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

JUnit 5 is the modern successor to JUnit 4, offering a modular architecture and new annotations like @Test with improved features. It supports Java 8+ and provides better extension models, while JUnit 4 is simpler but less flexible and supports older Java versions.

Quick Comparison

Here is a quick side-by-side comparison of JUnit 4 and JUnit 5 across key factors.

Factor	JUnit 4	JUnit 5
Release Year	2006	2017
Java Version Support	Java 5+	Java 8+
Architecture	Monolithic	Modular (JUnit Platform, Jupiter, Vintage)
Annotations	Basic set (e.g., @Test, @Before)	Expanded set (e.g., @Test, @BeforeEach, @DisplayName)
Extension Model	Rules and Runners	Extensions API
Backward Compatibility	N/A	Supports JUnit 4 tests via Vintage engine

Key Differences

JUnit 5 introduces a modular architecture split into three main components: the JUnit Platform for launching tests, JUnit Jupiter for writing tests and extensions, and JUnit Vintage to run legacy JUnit 4 tests. This design allows more flexibility and easier integration with build tools and IDEs.

Unlike JUnit 4, which uses annotations like @Before and @After, JUnit 5 replaces these with more descriptive annotations such as @BeforeEach and @AfterEach. It also adds new features like @DisplayName for better test reporting and supports lambda expressions for dynamic tests.

The extension model in JUnit 5 is more powerful and replaces the older rules and runners system from JUnit 4. This allows developers to create reusable extensions for lifecycle callbacks, parameter resolution, and conditional test execution, improving test customization and maintainability.

Code Comparison

Here is a simple test example showing how to write a test in JUnit 4 that checks if a number is positive.

java

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

public class NumberTest {
    @Test
    public void testIsPositive() {
        int number = 5;
        assertTrue(number > 0);
    }
}

Output

Test passes if number is greater than zero.

JUnit 5 Equivalent

The same test in JUnit 5 uses updated annotations and supports Java 8 features.

java

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

public class NumberTest {
    @Test
    void testIsPositive() {
        int number = 5;
        assertTrue(number > 0);
    }
}

Output

Test passes if number is greater than zero.

When to Use Which

Choose JUnit 5 when you want modern features, better extension support, and are using Java 8 or newer. It is ideal for new projects needing flexibility and improved test reporting.

Use JUnit 4 if you maintain legacy projects or need compatibility with older Java versions. It remains stable and widely supported but lacks the advanced features of JUnit 5.

Key Takeaways

JUnit 5 offers a modular, modern architecture with improved annotations and extensions.

JUnit 4 is simpler but limited to older Java versions and less flexible extension models.

JUnit 5 supports running JUnit 4 tests via the Vintage engine for backward compatibility.

Choose JUnit 5 for new projects and JUnit 4 for legacy support or older Java environments.