Why extensions customize JUnit behavior - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

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

JUnit

@[1] MyExtension extension = new MyExtension();

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

ARegisterExtension

BExtendWith

CTestExtension

DEnableExtension

Attempts:

3 left

💡 Hint

Common Mistakes

Using @ExtendWith instead of @RegisterExtension for instance fields.

Misspelling the annotation name.

The @RegisterExtension annotation is used to register an extension instance in JUnit 5.

2fill in blank

medium

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

JUnit

public class MyExtension implements [1] { }

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AAfterTestExecutionCallback

BBeforeAllCallback

CTestExecutionExceptionHandler

DBeforeEachCallback

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing AfterTestExecutionCallback which runs after tests.

Confusing BeforeAllCallback which runs once before all tests.

The BeforeEachCallback interface allows code to run before each test method.

3fill in blank

hard

Fix the error in the extension method signature to correctly override the beforeEach method.

JUnit

@Override
public void beforeEach([1] context) throws Exception { }

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AExecutionContext

BExtensionContext

CTestContext

DTestInfo

Attempts:

3 left

💡 Hint

Common Mistakes

Using TestContext which is not part of JUnit 5 extension API.

Using TestInfo which is for test methods, not extension callbacks.

The beforeEach method receives an ExtensionContext parameter in JUnit 5 extensions.

4fill in blank

hard

Fill both blanks to implement an extension that prints a message before and after each test.

JUnit

public class MyExtension implements [1], [2] {
  @Override
  public void beforeEach(ExtensionContext context) {
    System.out.println("Starting test");
  }
  @Override
  public void afterEach(ExtensionContext context) {
    System.out.println("Finished test");
  }
}

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

ABeforeEachCallback

BAfterAllCallback

CAfterEachCallback

DBeforeAllCallback

Attempts:

3 left

💡 Hint

Common Mistakes

Using AfterAllCallback or BeforeAllCallback which run once per test class, not per test method.

The extension implements BeforeEachCallback and AfterEachCallback to run code before and after each test.

5fill in blank

hard

Fill all three blanks to create a dynamic test factory method using JUnit extension.

JUnit

@TestFactory
Stream<DynamicTest> dynamicTests() {
  return Stream.of("A", "B", "C")
    .map([1] -> DynamicTest.dynamicTest(
      "Test " + [2],
      () -> assertTrue([3].length() > 0)
    ));
}

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

Aname

Dvalue

Attempts:

3 left

💡 Hint

Common Mistakes

Using different variable names inconsistently inside the lambda.

Using 'name' which is not declared as the lambda parameter.

The lambda parameter is value, used to name and test each dynamic test.

Practice

1. What is the main reason to use extensions in JUnit tests?

easy

A. To add extra behavior without changing the test code

B. To write test cases faster

C. To replace the JUnit framework

D. To create user interfaces for tests

Solution

1. Step 1: Understand the purpose of extensions

   Extensions allow adding extra behavior like setup or cleanup without modifying test methods.

2. Step 2: Compare options with this purpose

   Only To add extra behavior without changing the test code matches this purpose; others describe unrelated goals.

3. Final Answer:

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

4. Quick Check:

   Extensions add behavior = D [OK]

Hint: Extensions add behavior without changing tests [OK]

Common Mistakes:

• Thinking extensions speed up writing tests
• Confusing extensions with replacing JUnit
• Assuming extensions create UI

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

easy

A. @TestExtension

B. @UseExtension

C. @ExtendWith

D. @EnableExtension

Solution

1. Step 1: Recall the correct annotation for extensions

   The standard JUnit annotation to apply extensions is @ExtendWith.

2. Step 2: Verify other options

   Other options are not valid JUnit annotations for extensions.

3. Final Answer:

   @ExtendWith -> Option C

4. Quick Check:

   @ExtendWith = C [OK]

Hint: Remember '@ExtendWith' applies extensions [OK]

Common Mistakes:

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

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

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

Assuming MyExtension prints "Before test" before the test and "After test" after the test.

medium

A. Test running\nBefore test\nAfter test

B. Before test\nTest running\nAfter test

C. After test\nTest running\nBefore test

D. Test running only

Solution

1. Step 1: Understand extension behavior

   MyExtension runs code before and after the test method, printing "Before test" and "After test" respectively.

2. Step 2: Order of output

   Output order is: before test message, test method message, after test message.

3. Final Answer:

   Before test\nTest running\nAfter test -> Option B

4. Quick Check:

   Extension runs before/after test = A [OK]

Hint: Extensions run code before and after tests [OK]

Common Mistakes:

• Assuming extension runs after test only
• Mixing order of printed lines
• Ignoring extension output

4. Identify the error in this JUnit test using an extension:

@ExtendWith(MyExtension.class)
class TestClass {
  @Test
  void test() {
    System.out.println("Running test");
  }

  void helper() {
    System.out.println("Helper method");
  }
}

Why might helper() not be called during test execution?

medium

A. Extension disables helper methods

B. Helper method must be private

C. Helper method must be static

D. Helper method is not annotated with @Test

Solution

1. Step 1: Check method annotations

   Only methods annotated with @Test run as tests; helper() lacks this annotation.

2. Step 2: Understand extension role

   Extensions do not disable helper methods; they only affect test lifecycle.

3. Final Answer:

   Helper method is not annotated with @Test -> Option D

4. Quick Check:

   Missing @Test means no run = B [OK]

Hint: Only @Test methods run automatically [OK]

Common Mistakes:

• Thinking extensions disable helper methods
• Believing helper must be static or private
• Assuming all methods run as tests

5. You want to create a JUnit extension that changes test data before each test runs. Which approach best uses extensions to achieve this?

hard

A. Implement BeforeEachCallback and modify test data in beforeEach()

B. Override the test method and change data inside it

C. Use @Test annotation with parameters to change data

D. Create a separate test class for each data variation

Solution

1. Step 1: Identify extension interface for pre-test actions

   The BeforeEachCallback interface allows running code before each test method.

2. Step 2: Apply this to modify test data

   Modifying test data in beforeEach() method is the correct way to customize data before tests.

3. Final Answer:

   Implement BeforeEachCallback and modify test data in beforeEach() -> Option A

4. Quick Check:

   BeforeEachCallback modifies data before tests = A [OK]

Hint: Use BeforeEachCallback to run code before tests [OK]

Common Mistakes:

• Trying to override test methods directly
• Misusing @Test parameters for data changes
• Creating many classes instead of using extensions