call MockitoAnnotations.openMocks(this)
• Thinking manual assignment replaces initialization
• Believing doReturn can't be used with spies

5. You want to partially mock a complex service class PaymentProcessor to test only the processPayment() method while keeping other methods real. Which approach using @Spy is best to achieve this?

hard

A. Create a new PaymentProcessor instance without @Spy and call processPayment() directly.

B. Mock the entire PaymentProcessor class with @Mock and implement processPayment() manually.

C. Use @Spy but override all methods except processPayment() to avoid side effects.

D. Create a spy of PaymentProcessor and stub only processPayment() method with Mockito.doReturn().

Solution

1. Step 1: Understand partial mocking goal

   Partial mocking means real methods run except those stubbed.

2. Step 2: Choose spy with selective stubbing

   Using @Spy and stubbing only processPayment() lets you test that method while others behave normally.

3. Step 3: Evaluate other options

   Mocking entire class loses real behavior; overriding all except one is complex; no spy means no partial mocking.

4. Final Answer:

   Create a spy of PaymentProcessor and stub only processPayment() method with Mockito.doReturn(). -> Option D

5. Quick Check:

   @Spy + stub target method = partial mocking [OK]

Hint: Spy real object, stub only target method for partial mocking [OK]

Common Mistakes:

• Mocking entire class losing real method calls
• Overriding too many methods manually
• Not using @Spy for partial mocking