JUnit 4 to JUnit 5 changes and migration steps.

Differences Between JUnit 4 and JUnit 5: Why Upgrade?

JUnit 4 has some limitations that become evident when compared to JUnit 5:

1. Library Structure: JUnit 4 comprises a single JAR library, making it somewhat cumbersome to use. Even if you need just one feature, the entire library must be imported. Conversely, JUnit 5 offers a more modular approach, providing greater ease of use, especially with Maven and Gradle build systems.
2. Test Execution: JUnit 4 restricts you to one test runner at a time. JUnit 5, however, supports multiple extensions simultaneously, allowing for parallel test execution. The Spring extension in JUnit 5 can be seamlessly integrated with third-party or custom extensions.
3. Java 8 Features: JUnit 5 supports Java 8 features like lambda expressions, enabling the creation of more powerful and maintainable tests.
4. Test Organization: JUnit 4 lacks some of the useful features for describing, organizing, and executing tests that JUnit 5 offers. For instance, JUnit 5 allows tests to be organized hierarchically with more descriptive display names.
5. Migration Flexibility: The good news is that you don't need to migrate entirely to benefit from JUnit 5. JUnit 4 and JUnit 5 can coexist, with JUnit 4 artifacts in the old org.junit package and JUnit 5 introducing new annotations and classes in the org.junit.jupiter package. This allows for a gradual and hybrid migration approach.

JUnit 4 to JUnit 5 Migration: A Step-by-Step Guide

Before we begin, it's important to understand that there is a fundamental architectural shift from JUnit 4 to JUnit 5, impacting the migration process. JUnit 4 was a single module, whereas JUnit 5 comprises three separate sub-projects or modules:

1. JUnit Jupiter: Introduces a new programming and extension model with new assertions, annotations, and support for Java 8 Lambda Expressions.
2. JUnit Platform: Acts as the foundation for launching test frameworks on the JVM and defines the TestEngine API for testing frameworks.
3. JUnit Vintage: Ensures backward compatibility by supporting the execution of legacy JUnit 3 and 4-based tests within the JUnit 5 framework, allowing a smooth migration path.

The migration process involves four key steps:

1. Replacing Dependencies: Update your build files to include JUnit 5 dependencies.
2. Updating Annotations: Revise your testing classes and methods to use the new JUnit 5 annotations.
3. Replacing Rules and Runners: Adapt JUnit 4 rules and runners to JUnit 5’s new extension model.
4. Running and Verifying Tests: Execute your updated tests to ensure they run correctly under JUnit 5.

JUnit 5 POM Dependencies: Removing JUnit 4 and Adding JUnit 5

Due to JUnit 4's monolithic architecture, running JUnit 4 tests requires a single dependency in your Maven configuration. When migrating to JUnit 5, this dependency will be replaced with the JUnit Vintage dependency. JUnit Vintage ensures backward compatibility with JUnit 4 tests, allowing JUnit 4 and JUnit 5 tests to coexist during the migration process.

Here's how to update your pom.xml file:

1. Remove the JUnit 4 Dependency:

<dependency>
    <groupId>junit</groupId>
    <artifactId>junit</artifactId>
    <version>4.x.x</version>
    <scope>test</scope>
</dependency>        

2. Add JUnit 5 Dependencies:

<dependency>
    <groupId>org.junit.jupiter</groupId>
    <artifactId>junit-jupiter-api</artifactId>
    <version>5.x.x</version>
    <scope>test</scope>
</dependency>
<dependency>
    <groupId>org.junit.jupiter</groupId>
    <artifactId>junit-jupiter-engine</artifactId>
    <version>5.x.x</version>
    <scope>test</scope>
</dependency>
<dependency>
    <groupId>org.junit.vintage</groupId>
    <artifactId>junit-vintage-engine</artifactId>
    <version>5.x.x</version>
    <scope>test</scope>
</dependency>        

By incorporating these dependencies, you can run JUnit 5 tests and maintain backward compatibility with JUnit 4 tests during the migration.

