Writing Templates for Test Cases Using JUnit 5

Writing consistent and reusable test case templates is essential for maintaining quality and ensuring uniformity in large-scale projects. Test case templates provide a structured approach for documenting and executing test scenarios, making it easier to identify issues, automate testing, and validate functionality across different modules of an application.

This article will guide you through creating effective test case templates using JUnit 5, focusing on standard annotations, assertions, and reusable testing patterns that can streamline your testing efforts. Whether you are writing tests for small modules or larger systems, having a well-defined template ensures consistency and reduces maintenance overhead.

Why Use Test Case Templates?

Using test case templates offers several advantages:

• Consistency: Templates ensure that all test cases follow the same format, making them easier to read and understand.
• Efficiency: Reusable templates save time by reducing the need to write boilerplate code for each test case.
• Clarity: Well-defined templates help in clearly documenting the purpose and expected outcome of each test case, facilitating better communication among team members.
• Ease of Maintenance: When the format is standardized, updating or modifying test cases becomes a straightforward task.

Basic Structure of a Test Case Template

A typical test case template for JUnit 5 might include the following sections:

• Test Case ID: A unique identifier for each test case.
• Test Description: A brief overview of what the test case verifies.
• Preconditions: Any setup or initial conditions required before running the test.
• Test Steps: Step-by-step instructions to execute the test.
• Expected Result: The anticipated outcome if the test passes.
• Actual Result: The actual outcome when the test is executed (filled out during testing).
• Status: Pass/Fail based on the comparison of expected results with actual results.

Setting Up JUnit 5 with Maven

Before starting with JUnit 5, ensure you have the following:

1. Java Development Kit (JDK): JDK 8 or higher is required.
2. Maven or Gradle: Ensure you have a build tool like Maven or Gradle set up in your project to manage dependencies.
3. JUnit 5 Dependency: Add the JUnit 5 dependency to your pom.xml (for Maven) or build.gradle (for Gradle) file.

For Maven, add the following dependencies:

<dependencies>
    <dependency>
        <groupId>org.junit.jupiter</groupId>
        <artifactId>junit-jupiter-api</artifactId>
        <version>5.7.0</version>
        <scope>test</scope>
    </dependency>
    <dependency>
        <groupId>org.junit.jupiter</groupId>
        <artifactId>junit-jupiter-engine</artifactId>
        <version>5.7.0</version>
        <scope>test</scope>
    </dependency>
</dependencies>

Writing Test Cases in JUnit 5

JUnit 5 introduces the Jupiter API as the foundation for writing tests. The key components of JUnit 5 test writing are:

• Test Methods: The methods where you define your tests.
• Annotations: Special markers that JUnit uses to recognize test methods, setup, teardown, etc.
• Assertions: Statements used to verify the expected outcome of a test.

Key Annotations

• @Test: Marks a method as a test case.
• @BeforeEach: Runs before each test, useful for setting up the environment.
• @AfterEach: Runs after each test, used for cleanup.
• @BeforeAll and @AfterAll: Run once before/after all tests in the class.
• @Disabled: Skips the test method.

JUnit Jupiter annotations can be used as meta-annotations, allowing you to define custom annotations that inherit the semantics of their meta-annotations. For example, to create a custom annotation named @Fast that combines the @Tag and @Test annotations:

import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
import org.junit.jupiter.api.Tag;
import org.junit.jupiter.api.Test;

@Target(ElementType.METHOD)
@Retention(RetentionPolicy.RUNTIME)
@Tag("fast")
@Test
public @interface FastTest {
}

The following @Test method demonstrates usage of the @Fast annotation.

@Fast @Test void myFastTest()
{
    // ...
}

You can even take that one step further by introducing a custom @FastTest annotation that can be used as a drop-in replacement for @Tag("fast") and @Test.

import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
import org.junit.jupiter.api.Tag;
import org.junit.jupiter.api.Test;

@Target(ElementType.METHOD)
@Retention(RetentionPolicy.RUNTIME)
@Tag("fast")
@Test
public @interface FastTest
{
}

Usage in a test method:

@Fast 
@Test 
void myFastTest() {
    // ...
}

You can also create a custom composed annotation that combines @Tag("fast") and @Test:

import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
import org.junit.jupiter.api.Tag;
import org.junit.jupiter.api.Test;

@Target(ElementType.METHOD)
@Retention(RetentionPolicy.RUNTIME)
@Tag("fast")
@Test
public @interface FastTest {}

Using Assertions

JUnit Jupiter comes with various assertion methods, enhancing the testing experience with static methods found in the org.junit.jupiter.api.Assertions class. This guide explores standard assertions, grouped assertions, exception testing, timeout testing, and how to use third-party assertion libraries like AssertJ and Hamcrest.

