BeforeEachCallback and AfterEachCallback in JUnit

Introduction

BeforeEachCallback and AfterEachCallback help run code before and after each test. This keeps tests clean and avoids repeating setup or cleanup steps.

You want to open a database connection before each test and close it after.

You need to reset some data or state before every test runs.

You want to log information before and after each test for debugging.

You want to prepare test data fresh for each test case.

You want to clean temporary files after each test finishes.

Syntax

JUnit

public class MyExtension implements BeforeEachCallback, AfterEachCallback {
    @Override
    public void beforeEach(ExtensionContext context) throws Exception {
        // code to run before each test
    }

    @Override
    public void afterEach(ExtensionContext context) throws Exception {
        // code to run after each test
    }
}

Implement both interfaces in one class to run code before and after each test.

The ExtensionContext gives info about the test being run.

Examples

This example prints the test name before each test starts.

JUnit

public class MyExtension implements BeforeEachCallback {
    @Override
    public void beforeEach(ExtensionContext context) throws Exception {
        System.out.println("Starting test: " + context.getDisplayName());
    }
}

This example prints the test name after each test finishes.

JUnit

public class MyExtension implements AfterEachCallback {
    @Override
    public void afterEach(ExtensionContext context) throws Exception {
        System.out.println("Finished test: " + context.getDisplayName());
    }
}

This example shows both setup and cleanup messages around each test.

JUnit

public class MyExtension implements BeforeEachCallback, AfterEachCallback {
    @Override
    public void beforeEach(ExtensionContext context) throws Exception {
        System.out.println("Setup before " + context.getDisplayName());
    }

    @Override
    public void afterEach(ExtensionContext context) throws Exception {
        System.out.println("Cleanup after " + context.getDisplayName());
    }
}

Sample Program

This test class uses MyExtension to print messages before and after each test. It runs two simple tests that print their own messages.

JUnit

import org.junit.jupiter.api.extension.BeforeEachCallback;
import org.junit.jupiter.api.extension.AfterEachCallback;
import org.junit.jupiter.api.extension.ExtensionContext;
import org.junit.jupiter.api.extension.RegisterExtension;
import org.junit.jupiter.api.Test;

public class ExampleTest {

    static class MyExtension implements BeforeEachCallback, AfterEachCallback {
        @Override
        public void beforeEach(ExtensionContext context) throws Exception {
            System.out.println("Before test: " + context.getDisplayName());
        }

        @Override
        public void afterEach(ExtensionContext context) throws Exception {
            System.out.println("After test: " + context.getDisplayName());
        }
    }

    @RegisterExtension
    static MyExtension extension = new MyExtension();

    @Test
    void testOne() {
        System.out.println("Running testOne");
    }

    @Test
    void testTwo() {
        System.out.println("Running testTwo");
    }
}

Important Notes

Use @RegisterExtension to apply your extension in the test class.

BeforeEachCallback runs before each test method, AfterEachCallback runs after.

Extensions help keep test code clean by handling setup and cleanup in one place.

Summary

BeforeEachCallback and AfterEachCallback run code before and after each test.

They help avoid repeating setup and cleanup code in tests.

Use ExtensionContext to get info about the current test.

Practice

1. What is the main purpose of implementing BeforeEachCallback and AfterEachCallback in JUnit 5?

easy

A. To skip tests based on conditions

B. To run code before and after each test method for setup and cleanup

C. To replace the test methods with custom logic

D. To run code only once before all tests in a class

Solution

1. Step 1: Understand the role of BeforeEachCallback and AfterEachCallback

   These interfaces allow running code before and after each test method, typically for setup and cleanup.

2. Step 2: Compare with other lifecycle callbacks

   Unlike @BeforeAll or @AfterAll, these run before and after every test, not once per class.

3. Final Answer:

   To run code before and after each test method for setup and cleanup -> Option B

4. Quick Check:

   BeforeEachCallback and AfterEachCallback = setup and cleanup per test [OK]

Hint: They run code before and after each test method [OK]

Common Mistakes:

• Confusing with @BeforeAll/@AfterAll which run once per class
• Thinking they replace test methods
• Assuming they skip tests

2. Which method must be implemented when using the BeforeEachCallback interface in JUnit 5?

easy

A. void beforeEach(ExtensionContext context)

B. void beforeAll(ExtensionContext context)

C. void afterEach(ExtensionContext context)

D. void afterAll(ExtensionContext context)

Solution

1. Step 1: Recall the method signature for BeforeEachCallback

   The interface requires implementing beforeEach(ExtensionContext context) to run code before each test.

