Spy objects in JUnit - Framework Patterns

Framework Mode - Spy objects

Folder Structure for JUnit Test Framework with Spy Objects

src/
├── main/
│   └── java/
│       └── com/example/app/
│           └── service/
│               └── UserService.java
└── test/
    └── java/
        └── com/example/app/
            ├── service/
            │   └── UserServiceTest.java  <-- Tests using spy objects here
            └── utils/
                └── TestUtils.java

Test Framework Layers

• Test Classes: Contain JUnit test methods. Use spy objects to monitor real objects' behavior.
• Spy Objects: Partial mocks that wrap real objects to track method calls and arguments.
• Service Layer: Application logic classes under test (e.g., UserService).
• Utilities: Helper methods for test setup, common assertions, or spy creation.
• Configuration: Settings for test environment, dependencies, and mocking framework setup.

Configuration Patterns

• Dependency Injection: Inject real objects into spies to wrap them easily.
• Mockito Setup: Use @ExtendWith(MockitoExtension.class) and @Spy annotations for spy creation.
• Environment Profiles: Use Maven or Gradle profiles to run tests in different environments if needed.
• Test Properties: Store test-specific configs (e.g., test database URLs) in src/test/resources.

Test Reporting and CI/CD Integration

• JUnit Reports: Use built-in JUnit XML reports for test results.
• Mockito Verification: Spy objects allow verification of method calls, which appear as assertion results.
• CI/CD Integration: Integrate with Jenkins, GitHub Actions, or GitLab CI to run tests automatically on code push.
• Fail Fast: Tests fail immediately if spy verification fails, providing clear feedback.

Best Practices for Using Spy Objects in JUnit

1. Use Spies Only When Needed: Prefer mocks or real objects unless you need to track real method calls.
2. Keep Tests Clear: Verify only relevant interactions to avoid brittle tests.
3. Use Annotations: Use @Spy and @InjectMocks for clean setup.
4. Reset Spies: Reset spy state between tests to avoid cross-test interference.
5. Combine with Real Logic: Spies allow partial mocking, so real methods run unless stubbed.

Self Check Question

Where in this folder structure would you add a new spy object for a PaymentService class to monitor its method calls during tests?

Key Result

Use spy objects in JUnit tests to monitor real object behavior while running actual methods.

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