Extension model overview in JUnit - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to register a JUnit 5 extension using the annotation.

JUnit

@ExtendWith([1].class)
public class MyTest {
    @Test
    void testExample() {
        // test code
    }
}

Drag options to blanks, or click blank then click option'

AJUnitExtension

BTestExtension

CExtension

DMyExtension

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting to add .class after the extension name

Using the wrong class name in @ExtendWith

The @ExtendWith annotation requires the class name of the extension to register it. Here, 'MyExtension' is the correct extension class.

2fill in blank

medium

Complete the code to implement the JUnit 5 extension interface for before each test callback.

JUnit

public class MyExtension implements [1] {
    @Override
    public void beforeEach(ExtensionContext context) throws Exception {
        // setup code
    }
}

Drag options to blanks, or click blank then click option'

ABeforeEachCallback

BTestExecutionExceptionHandler

CBeforeAllCallback

DAfterEachCallback

Attempts:

3 left

💡 Hint

Common Mistakes

Using BeforeAllCallback which runs once before all tests

Confusing AfterEachCallback with BeforeEachCallback

To run code before each test, implement the BeforeEachCallback interface.

3fill in blank

hard

Fix the error in the extension registration code to properly enable the extension.

JUnit

@ExtendWith([1])
public class SampleTest {
    @Test
    void testSomething() {}
}

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AMyExtension.class

BMyExtension

CMyExtension()

Dnew MyExtension()

Attempts:

3 left

💡 Hint

Common Mistakes

Omitting .class after the extension class name

Trying to instantiate the extension inside @ExtendWith

The @ExtendWith annotation requires the class literal with .class, not an instance or just the class name.

4fill in blank

hard

Fill both blanks to create a custom extension that implements both before and after each test callbacks.

JUnit

public class CustomExtension implements [1], [2] {
    @Override
    public void beforeEach(ExtensionContext context) throws Exception {
        // before test
    }

    @Override
    public void afterEach(ExtensionContext context) throws Exception {
        // after test
    }
}

Drag options to blanks, or click blank then click option'

ABeforeEachCallback

BAfterAllCallback

CAfterEachCallback

DBeforeAllCallback

Attempts:

3 left

💡 Hint

Common Mistakes

Using BeforeAllCallback or AfterAllCallback instead of per-test callbacks

Implementing only one of the two interfaces

To run code before and after each test, implement BeforeEachCallback and AfterEachCallback interfaces.

5fill in blank

hard

Fill all three blanks to create a parameter resolver extension that supports injecting a String parameter with value "Hello".

JUnit

public class StringParameterResolver implements [1] {

    @Override
    public boolean supportsParameter(ParameterContext parameterContext, ExtensionContext extensionContext) throws ParameterResolutionException {
        return parameterContext.getParameter().getType() == [2];
    }

    @Override
    public Object resolveParameter(ParameterContext parameterContext, ExtensionContext extensionContext) throws ParameterResolutionException {
        return [3];
    }
}

Drag options to blanks, or click blank then click option'

AParameterResolver

BString.class

C"Hello"

DObject.class

Attempts:

3 left

💡 Hint

Common Mistakes

Returning wrong type in supportsParameter check

Not implementing ParameterResolver interface

Returning wrong value in resolveParameter

To create a parameter resolver, implement ParameterResolver. Check if parameter type is String.class and return "Hello" as the resolved value.

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