Why extensions customize JUnit behavior - Automation Benefits in Action

Verify custom JUnit extension modifies test behavior

Preconditions (2)

Step 1: Create a test class using the custom extension with @ExtendWith annotation

Step 2: Write a test method that relies on the extension's behavior (e.g., modifies a value or logs info)

Step 3: Run the test class

Step 4: Observe the test output or state changes caused by the extension

✅ Expected Result: The test method runs with the custom behavior applied by the extension, verified by assertions or output changes

Automation Requirements - JUnit 5

Assertions Needed:

Verify the extension modifies the test state or output as expected

Confirm the test passes with the extension applied

Best Practices:

Use @ExtendWith annotation to apply the extension

Implement BeforeEachCallback or other extension interfaces properly

Use assertions to verify extension effects

Keep test code clean and focused on verifying extension behavior

Automated Solution

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.assertEquals;

// Custom extension that sets a value before each test
class CustomValueExtension implements BeforeEachCallback {
    @Override
    public void beforeEach(ExtensionContext context) {
        context.getStore(ExtensionContext.Namespace.GLOBAL).put("value", 42);
    }
}

@ExtendWith(CustomValueExtension.class)
public class CustomExtensionTest {

    @Test
    void testValueSetByExtension(ExtensionContext context) {
        Integer value = context.getStore(ExtensionContext.Namespace.GLOBAL).get("value", Integer.class);
        assertEquals(42, value, "Extension should set value to 42 before each test");
    }

    @Test
    void testAnotherValueCheck(ExtensionContext context) {
        Integer value = context.getStore(ExtensionContext.Namespace.GLOBAL).get("value", Integer.class);
        assertEquals(42, value);
    }
}

This code defines a custom JUnit 5 extension CustomValueExtension that implements BeforeEachCallback. It sets a value of 42 in the global store before each test runs.

The test class CustomExtensionTest uses @ExtendWith(CustomValueExtension.class) to apply the extension. Each test method accesses the stored value from the extension context and asserts it equals 42.

This shows how extensions customize JUnit behavior by injecting or modifying test state before execution. Assertions verify the extension's effect, ensuring the test passes only if the extension works correctly.

Common Mistakes - 3 Pitfalls

{'mistake': 'Not using @ExtendWith annotation to apply the extension', 'why_bad': "The extension will not be applied and tests won't have the custom behavior", 'correct_approach': 'Always annotate the test class or method with @ExtendWith to activate the extension'}

Trying to access extension context parameters incorrectly in test methods

Modifying static or shared state without cleanup

Bonus Challenge

Now add data-driven testing with 3 different values set by the extension

Show Hint

Practice

1. What is the main reason to use extensions in JUnit tests?

easy

A. To add extra behavior without changing the test code

B. To write test cases faster

C. To replace the JUnit framework

D. To create user interfaces for tests

Solution

1. Step 1: Understand the purpose of extensions

   Extensions allow adding extra behavior like setup or cleanup without modifying test methods.

2. Step 2: Compare options with this purpose

   Only To add extra behavior without changing the test code matches this purpose; others describe unrelated goals.

3. Final Answer:

   To add extra behavior without changing the test code -> Option A

4. Quick Check:

   Extensions add behavior = D [OK]

Hint: Extensions add behavior without changing tests [OK]

Common Mistakes:

• Thinking extensions speed up writing tests
• Confusing extensions with replacing JUnit
• Assuming extensions create UI

2. Which annotation is used to apply an extension to a JUnit test class or method?

easy

A. @TestExtension

B. @UseExtension

C. @ExtendWith

D. @EnableExtension

Solution

1. Step 1: Recall the correct annotation for extensions

   The standard JUnit annotation to apply extensions is @ExtendWith.

2. Step 2: Verify other options

   Other options are not valid JUnit annotations for extensions.

3. Final Answer:

   @ExtendWith -> Option C

4. Quick Check:

   @ExtendWith = C [OK]

Hint: Remember '@ExtendWith' applies extensions [OK]

Common Mistakes:

• Using @TestExtension which does not exist
• Confusing with @UseExtension or @EnableExtension
• Forgetting the '@' symbol

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

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

Assuming MyExtension prints "Before test" before the test and "After test" after the test.

medium

A. Test running\nBefore test\nAfter test

B. Before test\nTest running\nAfter test

C. After test\nTest running\nBefore test

D. Test running only

Solution

1. Step 1: Understand extension behavior

   MyExtension runs code before and after the test method, printing "Before test" and "After test" respectively.

2. Step 2: Order of output

   Output order is: before test message, test method message, after test message.

3. Final Answer:

   Before test\nTest running\nAfter test -> Option B

4. Quick Check:

   Extension runs before/after test = A [OK]

Hint: Extensions run code before and after tests [OK]

Common Mistakes:

• Assuming extension runs after test only
• Mixing order of printed lines
• Ignoring extension output

4. Identify the error in this JUnit test using an extension:

@ExtendWith(MyExtension.class)
class TestClass {
  @Test
  void test() {
    System.out.println("Running test");
  }

  void helper() {
    System.out.println("Helper method");
  }
}

Why might helper() not be called during test execution?

medium

A. Extension disables helper methods

B. Helper method must be private

C. Helper method must be static

D. Helper method is not annotated with @Test

Solution

1. Step 1: Check method annotations

   Only methods annotated with @Test run as tests; helper() lacks this annotation.

2. Step 2: Understand extension role

   Extensions do not disable helper methods; they only affect test lifecycle.

3. Final Answer:

   Helper method is not annotated with @Test -> Option D

4. Quick Check:

   Missing @Test means no run = B [OK]

Hint: Only @Test methods run automatically [OK]

Common Mistakes:

• Thinking extensions disable helper methods
• Believing helper must be static or private
• Assuming all methods run as tests

5. You want to create a JUnit extension that changes test data before each test runs. Which approach best uses extensions to achieve this?

hard

A. Implement BeforeEachCallback and modify test data in beforeEach()

B. Override the test method and change data inside it

C. Use @Test annotation with parameters to change data

D. Create a separate test class for each data variation

Solution

1. Step 1: Identify extension interface for pre-test actions

   The BeforeEachCallback interface allows running code before each test method.

2. Step 2: Apply this to modify test data

   Modifying test data in beforeEach() method is the correct way to customize data before tests.

3. Final Answer:

   Implement BeforeEachCallback and modify test data in beforeEach() -> Option A

4. Quick Check:

   BeforeEachCallback modifies data before tests = A [OK]

Hint: Use BeforeEachCallback to run code before tests [OK]

Common Mistakes:

• Trying to override test methods directly
• Misusing @Test parameters for data changes
• Creating many classes instead of using extensions