Introducing JUnit 5 Jupiter Annotations, Assertions, and Assumptions

JUnit 5 introduces new annotations, assertions, and assumptions that differ from those in JUnit 4. This requires updating your imports and test code to align with JUnit 5 conventions. Notably, JUnit 5 does not require test classes and methods to be public.

Core Annotations and Their Imports

Here's a mapping of JUnit 4 annotations to their JUnit 5 counterparts:

• @Test:
• @Before (JUnit 4) -> @BeforeEach (JUnit 5)
• @After (JUnit 4) -> @AfterEach (JUnit 5)
• @BeforeClass (JUnit 4) -> @BeforeAll (JUnit 5)
• @AfterClass (JUnit 4) -> @AfterAll (JUnit 5)
• @Ignore (JUnit 4) -> @Disabled (JUnit 5)

Assertions and Assumptions

JUnit 5 provides a rich set of assertions and assumptions to validate test conditions and assumptions:

Assertions:

• JUnit 4: import org.junit.Assert;
• JUnit 5: import org.junit.jupiter.api.Assertions;

// JUnit 4
Assert.assertEquals(expected, actual);
Assert.assertTrue(condition);

// JUnit 5
Assertions.assertEquals(expected, actual);
Assertions.assertTrue(condition);        

Assumptions:

• JUnit 4: import org.junit.Assume;
• JUnit 5: import org.junit.jupiter.api.Assumptions;

// JUnit 4
Assume.assumeTrue(condition);

// JUnit 5
Assumptions.assumeTrue(condition);        

By updating your test classes and methods to use these new annotations, assertions, and assumptions, you'll leverage JUnit 5's enhanced features and more flexible testing framework.

Here is a list of all available JUnit 5 annotations:

https://junit.org/junit5/docs/snapshot/user-guide/#writing-tests-annotations

Using JUnit 5’s New Rules and Runners

In JUnit 5, the JUnit Jupiter extension model replaces the runners and rules extension points familiar from JUnit 4. Instead of using rules, you will now use the Extension API.

Replacing @RunWith with @ExtendWith

JUnit 4’s @RunWith annotation is replaced by @ExtendWith in JUnit 5. This requires importing the org.junit.jupiter.api.extension package.

For example, if you previously used the Spring test runner with JUnit 4:

• JUnit 4:

import org.junit.runner.RunWith;
import org.springframework.test.context.junit4.SpringJUnit4ClassRunner;

@RunWith(SpringJUnit4ClassRunner.class)
public class MySpringTest {
    // test methods
}        

• JUnit 5:

import org.junit.jupiter.api.extension.ExtendWith;
import org.springframework.test.context.junit.jupiter.SpringExtension;

@ExtendWith(SpringExtension.class)
public class MySpringTest {
    // test methods
}        

Bonus: Leveraging Modern IDEs for Migration

After learning and understanding all the changes between JUnit 4 and JUnit 5, it's beneficial not to overlook the current capabilities of modern IDEs. IDEs like IntelliJ IDEA, Eclipse, and Visual Studio Code offer a plethora of features that can expedite the development and migration process. These tools provide advanced refactoring options, intelligent code completion, and integrated testing support, which can streamline the transition to JUnit 5.

By utilizing these IDE features, you can save valuable time and effort, allowing you to focus more on writing efficient and effective tests. Plus, you'll have some extra time to enjoy a coffee break. Embracing the synergy between JUnit 5 and your favorite IDE will not only enhance your productivity but also improve the overall quality of your testing framework.

https://www.jetbrains.com/guide/java/tutorials/migrating-junit4-junit5/

Conclusion

Migrating from JUnit 4 to JUnit 5 offers numerous benefits, including improved modularity, enhanced support for Java 8 features, and a more flexible extension model. By following a structured migration process—updating dependencies, transitioning to new annotations, and leveraging the new extension API—you can ensure a smooth and efficient upgrade. The coexistence of JUnit 4 and JUnit 5 during the transition period allows for a gradual migration, minimizing disruptions to your workflow. Embracing JUnit 5 will not only modernize your testing framework but also enhance the maintainability and robustness of your test suites.