JUnit 4 vs JUnit 5: Key Differences and When to Use Each

JUnit 5 is a modern, modular testing framework that replaces JUnit 4 with improved architecture and new features like JUnit Platform and Jupiter. Unlike JUnit 4, which uses a single monolithic jar, JUnit 5 separates concerns into modules and supports more flexible extensions and annotations.

Quick Comparison

This table summarizes the main differences between JUnit 4 and JUnit 5.

Aspect	JUnit 4	JUnit 5
Architecture	Single monolithic jar	Modular: Platform, Jupiter, Vintage
Annotations	Uses @Test, @Before, @After	Adds @BeforeEach, @AfterEach, @Test with more options
Extension Model	Limited, uses Rules and Runners	Powerful extension model with Extension API
Java Version Support	Java 5+	Java 8+ (uses lambdas and streams)
Test Engine Support	Only JUnit 4 tests	Supports multiple test engines including JUnit 4 via Vintage
Dynamic Tests	Not supported	Supports dynamic tests with @TestFactory

Key Differences

JUnit 5 introduces a new architecture split into three main modules: JUnit Platform for launching tests, JUnit Jupiter for writing tests and extensions, and JUnit Vintage to run legacy JUnit 4 tests. This modular design allows better flexibility and integration with modern tools.

The annotation model in JUnit 5 is more expressive and consistent. For example, @BeforeEach replaces @Before, and @AfterEach replaces @After, making test lifecycle methods clearer. Also, JUnit 5 supports new annotations like @DisplayName for better test reporting.

Extension support is a major upgrade in JUnit 5. Instead of the older Rules and Runners in JUnit 4, JUnit 5 uses a powerful Extension API that allows developers to customize test behavior more flexibly, including parameter injection, conditional test execution, and lifecycle callbacks.

Code Comparison

java

import org.junit.Test;
import static org.junit.Assert.assertEquals;

public class CalculatorTest {

    @Test
    public void addition() {
        int result = 2 + 3;
        assertEquals(5, result);
    }
}

Output

Test passes if 2 + 3 equals 5

JUnit 5 Equivalent

java

import org.junit.jupiter.api.Test;
import static org.junit.jupiter.api.Assertions.assertEquals;

class CalculatorTest {

    @Test
    void addition() {
        int result = 2 + 3;
        assertEquals(5, result);
    }
}

Output

Test passes if 2 + 3 equals 5

When to Use Which

Choose JUnit 5 when starting new projects or when you want modern features like modularity, better extension support, and Java 8+ compatibility. It offers more flexibility and future-proofing.

Use JUnit 4 if you maintain legacy projects that already depend on it and migration is costly or risky. JUnit 5 supports running JUnit 4 tests via the Vintage engine, easing gradual migration.

Key Takeaways

JUnit 5 is modular and supports multiple test engines, unlike JUnit 4's monolithic design.

JUnit 5 uses improved annotations and a powerful extension model for flexible testing.

JUnit 5 requires Java 8 or higher, leveraging modern language features.

JUnit 4 is suitable for legacy projects, while JUnit 5 is recommended for new development.

JUnit 5 can run JUnit 4 tests using the Vintage engine, easing migration.