@ExtendWith annotation in JUnit - Test Execution Trace

Test Overview

This test uses the @ExtendWith annotation in JUnit 5 to register a custom extension. It verifies that the extension's callback method is called during test execution.

Test Code - JUnit 5

JUnit

import org.junit.jupiter.api.Test;
import org.junit.jupiter.api.extension.ExtendWith;
import org.junit.jupiter.api.extension.BeforeEachCallback;
import org.junit.jupiter.api.extension.ExtensionContext;
import static org.junit.jupiter.api.Assertions.assertTrue;

class MyExtension implements BeforeEachCallback {
    static boolean beforeEachCalled = false;

    @Override
    public void beforeEach(ExtensionContext context) {
        beforeEachCalled = true;
    }
}

@ExtendWith(MyExtension.class)
public class ExtendWithTest {

    @Test
    void testExtensionCalled() {
        assertTrue(MyExtension.beforeEachCalled, "Extension beforeEach should have been called");
    }
}

Execution Trace - 5 Steps

Step	Action	System State	Assertion	Result
1	JUnit test runner starts the test class ExtendWithTest	JUnit framework initialized, test class loaded	—	PASS
2	JUnit detects @ExtendWith(MyExtension.class) and registers MyExtension as an extension	Extension MyExtension is ready to be called before each test	—	PASS
3	JUnit calls MyExtension.beforeEach() before running testExtensionCalled()	MyExtension.beforeEachCalled is set to true	—	PASS
4	JUnit runs testExtensionCalled() method	Test method executing	assertTrue(MyExtension.beforeEachCalled) verifies beforeEachCalled is true	PASS
5	JUnit completes testExtensionCalled() successfully	Test passed	—	PASS

Failure Scenario

Failing Condition: MyExtension.beforeEach() is not called before the test method

Execution Trace Quiz - 3 Questions

What does the @ExtendWith annotation do in this test?

A Marks the test method as disabled

B Registers MyExtension to run before each test method

C Specifies the test method's timeout

D Injects a mock object into the test

Key Result

Use @ExtendWith to add custom behavior before or after tests. Always verify that your extension's callbacks are executed as expected.

Practice

1. What is the primary purpose of the @ExtendWith annotation in JUnit 5?

easy

A. To mark a method as a test case

B. To disable a test method temporarily

C. To register extensions that add extra behavior to tests

D. To specify the order of test execution

Solution

1. Step 1: Understand the role of @ExtendWith

   This annotation is used to register extensions that can add behavior before, after, or around tests.

2. Step 2: Compare with other annotations

   Other annotations like @Test mark test methods, but @ExtendWith is specifically for extensions.

3. Final Answer:

   To register extensions that add extra behavior to tests -> Option C

4. Quick Check:

   @ExtendWith = register extensions [OK]

Hint: Remember: @ExtendWith adds extra test behavior [OK]

Common Mistakes:

• Confusing @ExtendWith with @Test
• Thinking it disables tests
• Assuming it controls test order

2. Which of the following is the correct syntax to use @ExtendWith to register a custom extension named MyExtension.class on a test class?

easy

A. @ExtendWith(MyExtension.class) public class TestClass {}

B. @ExtendWith{MyExtension.class} public class TestClass {}

C. @ExtendWith[MyExtension.class] public class TestClass {}

D. @ExtendWith public class TestClass {}

Solution

1. Step 1: Recall correct annotation syntax

   Annotations use parentheses with class references, like @ExtendWith(MyExtension.class).

2. Step 2: Identify invalid syntax

   Curly braces, square brackets, or angle brackets are not valid for this annotation.

3. Final Answer:

   @ExtendWith(MyExtension.class) public class TestClass {} -> Option A

4. Quick Check:

   Annotation syntax uses parentheses () [OK]

Hint: Use parentheses () with class name for annotations [OK]

Common Mistakes:

• Using curly braces {} instead of parentheses
• Using square brackets [] or angle brackets <>
• Omitting .class after the class name

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

@ExtendWith(MyExtension.class)
public class SampleTest {
    @Test
    void testMethod() {
        System.out.println("Test running");
    }
}

class MyExtension implements BeforeTestExecutionCallback {
    public void beforeTestExecution(ExtensionContext context) {
        System.out.println("Before test");
    }
}

medium

A. Before test

B. Test running\nBefore test

C. Test running

D. Before test\nTest running

Solution

1. Step 1: Understand extension behavior

   The MyExtension implements BeforeTestExecutionCallback, so it runs code before the test method.

2. Step 2: Trace output order

   First, "Before test" prints from the extension, then "Test running" prints from the test method.

3. Final Answer:

   Before test\nTest running -> Option D

4. Quick Check:

   Extension beforeTestExecution runs before test method [OK]

Hint: BeforeTestExecutionCallback runs before test method [OK]

Common Mistakes:

• Assuming extension runs after test
• Ignoring extension output
• Confusing output order

4. Identify the error in the following JUnit 5 test class using @ExtendWith:

@ExtendWith
public class FaultyTest {
    @Test
    void testSomething() {}
}

medium

A. @ExtendWith cannot be used on classes

B. @ExtendWith is missing the extension class argument

C. The test method must be static

D. The test method must return a value

Solution

1. Step 1: Check @ExtendWith usage

   The annotation requires specifying which extension class to use, e.g., @ExtendWith(MyExtension.class).

2. Step 2: Validate other options

   It is valid on classes, test methods do not need to be static or return values.

3. Final Answer:

   @ExtendWith is missing the extension class argument -> Option B

4. Quick Check:

   @ExtendWith needs extension class specified [OK]

Hint: Always specify extension class inside @ExtendWith() [OK]

Common Mistakes:

• Omitting the extension class argument
• Thinking @ExtendWith can't be on classes
• Believing test methods must be static

5. You want to reuse a setup logic that prints "Setup done" before each test method in multiple test classes. How should you apply @ExtendWith to achieve this cleanly?

hard

A. Create a custom extension class implementing BeforeEachCallback and annotate each test class with @ExtendWith(YourExtension.class)

B. Add a static method printing "Setup done" and call it manually in each test method

C. Use @BeforeAll annotation in each test class to print "Setup done"

D. Annotate each test method with @ExtendWith(YourExtension.class) instead of the class

Solution

1. Step 1: Identify reusable setup approach

   Implementing a custom extension with BeforeEachCallback allows running code before each test method.

2. Step 2: Apply @ExtendWith on test classes

   Annotating test classes with @ExtendWith(YourExtension.class) reuses the setup logic cleanly across classes.

3. Step 3: Evaluate other options

   Manually calling methods or using @BeforeAll is less reusable or runs only once per class. Annotating methods individually is repetitive.

4. Final Answer:

   Create a custom extension class implementing BeforeEachCallback and annotate each test class with @ExtendWith(YourExtension.class) -> Option A

5. Quick Check:

   Custom extension + @ExtendWith on class = reusable setup [OK]

Hint: Use custom extension + @ExtendWith on class for reusable setup [OK]

Common Mistakes:

• Calling setup manually in each test
• Using @BeforeAll for per-test setup
• Annotating each test method instead of class