Custom extensions in JUnit - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a JUnit 5 custom extension with BeforeEachCallback

What will be printed when this JUnit 5 test class runs with the custom extension?

JUnit

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

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

@ExtendWith(MyExtension.class)
class SampleTest {
    @Test
    void testOne() {
        System.out.println("Running testOne");
    }
    @Test
    void testTwo() {
        System.out.println("Running testTwo");
    }
}

A Running testOne\nRunning testTwo\nSetup before each test\nSetup before each test

B Running testOne\nSetup before each test\nRunning testTwo\nSetup before each test

C Setup before each test\nSetup before each test\nRunning testOne\nRunning testTwo

D Setup before each test\nRunning testOne\nSetup before each test\nRunning testTwo

💡 Hint

Remember that BeforeEachCallback runs before each test method.

The custom extension implements BeforeEachCallback, so its beforeEach method runs before each test method. Therefore, the message 'Setup before each test' prints before each test's output.

❓ assertion

intermediate

Correct assertion to verify extension invocation count

Given a custom JUnit 5 extension that increments a counter each time beforeEach is called, which assertion correctly verifies that the counter equals 3 after running 3 tests?

JUnit

class CountingExtension implements BeforeEachCallback {
    static int count = 0;
    @Override
    public void beforeEach(ExtensionContext context) {
        count++;
    }
}

// After running 3 tests, check count value

A assertEquals(3, CountingExtension.count);

B assertTrue(CountingExtension.count == 3);

C assertFalse(CountingExtension.count != 3);

D assertEquals(CountingExtension.count, 3);

💡 Hint

Use the standard JUnit assertion method with expected value first.

assertEquals(expected, actual) expects the expected value as the first argument. Option A uses this correctly. Option A reverses arguments which is not recommended and may cause confusing error messages.

🔧 Debug

advanced

Identify the error in this custom extension implementation

What error will occur when running this JUnit 5 extension code?

JUnit

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

public class FaultyExtension implements BeforeAllCallback {
    @Override
    public void beforeAll(ExtensionContext context) {
        System.out.println("Before all tests");
    }

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

A No error; both methods run correctly

B beforeEach method is never called because it does not override any interface method

C Compilation error due to missing @Override on beforeEach method

D Runtime error: Method beforeEach causes IllegalStateException

💡 Hint

Check which interfaces the class implements and which methods are called by JUnit.

The class implements BeforeAllCallback, so only beforeAll is called by JUnit. The beforeEach method is a plain method without @Override and not part of any interface implemented, so it is never called.

❓ framework

advanced

Which JUnit 5 extension point allows modifying test instance before each test?

You want to create a custom extension that modifies the test class instance before each test method runs. Which extension point should you implement?

A TestInstancePostProcessor

B TestExecutionExceptionHandler

C BeforeAllCallback

D BeforeEachCallback

💡 Hint

Consider which extension point allows you to access and modify the test instance itself.

TestInstancePostProcessor allows you to modify the test instance after it is created but before any tests run. BeforeEachCallback runs before each test but does not provide the test instance for modification.

🧠 Conceptual

expert

Effect of registering multiple extensions with conflicting callbacks

If two JUnit 5 extensions both implement BeforeEachCallback and are registered on the same test class, what is the expected behavior during test execution?

A JUnit throws an exception due to conflicting extensions

B Only the first registered extension's beforeEach method is called

C Both extensions' beforeEach methods are called in registration order before each test

D Only the last registered extension's beforeEach method is called

💡 Hint

Think about how JUnit 5 handles multiple extensions implementing the same callback.

JUnit 5 calls all registered extensions' callback methods in the order they are registered. So both beforeEach methods run before each test.