Custom extensions in JUnit - Framework Patterns

Framework Mode - Custom extensions

Folder Structure

src/
├── main/
│   └── java/
│       └── com/example/extensions/   # Custom JUnit extensions
│           ├── RetryExtension.java
│           └── TimingExtension.java
└── test/
    └── java/
        └── com/example/tests/         # Test classes using extensions
            └── SampleTest.java

Test Framework Layers

• Custom Extensions Layer: Contains reusable JUnit extension classes implementing interfaces like BeforeEachCallback, AfterEachCallback, or TestExecutionExceptionHandler.
• Test Layer: Test classes that use @ExtendWith annotation to apply custom extensions.
• Utilities Layer: Helper classes or methods used by extensions or tests (e.g., logging, retry logic).
• Configuration Layer: Properties or environment settings that can influence extension behavior (e.g., retry count).

Configuration Patterns

• Use java.util.Properties or System.getProperty to configure extension parameters like retry count or timeout.
• Allow environment variables or Maven/Gradle profiles to override default extension settings.
• Keep configuration centralized in a config.properties file or dedicated config class.
• Pass configuration values to extensions via constructor injection or static access.

Test Reporting and CI/CD Integration

• JUnit extensions can log additional info (e.g., retry attempts, test duration) to console or files.
• Integrate with CI tools (Jenkins, GitHub Actions) to capture test results including extension effects.
• Use JUnit XML reports to show retries or failures clearly.
• Extensions can add tags or metadata to tests for filtering in reports.

Best Practices for Custom Extensions

• Keep extensions focused on a single responsibility (e.g., retry, timing, logging).
• Use JUnit 5 extension interfaces properly (e.g., BeforeEachCallback, AfterTestExecutionCallback).
• Make extensions configurable and reusable across multiple test classes.
• Handle exceptions gracefully inside extensions to avoid masking test failures.
• Document extension usage clearly for test developers.

Self Check

Where in this folder structure would you add a new extension that logs test start and end times?

Key Result

Organize custom JUnit extensions in a dedicated package and apply them via @ExtendWith for reusable, configurable test behavior.

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