2. Step 2: Differentiate from other lifecycle methods

   Methods like beforeAll or afterAll belong to other interfaces or annotations, not BeforeEachCallback.

3. Final Answer:

   void beforeEach(ExtensionContext context) -> Option A

4. Quick Check:

   BeforeEachCallback method = beforeEach() [OK]

Hint: BeforeEachCallback requires beforeEach() method [OK]

Common Mistakes:

• Using beforeAll or afterAll methods incorrectly
• Confusing AfterEachCallback method with BeforeEachCallback
• Missing the ExtensionContext parameter

3. Given the following JUnit 5 extension code, what will be printed when running a test method?

public class MyExtension implements BeforeEachCallback, AfterEachCallback {
    @Override
    public void beforeEach(ExtensionContext context) {
        System.out.println("Start " + context.getDisplayName());
    }
    @Override
    public void afterEach(ExtensionContext context) {
        System.out.println("End " + context.getDisplayName());
    }
}

Assuming the test method is named testExample() in class ExampleTest.

medium

A. Start ExampleTest
testExample End

B. Only Start testExample(ExampleTest) is printed

C. testExample Start
ExampleTest End

D. Start testExample(ExampleTest)
End testExample(ExampleTest)

Solution

1. Step 1: Understand what context.getDisplayName() returns

   It returns the display name of the current test, typically in the format "methodName(className)".

2. Step 2: Analyze the print statements in beforeEach and afterEach

   Before the test, it prints "Start " plus the display name; after the test, it prints "End " plus the same display name.

3. Final Answer:

   Start testExample(ExampleTest) End testExample(ExampleTest) -> Option D

4. Quick Check:

   beforeEach prints Start + displayName, afterEach prints End + displayName [OK]

Hint: context.getDisplayName() shows method and class name [OK]

Common Mistakes:

• Assuming displayName returns only class or method name
• Thinking only one print runs
• Mixing order of prints

4. Identify the error in this JUnit 5 extension code implementing AfterEachCallback:

public class CleanupExtension implements AfterEachCallback {
    public void afterEach() {
        System.out.println("Cleaning up after test");
    }
}

medium

A. Class must implement BeforeEachCallback too

B. afterEach method should be static

C. Missing ExtensionContext parameter in afterEach method

D. System.out.println is not allowed in extensions

Solution

1. Step 1: Check method signature for AfterEachCallback

   The interface requires implementing afterEach(ExtensionContext context) with the ExtensionContext parameter.

2. Step 2: Identify the missing parameter error

   The code's afterEach method has no parameters, so it does not correctly override the interface method.

3. Final Answer:

   Missing ExtensionContext parameter in afterEach method -> Option C

4. Quick Check:

   AfterEachCallback.afterEach() requires ExtensionContext parameter [OK]

Hint: Check method parameters match interface exactly [OK]

Common Mistakes:

• Omitting ExtensionContext parameter
• Making method static incorrectly
• Thinking BeforeEachCallback is mandatory
• Believing print statements are disallowed

5. You want to create a JUnit 5 extension that logs the start time before each test and logs the elapsed time after each test. Which approach correctly uses BeforeEachCallback and AfterEachCallback to achieve this?

hard

A. Store start time in a field in beforeEach, then calculate elapsed time in afterEach using the same field

B. Store start time in a local variable inside beforeEach and reuse it in afterEach

C. Use static variables to store start time shared across all tests

D. Only implement BeforeEachCallback and log elapsed time after test in test method

Solution

1. Step 1: Understand lifecycle of BeforeEachCallback and AfterEachCallback

   They run before and after each test method, so storing start time in a field allows sharing data between these callbacks for the same test.

2. Step 2: Evaluate options for storing start time

   Local variables in beforeEach cannot be accessed in afterEach; static variables cause data sharing issues across tests; logging elapsed time in test method breaks separation of concerns.

3. Final Answer:

   Store start time in a field in beforeEach, then calculate elapsed time in afterEach using the same field -> Option A

4. Quick Check:

   Use instance field to share data between beforeEach and afterEach [OK]

Hint: Use instance fields to share data between callbacks [OK]

Common Mistakes:

• Trying to share data via local variables
• Using static fields causing test interference
• Logging elapsed time inside test methods