Extension model overview in JUnit - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the JUnit Extension Model?

The JUnit Extension Model allows you to add custom behavior to tests, such as setup, teardown, or modifying test execution, without changing the test code itself.

beginner

Name two common extension points in JUnit 5's Extension Model.

Two common extension points are BeforeEachCallback and AfterEachCallback, which run code before and after each test method respectively.

beginner

How do you register an extension in JUnit 5?

You can register an extension by annotating the test class or method with <code>@ExtendWith(YourExtension.class)</code>.

intermediate

What is the difference between BeforeAllCallback and BeforeEachCallback?

BeforeAllCallback runs once before all tests in a class, while BeforeEachCallback runs before each individual test method.

intermediate

Why is the JUnit Extension Model useful for test code maintenance?

It helps keep test code clean by separating reusable setup or teardown logic into extensions, making tests easier to read and maintain.

Which annotation is used to apply a JUnit 5 extension to a test class?

A @ExtendWith

B @TestExtension

C @UseExtension

D @EnableExtension

The correct annotation to register an extension in JUnit 5 is @ExtendWith.

What does the AfterEachCallback interface allow you to do?

A Run code before all tests

B Modify test method parameters

C Run code after each test method

D Skip tests conditionally

AfterEachCallback runs code after each test method finishes.

Which of these is NOT a valid JUnit 5 extension point?

A BeforeAllCallback

B AfterEachCallback

C BeforeTestExecutionCallback

D TestCaseListener

TestCaseListener is not a JUnit 5 extension point.

How often does BeforeAllCallback run in a test class?

A Before each test method

B Once before all test methods

C After each test method

D After all test methods

BeforeAllCallback runs once before all tests in the class.

What is a key benefit of using the JUnit Extension Model?

A Adding reusable behavior without cluttering test code

B Writing tests without assertions

C Automatically generating test data

D Running tests in parallel

Extensions let you add reusable setup or teardown logic cleanly.

Explain how the JUnit Extension Model helps improve test code organization.

Think about how extensions reduce repeated code in tests.

Describe the difference between BeforeEachCallback and BeforeAllCallback in JUnit extensions.

Consider how often each callback runs during testing.

Practice

1. What is the main purpose of the JUnit extension model?

easy

A. To add extra behavior to tests without changing the test code

B. To write test cases faster using templates

C. To automatically generate test data

D. To replace assertions with custom logic

Solution

1. Step 1: Understand the extension model concept

   The extension model allows adding extra behavior to tests without modifying the original test code.

2. Step 2: Compare options with the concept

   Only To add extra behavior to tests without changing the test code correctly describes this purpose. Other options describe unrelated features.

3. Final Answer:

   To add extra behavior to tests without changing the test code -> Option A

4. Quick Check:

   Extension model purpose = add behavior without code change [OK]

Hint: Extension model = add behavior without changing tests [OK]

Common Mistakes:

• Confusing extension model with test data generation
• Thinking it replaces assertions
• Assuming it speeds up test writing

2. Which annotation is used to apply an extension to a JUnit test class?

easy

A. @ExtendWith

B. @TestExtension

C. @UseExtension

D. @EnableExtension

Solution

1. Step 1: Recall the correct annotation for extensions

   JUnit uses @ExtendWith to apply extensions to test classes or methods.

2. Step 2: Verify other options

   Options B, C, and D are not valid JUnit annotations for extensions.

3. Final Answer:

   @ExtendWith -> Option A

4. Quick Check:

   Extension annotation = @ExtendWith [OK]

Hint: Remember extension annotation is @ExtendWith [OK]

Common Mistakes:

• Using @TestExtension which does not exist
• Confusing with @UseExtension or @EnableExtension
• Forgetting the '@' symbol

3. Given the following code snippet, what will be printed when the test runs?

@ExtendWith(MyExtension.class)
class MyTest {
  @Test
  void testMethod() {
    System.out.println("Test running");
  }
}

class MyExtension implements BeforeTestExecutionCallback {
  public void beforeTestExecution(ExtensionContext context) {
    System.out.println("Before test");
  }
}

medium

A. Test running
Before test

B. Before test
Test running

C. Test running

D. Before test

Solution

1. Step 1: Understand the extension callback

   The BeforeTestExecutionCallback runs its method before the test method executes, printing "Before test" first.

2. Step 2: Order of print statements

   First "Before test" prints, then the test method prints "Test running".

3. Final Answer:

   Before test\nTest running -> Option B

4. Quick Check:

   BeforeTestExecutionCallback runs before test method [OK]

Hint: BeforeTestExecutionCallback runs before test method [OK]

Common Mistakes:

• Assuming test method prints first
• Confusing callback timing
• Ignoring extension execution order

4. Identify the error in this extension implementation:

public class MyExtension implements BeforeTestExecutionCallback {
  public void beforeTestExecution() {
    System.out.println("Before test");
  }
}

medium

A. Method beforeTestExecution should return boolean

B. Method name should be beforeTestStart

C. Class must extend AbstractExtension instead of implementing interface

D. Missing parameter ExtensionContext in beforeTestExecution method

Solution

1. Step 1: Check method signature for interface implementation

   The interface BeforeTestExecutionCallback requires beforeTestExecution(ExtensionContext context) method with one parameter.

2. Step 2: Identify missing parameter

   The method in the code has no parameters, so it does not correctly implement the interface.

3. Final Answer:

   Missing parameter ExtensionContext in beforeTestExecution method -> Option D

4. Quick Check:

   Interface method requires ExtensionContext parameter [OK]

Hint: Check method parameters match interface exactly [OK]

Common Mistakes:

• Omitting required method parameters
• Changing method return type incorrectly
• Using wrong method names

5. You want to create a JUnit extension that logs a message before and after each test method runs. Which combination of interfaces should your extension implement?

hard

A. BeforeTestExecutionCallback and AfterTestExecutionCallback

B. BeforeAllCallback and AfterAllCallback

C. BeforeEachCallback and AfterEachCallback

D. TestExecutionExceptionHandler and BeforeEachCallback

Solution

1. Step 1: Understand callback interfaces for test methods

   BeforeEachCallback runs before each test method, and AfterEachCallback runs after each test method.

2. Step 2: Match requirement to interfaces

   To log before and after each test method, implement both BeforeEachCallback and AfterEachCallback.

3. Final Answer:

   BeforeEachCallback and AfterEachCallback -> Option C

4. Quick Check:

   Before/After each test = BeforeEachCallback + AfterEachCallback [OK]

Hint: Before/After each test = BeforeEachCallback + AfterEachCallback [OK]

Common Mistakes:

• Using BeforeAllCallback which runs once per class
• Confusing AfterTestExecutionCallback with AfterEachCallback
• Choosing exception handler interfaces incorrectly