Custom extensions in JUnit - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a custom extension in JUnit?

A custom extension in JUnit is a user-defined class that adds extra behavior to tests, such as setup, teardown, or conditional test execution, by implementing JUnit's Extension API.

beginner

Which JUnit interface do you implement to create a custom extension that runs code before and after each test?

You implement the BeforeEachCallback and AfterEachCallback interfaces to run code before and after each test method.

beginner

How do you register a custom extension in a JUnit 5 test class?

You register a custom extension by annotating the test class with <code>@ExtendWith(YourExtension.class)</code>.

intermediate

What is the purpose of the ExtensionContext parameter in JUnit custom extensions?

The ExtensionContext provides information about the current test, such as test method, test class, and a store for sharing data between callbacks.

intermediate

Name two common use cases for creating custom JUnit extensions.

Common use cases include managing external resources (like databases or servers) and implementing conditional test execution based on environment or configuration.

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

A @UseExtension

B @CustomExtension

C @ExtendWith

D @RegisterExtension

The correct annotation to register a custom extension is @ExtendWith. @RegisterExtension is used for field-level extensions.

Which interface should you implement to run code before all tests in a test class?

A BeforeEachCallback

B TestWatcher

C TestExecutionListener

D BeforeAllCallback

BeforeAllCallback runs code once before all tests. BeforeEachCallback runs before each test.

What does the ExtensionContext NOT provide in a custom extension?

A Ability to modify test source code

B Information about the current test method

C Access to test class instance

D A store to share data between callbacks

ExtensionContext provides test info and data sharing but cannot modify test source code.

How can you share data between different callbacks in a custom extension?

A Using the ExtensionContext's store

B Using static variables

C Using global variables

D Using environment variables

The ExtensionContext provides a Store to safely share data between callbacks.

Which of the following is NOT a typical use case for custom JUnit extensions?

A Conditional test execution

B Changing Java language syntax

C Managing external resources

D Logging test lifecycle events

Custom extensions cannot change Java syntax; they add behavior around tests.

Explain how to create and register a custom extension in JUnit 5.

Think about lifecycle interfaces and how JUnit knows to use your extension.

Describe two practical scenarios where custom JUnit extensions improve testing.

Consider what repetitive tasks or conditions you want to automate in tests.

Practice

1. What is the main purpose of using a custom extension in JUnit with @ExtendWith?

easy

A. To generate test reports in different formats

B. To write test cases faster by skipping assertions

C. To change the Java version used by the test runner

D. To add reusable code that runs before or after tests automatically

Solution

1. Step 1: Understand the role of custom extensions

   Custom extensions allow adding reusable code that hooks into the test lifecycle, such as setup or teardown actions.

2. Step 2: Recognize the use of @ExtendWith

   The annotation @ExtendWith applies these extensions to tests so the extra code runs automatically before or after tests.

3. Final Answer:

   To add reusable code that runs before or after tests automatically -> Option D

4. Quick Check:

   Custom extensions = reusable lifecycle code [OK]

Hint: Remember @ExtendWith adds reusable code around tests [OK]

Common Mistakes:

• Thinking extensions skip assertions
• Confusing extensions with Java version settings
• Assuming extensions generate reports

2. Which of the following is the correct syntax to apply a custom extension class named MyExtension to a JUnit 5 test class?

easy

A. @ExtendWith(MyExtension.class)

B. @UseExtension(MyExtension)

C. @EnableExtension(MyExtension.class)

D. @ApplyExtension(MyExtension.class)

Solution

1. Step 1: Recall the correct annotation for extensions

   JUnit 5 uses @ExtendWith to apply extensions to test classes or methods.

2. Step 2: Check the syntax for specifying the extension class

   The extension class must be passed as a class literal with .class, so @ExtendWith(MyExtension.class) is correct.

3. Final Answer:

   @ExtendWith(MyExtension.class) -> Option A

4. Quick Check:

   @ExtendWith + .class = correct syntax [OK]

Hint: Use @ExtendWith with .class for extension classes [OK]

Common Mistakes:

• Omitting .class after the extension name
• Using wrong annotation names like @UseExtension
• Passing extension name without parentheses

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

public class PrintExtension implements BeforeEachCallback {
    @Override
    public void beforeEach(ExtensionContext context) {
        System.out.println("Before test: " + context.getDisplayName());
    }
}

@ExtendWith(PrintExtension.class)
public class SampleTest {
    @Test
    void testOne() {}
}

medium

A. Before test: testOne()

B. Before test: SampleTest

C. No output printed

D. Compilation error due to missing interface

Solution

1. Step 1: Understand BeforeEachCallback behavior

   The beforeEach method runs before each test method and prints the test's display name.

2. Step 2: Identify the display name for the test method

   The display name is the test method name with parentheses, so it prints "Before test: testOne()".

3. Final Answer:

   Before test: testOne() -> Option A

4. Quick Check:

   beforeEach prints test method name [OK]

Hint: BeforeEachCallback runs before each test method [OK]

Common Mistakes:

• Thinking it prints class name instead of method
• Assuming no output is printed
• Confusing interface implementation

4. Consider this extension implementation:

public class FaultyExtension implements BeforeEachCallback {
    @Override
    public void beforeEach(ExtensionContext context) {
        int x = 1 / 0;
    }
}

@ExtendWith(FaultyExtension.class)
public class TestClass {
    @Test
    void testMethod() {}
}

What will happen when running testMethod?

medium

A. The test will be skipped silently

B. The test will fail with an ArithmeticException before running

C. The test will pass normally

D. Compilation error due to division by zero

Solution

1. Step 1: Analyze the extension code for errors

   The extension's beforeEach method divides by zero, causing an ArithmeticException at runtime.

2. Step 2: Understand effect on test execution

   Since the exception occurs before the test runs, the test fails immediately with that exception.

3. Final Answer:

   The test will fail with an ArithmeticException before running -> Option B

4. Quick Check:

   Runtime error in extension causes test failure [OK]

Hint: Runtime errors in extensions fail tests before execution [OK]

Common Mistakes:

• Thinking division by zero causes compile error
• Assuming test passes despite extension error
• Believing test is skipped silently

5. You want to create a custom extension that sets up a database connection before all tests and closes it after all tests in a class. Which two JUnit 5 extension interfaces should you implement together?

hard

A. BeforeEachCallback and AfterEachCallback

B. TestInstancePostProcessor and ParameterResolver

C. BeforeAllCallback and AfterAllCallback

D. BeforeTestExecutionCallback and AfterTestExecutionCallback

Solution

1. Step 1: Identify lifecycle timing for setup/teardown

   Setting up before all tests and closing after all tests requires hooks that run once per test class, not per test method.

2. Step 2: Match interfaces to lifecycle events

   BeforeAllCallback runs before all tests in a class, and AfterAllCallback runs after all tests in a class, perfect for this use case.

3. Final Answer:

   BeforeAllCallback and AfterAllCallback -> Option C

4. Quick Check:

   BeforeAll + AfterAll = class-level setup/teardown [OK]

Hint: Use BeforeAllCallback and AfterAllCallback for class-level setup [OK]

Common Mistakes:

• Using BeforeEachCallback for class-level setup
• Confusing TestInstancePostProcessor with lifecycle callbacks
• Choosing test execution callbacks instead of all callbacks