Custom extensions in JUnit

Introduction

Custom extensions let you add your own rules or actions to JUnit tests. This helps you run extra code before, after, or around tests easily.

You want to run setup or cleanup code that applies to many tests.

You need to change how tests behave, like retrying failed tests automatically.

You want to add logging or timing around tests without changing each test method.

You want to inject dependencies or test data into your tests.

You want to check conditions before running tests and skip them if needed.

Syntax

JUnit

public class MyExtension implements BeforeEachCallback, AfterEachCallback {
    @Override
    public void beforeEach(ExtensionContext context) throws Exception {
        // code to run before each test
    }

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

Implement interfaces like BeforeEachCallback or AfterEachCallback to hook into test lifecycle.

Use ExtensionContext to get info about the test or store data.

Examples

This extension measures and prints how long each test takes.

JUnit

public class TimingExtension implements BeforeEachCallback, AfterEachCallback {
    private long startTime;

    @Override
    public void beforeEach(ExtensionContext context) {
        startTime = System.currentTimeMillis();
    }

    @Override
    public void afterEach(ExtensionContext context) {
        long duration = System.currentTimeMillis() - startTime;
        System.out.println("Test took " + duration + " ms");
    }
}

Use @ExtendWith to apply your custom extension to a test class.

JUnit

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

This extension retries a test up to 3 times if it fails.

JUnit

public class RetryExtension implements TestExecutionExceptionHandler {
    private int maxRetries = 3;

    @Override
    public void handleTestExecutionException(ExtensionContext context, Throwable throwable) throws Throwable {
        int attempt = 0;
        while (attempt < maxRetries) {
            attempt++;
            System.out.println("Retry " + attempt + " for test " + context.getDisplayName());
            try {
                context.getRequiredTestMethod().invoke(context.getRequiredTestInstance());
                return;
            } catch (Throwable ignored) {
                // continue to next retry
            }
        }
        throw throwable;
    }
}

Sample Program

This test class uses the TimingExtension to print how long each test takes. The extension runs code before and after each test automatically.

JUnit

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

public class TimingExtension implements BeforeEachCallback, AfterEachCallback {
    private long startTime;

    @Override
    public void beforeEach(ExtensionContext context) {
        startTime = System.currentTimeMillis();
    }

    @Override
    public void afterEach(ExtensionContext context) {
        long duration = System.currentTimeMillis() - startTime;
        System.out.println("Test took " + duration + " ms");
    }
}

@ExtendWith(TimingExtension.class)
public class SampleTest {
    @Test
    void fastTest() throws InterruptedException {
        Thread.sleep(50);
    }

    @Test
    void slowTest() throws InterruptedException {
        Thread.sleep(150);
    }
}

Important Notes

Always keep extension code simple and fast to avoid slowing tests.

Use ExtensionContext to share data between callbacks.

Register extensions with @ExtendWith or programmatically in test suites.

Summary

Custom extensions let you add reusable code around tests.

Implement JUnit extension interfaces to hook into test lifecycle.

Apply extensions with @ExtendWith to run extra code automatically.

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