Custom extensions in JUnit - Test Execution Trace

Test Overview

This test demonstrates how to use a custom JUnit 5 extension to measure and print the execution time of a test method. It verifies that the extension runs and the test passes.

Test Code - JUnit 5

JUnit

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

class TimingExtension implements BeforeTestExecutionCallback, AfterTestExecutionCallback {
    private static final ExtensionContext.Namespace NAMESPACE = ExtensionContext.Namespace.create("timing");

    @Override
    public void beforeTestExecution(ExtensionContext context) throws Exception {
        context.getStore(NAMESPACE).put("startTime", System.currentTimeMillis());
    }

    @Override
    public void afterTestExecution(ExtensionContext context) throws Exception {
        long startTime = context.getStore(NAMESPACE).remove("startTime", Long.class);
        long duration = System.currentTimeMillis() - startTime;
        System.out.println("Test '" + context.getDisplayName() + "' took " + duration + " ms.");
    }
}

@ExtendWith(TimingExtension.class)
public class CustomExtensionTest {

    @Test
    void sampleTest() {
        Assertions.assertTrue(2 + 2 == 4);
    }
}

Execution Trace - 5 Steps

Step	Action	System State	Assertion	Result
1	JUnit test runner starts the test execution	JUnit framework initialized, test class loaded	—	PASS
2	JUnit calls TimingExtension.beforeTestExecution before sampleTest	Stores current time in extension context store	—	PASS
3	JUnit executes sampleTest method	Inside sampleTest, assertion checks if 2+2 equals 4	assertTrue(2 + 2 == 4)	PASS
4	JUnit calls TimingExtension.afterTestExecution after sampleTest	Calculates elapsed time and prints it to console	—	PASS
5	JUnit completes test execution and reports results	Test passed, timing output visible in console	—	PASS

Failure Scenario

Failing Condition: The assertion in sampleTest fails (e.g., 2 + 2 != 4)

Execution Trace Quiz - 3 Questions

What does the TimingExtension do before the test method runs?

A Stores the current time to measure test duration

B Prints the test name to the console

C Asserts that the test will pass

D Skips the test execution

Key Result

Using custom extensions in JUnit allows you to add reusable behavior around tests, such as timing, logging, or resource management, improving test maintainability and clarity.

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