Custom extensions in JUnit - Build an Automation Script

Verify custom JUnit extension logs test start and end

Preconditions (2)

Step 1: Create a test class and register the custom extension using @ExtendWith

Step 2: Write a simple test method that asserts true is true

Step 3: Run the test class

Step 4: Observe the console output for logs from the custom extension indicating test start and test end

✅ Expected Result: The console shows log messages from the custom extension before and after the test method runs, and the test passes

Automation Requirements - JUnit 5

Assertions Needed:

Assert that the test method passes successfully

Verify via console output that the extension logs test start and end

Best Practices:

Use @ExtendWith annotation to register the extension

Implement BeforeTestExecutionCallback and AfterTestExecutionCallback interfaces in the extension

Keep test methods simple and focused

Use standard JUnit assertions

Automated Solution

JUnit

import org.junit.jupiter.api.Test;
import org.junit.jupiter.api.extension.ExtendWith;
import org.junit.jupiter.api.extension.BeforeTestExecutionCallback;
import org.junit.jupiter.api.extension.AfterTestExecutionCallback;
import org.junit.jupiter.api.extension.ExtensionContext;

import static org.junit.jupiter.api.Assertions.assertTrue;

class LoggingExtension implements BeforeTestExecutionCallback, AfterTestExecutionCallback {
    @Override
    public void beforeTestExecution(ExtensionContext context) {
        System.out.println("[Extension] Starting test: " + context.getDisplayName());
    }

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

@ExtendWith(LoggingExtension.class)
public class CustomExtensionTest {

    @Test
    void simpleTest() {
        assertTrue(true, "True should be true");
    }
}

The LoggingExtension class implements two JUnit 5 extension interfaces: BeforeTestExecutionCallback and AfterTestExecutionCallback. These allow it to run code before and after each test method.

Inside beforeTestExecution, it prints a message indicating the test is starting. Inside afterTestExecution, it prints a message indicating the test finished.

The test class CustomExtensionTest uses @ExtendWith(LoggingExtension.class) to register the extension. It has one simple test method simpleTest that asserts true is true.

When running the test, the console will show the extension's log messages before and after the test method runs, confirming the extension works. The test assertion ensures the test passes.

Common Mistakes - 3 Pitfalls

Not registering the extension with @ExtendWith annotation

Implementing the wrong extension interfaces

Using System.out.println in production extensions without proper logging

Bonus Challenge

Modify the custom extension to measure and log the duration of each test execution

Show Hint

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