1. Standard Assertions

The following Java code demonstrates standard assertions in JUnit 5 using a Calculator and Person class.

import static java.time.Duration.ofMillis;
import static java.time.Duration.ofMinutes;
import static org.junit.jupiter.api.Assertions.*;

import example.domain.Person;
import example.util.Calculator;
import java.util.concurrent.CountDownLatch;
import org.junit.jupiter.api.Test;

class AssertionsDemo {

    private final Calculator calculator = new Calculator();
    private final Person person = new Person("Jane", "Doe");

    @Test
    void standardAssertions() {
        assertEquals(2, calculator.add(1, 1));
        assertEquals(4, calculator.multiply(2, 2), 
                     "The optional failure message is now the last parameter");
        assertTrue('a' < 'b', 
                   () -> "Assertion messages can be lazily evaluated -- "
                       + "to avoid constructing complex messages unnecessarily.");
    }

    @Test
    void groupedAssertions() {
        assertAll("person",
                  () -> assertEquals("Jane", person.getFirstName()),
                  () -> assertEquals("Doe", person.getLastName()));
    }

    @Test
    void dependentAssertions() {
        assertAll("properties",
                  () -> {
                      String firstName = person.getFirstName();
                      assertNotNull(firstName);
                      assertAll("first name",
                                () -> assertTrue(firstName.startsWith("J")),
                                () -> assertTrue(firstName.endsWith("e")));
                  },
                  () -> {
                      String lastName = person.getLastName();
                      assertNotNull(lastName);
                      assertAll("last name",
                                () -> assertTrue(lastName.startsWith("D")),
                                () -> assertTrue(lastName.endsWith("e")));
                  });
    }

    @Test
    void exceptionTesting() {
        Exception exception = assertThrows(ArithmeticException.class, 
                                            () -> calculator.divide(1, 0));
        assertEquals("/ by zero", exception.getMessage());
    }

    @Test
    void timeoutNotExceeded() {
        assertTimeout(ofMinutes(2), () -> {
            // Perform task that takes less than 2 minutes.
        });
    }

    @Test
    void timeoutExceeded() {
        assertTimeout(ofMillis(10), () -> {
            Thread.sleep(100); // Simulate task that takes more than 10 ms.
        });
    }

    @Test
    void timeoutExceededWithPreemptiveTermination() {
        assertTimeoutPreemptively(ofMillis(10), () -> {
            new CountDownLatch(1).await(); // Simulate long-running task.
        });
    }
}

• Standard Assertions: Use assertEquals and assertTrue to check basic outcomes.
• Grouped Assertions: assertAll is used to execute multiple assertions, reporting all failures together.
• Dependent Assertions: Further checks only execute if previous assertions pass.
• Exception Testing: assertThrows checks for expected exceptions.
• Timeout Testing: assertTimeout ensures tasks complete within specified time limits.

2. Kotlin Assertion Support

In Kotlin, JUnit Jupiter offers top-level assertion functions for a concise syntax. Below is an example.

import example.domain.Person
import example.util.Calculator
import org.junit.jupiter.api.Assertions.*
import org.junit.jupiter.api.Test
import java.time.Duration

class KotlinAssertionsDemo {
    private val person = Person("Jane", "Doe")
    private val people = setOf(person, Person("John", "Doe"))

    @Test
    fun `exception absence testing`() {
        val calculator = Calculator()
        val result = assertDoesNotThrow("Should not throw an exception") {
            calculator.divide(0, 1)
        }
        assertEquals(0, result)
    }

    @Test
    fun `expected exception testing`() {
        val calculator = Calculator()
        val exception = assertThrows<ArithmeticException>("Should throw an exception") {
            calculator.divide(1, 0)
        }
        assertEquals("/ by zero", exception.message)
    }

    @Test
    fun `grouped assertions`() {
        assertAll("Person properties",
            { assertEquals("Jane", person.firstName) },
            { assertEquals("Doe", person.lastName) })
    }
}

• The Kotlin code mirrors the Java assertions with more concise syntax.
• assertDoesNotThrow checks for methods that should not throw exceptions.

3. Third-party Assertion Libraries

For advanced assertions, you can leverage libraries like Hamcrest or AssertJ. Below is an example using Hamcrest.

import static org.hamcrest.CoreMatchers.equalTo;
import static org.hamcrest.CoreMatchers.is;
import static org.hamcrest.MatcherAssert.assertThat;

import example.util.Calculator;
import org.junit.jupiter.api.Test;

class HamcrestAssertionsDemo {
    private final Calculator calculator = new Calculator();

    @Test
    void assertWithHamcrestMatcher() {
        assertThat(calculator.subtract(4, 1), is(equalTo(3)));
    }
}

The Hamcrest library provides a fluent API for writing assertions, enhancing readability.

Step-by-Step Example

Step 1: Create the Class to be Tested

Create a simple class Calculator:

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

Step 2: Write JUnit 5 Test Cases

Create test cases to validate the Calculator class:

import static org.junit.jupiter.api.Assertions.*;

import org.junit.jupiter.api.BeforeEach;
import org.junit.jupiter.api.Test;

public class CalculatorTest {

    private Calculator calculator;

    @BeforeEach
    void setUp() {
        calculator = new Calculator();
    }

    @Test
    void testAddition() {
        int result = calculator.add(2, 3);
        assertEquals(5, result);
    }

    @Test
    void testSubtraction() {
        int result = calculator.subtract(5, 3);
        assertEquals(2, result);
    }
}

The test cases cover addition and subtraction functionality, using assertEquals to verify the outcomes.

Running and Verifying Tests

Run the tests using your IDE or Maven command:

mvn test

Output:

You should see the output indicating whether the tests passed successfully.

Conclusion

JUnit 5 simplifies writing and managing unit tests in Java with its clean syntax and powerful assertion capabilities. This guide covered various assertion methods, exception handling, timeout testing, and the integration of third-party assertion libraries. Expanding your tests with advanced scenarios and additional assertion features can enhance code quality and reliability.