Extension model overview in JUnit

Introduction

The extension model in JUnit helps you add extra features to your tests easily. It lets you run code before or after tests, change how tests behave, and share setup steps.

You want to run some code before or after each test automatically.

You need to change how tests are run without changing the test code itself.

You want to add custom behavior like retrying failed tests or logging extra info.

You want to share common setup or cleanup steps across many tests.

You want to control test execution order or conditions dynamically.

Syntax

JUnit

@ExtendWith(YourExtension.class)
public class YourTestClass {
    // test methods
}

Use @ExtendWith annotation to register an extension on a test class or method.

Extensions implement interfaces like BeforeEachCallback, AfterEachCallback, etc.

Examples

This example uses a custom extension to measure test execution time.

JUnit

@ExtendWith(TimingExtension.class)
public class MyTests {
    @Test
    void testExample() {
        // test code
    }
}

This shows a retry extension that can rerun a test if it fails.

JUnit

public class RetryExtension implements TestExecutionExceptionHandler {
    @Override
    public void handleTestExecutionException(ExtensionContext context, Throwable throwable) throws Throwable {
        // retry logic here
    }
}

@ExtendWith(RetryExtension.class)
public class RetryTests {
    @Test
    void flakyTest() {
        // test that might fail sometimes
    }
}

Sample Program

This test class uses a simple extension that prints messages before and after each test runs.

JUnit

import org.junit.jupiter.api.extension.*;
import org.junit.jupiter.api.*;

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

    @Override
    public void afterEach(ExtensionContext context) {
        System.out.println("Finished test: " + context.getDisplayName());
    }
}

@ExtendWith(SimpleExtension.class)
public class ExtensionModelTest {
    @Test
    void testOne() {
        Assertions.assertTrue(true);
    }

    @Test
    void testTwo() {
        Assertions.assertEquals(2, 1 + 1);
    }
}

Important Notes

Extensions can be reused across many test classes to keep code clean.

JUnit provides many built-in extension points like lifecycle callbacks and parameter resolvers.

Custom extensions must implement specific interfaces depending on what they do.

Summary

The extension model lets you add extra behavior to tests without changing test code.

Use @ExtendWith to apply extensions to tests.

Extensions can run code before/after tests, handle exceptions, or change test execution.

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