Extension model overview in JUnit - Test Execution Trace

Test Overview

This test demonstrates how to use JUnit 5 extension model to log test execution start and end. It verifies that the extension methods are called during the test lifecycle.

Test Code - JUnit 5

JUnit

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

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

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

@ExtendWith(LoggingExtension.class)
public class ExtensionModelTest {

    @Test
    void sampleTest() {
        Assertions.assertTrue(1 + 1 == 2);
    }
}

Execution Trace - 6 Steps

Step	Action	System State	Assertion	Result
1	JUnit test runner starts the test suite	JUnit framework initialized, test class ExtensionModelTest loaded	—	PASS
2	JUnit detects @ExtendWith annotation and registers LoggingExtension	LoggingExtension instance ready to intercept test lifecycle events	—	PASS
3	JUnit calls LoggingExtension.beforeTestExecution before sampleTest runs	Console prints: "Starting test: sampleTest()"	Verify beforeTestExecution callback is invoked	PASS
4	JUnit executes sampleTest method	Test method runs, assertion 1 + 1 == 2 evaluated	AssertTrue passes as 1 + 1 equals 2	PASS
5	JUnit calls LoggingExtension.afterTestExecution after sampleTest completes	Console prints: "Finished test: sampleTest()"	Verify afterTestExecution callback is invoked	PASS
6	JUnit test runner completes the test suite	Test report generated showing sampleTest passed	Test result is PASS	PASS

Failure Scenario

Failing Condition: If the LoggingExtension is not registered or its methods throw exceptions

Execution Trace Quiz - 3 Questions

What does the LoggingExtension.beforeTestExecution method do in this test?

A Prints a message before the test method runs

B Runs the test assertion

C Prints a message after the test method runs

D Skips the test method

Key Result

Using JUnit 5 extensions allows you to add custom behavior before and after tests, improving test reporting and setup without changing test code.

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