JUnit @DisplayNameGeneration Annotation

JUnit is a popular testing framework in the Java ecosystem that simplifies writing and running tests. The @DisplayNameGeneration annotation in JUnit 5 is used to specify a naming strategy for test methods and test classes. This guide covers the basics of using the @DisplayNameGeneration annotation to customize the display names of your tests.

Table of Contents

1. Introduction
2. Steps to Use @DisplayNameGeneration
3. Real-World Use Case
4. Conclusion

Introduction

The @DisplayNameGeneration annotation allows you to specify a custom naming strategy for your test classes and methods. This is useful for making your tests more readable and understandable, especially when you want a consistent naming convention across your test suite.

Steps to Use @DisplayNameGeneration

Step 1: Add Maven Dependency

To use JUnit in your project, you need to add the JUnit dependency to your pom.xml file. Use the latest version of JUnit 5:

<dependency>
    <groupId>org.junit.jupiter</groupId>
    <artifactId>junit-jupiter-engine</artifactId>
    <version>5.9.2</version>
    <scope>test</scope>
</dependency>

Step 2: Create the Class to be Tested

Create a Java class with methods that you want to test. For example, a simple Calculator class:

public class Calculator {
    public int add(int a, int b) {
        return a + b;
    }

    public int subtract(int a, int b) {
        return a - b;
    }
}

Step 3: Create the Display Name Generator

JUnit provides built-in display name generators such as ReplaceUnderscores and IndicativeSentences. You can also create a custom display name generator by implementing the DisplayNameGenerator interface.

Using Built-in Display Name Generators:

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

@DisplayNameGeneration(DisplayNameGenerator.ReplaceUnderscores.class)
public class CalculatorTest {
    private final Calculator calculator = new Calculator();

    @Test
    void test_addition_of_two_numbers() {
        assertEquals(5, calculator.add(2, 3), "2 + 3 should equal 5");
    }

    @Test
    void test_subtraction_of_two_numbers() {
        assertEquals(1, calculator.subtract(3, 2), "3 - 2 should equal 1");
    }
}

In this example, the CalculatorTest class is annotated with @DisplayNameGeneration(DisplayNameGenerator.ReplaceUnderscores.class), which replaces underscores in method names with spaces to generate more readable display names.

Creating a Custom Display Name Generator:

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

@DisplayNameGeneration(CamelCaseToSpaces.class)
public class CalculatorTest {
    private final Calculator calculator = new Calculator();

    @Test
    void testAdditionOfTwoNumbers() {
        assertEquals(5, calculator.add(2, 3), "2 + 3 should equal 5");
    }

    @Test
    void testSubtractionOfTwoNumbers() {
        assertEquals(1, calculator.subtract(3, 2), "3 - 2 should equal 1");
    }
}

class CamelCaseToSpaces extends DisplayNameGenerator.Standard {
    @Override
    public String generateDisplayNameForMethod(Class<?> testClass, java.lang.reflect.Method testMethod) {
        return testMethod.getName().replaceAll("([a-z])([A-Z]+)", "$1 $2");
    }
}

In this example, the CamelCaseToSpaces class implements the DisplayNameGenerator interface and provides a custom naming strategy that converts camel case method names into space-separated words.

Step 4: Run the Test

You can run the test using your IDE, Maven, or Gradle.

Using an IDE:

Most IDEs, like IntelliJ IDEA and Eclipse, have built-in support for running JUnit tests. Simply right-click on your test class or method and select “Run.”

Using Maven:

If you’re using Maven, you can run your tests with the following command:

mvn test

Using Gradle:

If you’re using Gradle, you can run your tests with the following command:

gradle test

Real-World Use Case

In real-world applications, custom display name generators can help enforce a consistent naming convention across your test suite, making it easier to understand and maintain. For example, a UserService class might have methods for creating, updating, and deleting users.

Create the Class to be Tested

Create a Java class that contains business logic. For example, a UserService class:

public class UserService {
    public boolean createUser(String username) {
        // Simulate user creation
        return true;
    }

    public boolean updateUser(String username) {
        // Simulate user update
        return true;
    }

    public boolean deleteUser(String username) {
        // Simulate user deletion
        return true;
    }
}

Create the Test Class with Custom Display Name Generator

Create a test class for the UserService in the src/test/java directory. Use the @DisplayNameGeneration annotation to specify a custom display name generator.

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

@DisplayNameGeneration(SentenceCaseDisplayNameGenerator.class)
public class UserServiceTest {
    private final UserService userService = new UserService();

    @Test
    void createUser() {
        assertTrue(userService.createUser("john_doe"), "User should be created successfully");
    }

    @Test
    void updateUser() {
        assertTrue(userService.updateUser("john_doe"), "User should be updated successfully");
    }

    @Test
    void deleteUser() {
        assertTrue(userService.deleteUser("john_doe"), "User should be deleted successfully");
    }
}

class SentenceCaseDisplayNameGenerator extends DisplayNameGenerator.Standard {
    @Override
    public String generateDisplayNameForMethod(Class<?> testClass, java.lang.reflect.Method testMethod) {
        return testMethod.getName().substring(0, 1).toUpperCase() + testMethod.getName().substring(1).replaceAll("([a-z])([A-Z]+)", "$1 $2");
    }
}

In this example, the SentenceCaseDisplayNameGenerator class implements the DisplayNameGenerator interface and provides a custom naming strategy that converts method names into sentence case with space-separated words.

Running the Tests

You can run the tests using your IDE, Maven, or Gradle.

Using an IDE:

Most IDEs, like IntelliJ IDEA and Eclipse, have built-in support for running JUnit tests. Simply right-click on your test class or method and select “Run.”

Using Maven:

If you’re using Maven, you can run your tests with the following command:

mvn test

Using Gradle:

If you’re using Gradle, you can run your tests with the following command:

gradle test

Conclusion

The @DisplayNameGeneration annotation in JUnit makes it easy to specify a custom naming strategy for your test classes and methods. By using @DisplayNameGeneration, you can enforce consistent and readable naming conventions across your test suite. Understanding and using the @DisplayNameGeneration annotation effectively is crucial for developing robust and maintainable Java applications.