Extension model overview in JUnit - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

What is the primary purpose of JUnit's Extension Model?

JUnit 5 introduced an extension model. What is its main purpose?

A To provide a graphical user interface for running tests

B To replace the need for assertions in test methods

C To automatically generate test data without user input

D To allow users to add custom behavior to tests like lifecycle callbacks and parameter resolution

💡 Hint

Think about how you can customize or extend test execution behavior in JUnit.

The extension model in JUnit 5 lets you add custom behavior such as before/after callbacks, parameter injection, and conditional test execution. It does not replace assertions or generate data automatically.

❓ Predict Output

intermediate

Output of a test using a simple JUnit extension

Given the following JUnit 5 extension and test class, what will be printed when the test runs?

JUnit

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

public class SimpleExtension implements BeforeEachCallback {
    @Override
    public void beforeEach(ExtensionContext context) {
        System.out.println("Before each test");
    }
}

import org.junit.jupiter.api.Test;
import org.junit.jupiter.api.extension.ExtendWith;

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

A

Test method running
Before each test

B

Before each test
Test method running

C Only Test method running

D No output printed

💡 Hint

Remember that BeforeEachCallback runs before each test method.

The extension's beforeEach method runs before the test method, so "Before each test" prints first, then the test method prints "Test method running".

❓ assertion

advanced

Correct assertion to verify extension invocation

You want to verify that a JUnit extension's beforeAll method was called exactly once before all tests. Which assertion correctly checks this?

A assertNull(extension.getBeforeAllCallCount())

B assertTrue(extension.getBeforeAllCallCount() > 1)

C assertEquals(1, extension.getBeforeAllCallCount())

D assertFalse(extension.getBeforeAllCallCount() == 0)

💡 Hint

Think about how to check the exact number of times a method was called.

To verify the method was called exactly once, assert that the call count equals 1. Other options check for wrong conditions or null.

🔧 Debug

advanced

Why does this JUnit extension not run before tests?

Consider this JUnit 5 extension class:

public class MyExtension implements BeforeEachCallback {
    public void beforeAll(ExtensionContext context) {
        System.out.println("Before all tests");
    }

    @Override
    public void beforeEach(ExtensionContext context) {
        System.out.println("Before each test");
    }
}

Why is "Before all tests" never printed?

A Because beforeAll is not annotated or overridden from an interface, so it is never called by JUnit

B Because the extension class is missing @ExtendWith annotation

C Because beforeAll method has wrong parameter type

D Because beforeEachCallback interface does not support beforeAll method

💡 Hint

Check which methods JUnit calls automatically in extensions.

JUnit calls lifecycle methods only if they override interface methods or are annotated properly. The beforeAll method here is just a normal method and not called automatically.

❓ framework

expert

How to implement conditional test execution with JUnit extensions?

You want to skip a test if a certain environment variable is not set. Which JUnit 5 extension interface should you implement to conditionally disable tests?

A ExecutionCondition

B TestInstancePostProcessor

C BeforeEachCallback

D ParameterResolver

💡 Hint

Think about how to enable or disable tests before they run.

The ExecutionCondition interface allows you to decide if a test should run or be skipped based on conditions like environment variables.