Test Main Method with JUnit

The entry point for any Java application is the main() method, and its structure can vary based on the application type. For typical web applications, the main() method initiates the context, while console applications may include business logic within it. Testing the main() method poses challenges due to its static nature, accepting only string arguments and having no return value. This complexity arises from the unique characteristics of the main() method, making it intricate to assess and validate effectively. Let us delve into a practical approach to Junit Test a Main method.

1. Introduction

JUnit is a powerful and widely used testing framework for Java that simplifies the process of writing and executing test cases. It plays a crucial role in ensuring the reliability and correctness of Java applications throughout the development lifecycle.

1.1 Features of JUnit

  • Annotations: JUnit provides a set of annotations such as @Test, @Before, @After, etc., making it easy to define and manage test cases and setup/teardown procedures.
  • Test Runners: JUnit supports various test runners, allowing developers to run tests in different environments and configurations. This flexibility is particularly useful for integration and system testing.
  • Assertions: JUnit offers a range of assertion methods for validating expected outcomes. These assertions make it straightforward to check whether the actual results match the anticipated results.

1.2 Benefits of JUnit

  • Automated Testing: JUnit facilitates the automation of test cases, enabling developers to quickly and repeatedly test code changes. This leads to faster development cycles and higher code quality.
  • Early Detection of Defects: By running tests automatically, JUnit helps identify issues early in the development process. This reduces the likelihood of defects reaching production, saving time and resources.
  • Improved Code Quality: Writing testable code is a good programming practice. JUnit encourages developers to create modular and loosely coupled code, resulting in improved overall code quality.

2. How to Test the Main Method?

Unit testing the main() method in Java can be challenging because it often involves interacting with the environment (e.g., reading command-line arguments). However, you can test the logic contained in the main() method by refactoring the code to separate concerns. Here’s an example using JUnit and Mockito for testing the main method.

2.1 pom.xml

If you are using Maven for your Java project, you may need to make changes to your project’s pom.xml file to include dependencies for testing frameworks (e.g., JUnit) and mocking frameworks (e.g., Mockito). Additionally, you might want to include a plugin for running tests during the build process.

Here’s an example of how you can modify your pom.xml file to include JUnit and Mockito dependencies and the Maven Surefire Plugin for running tests:

  • JUnit Dependency: The section includes the JUnit dependency. The scope is set to test to indicate that it is only needed during the testing phase.
  • Mockito Dependency: Similar to JUnit, the org.mockito dependency is included with a test scope.
  • Maven Surefire Plugin: The <build> section includes the Maven Surefire Plugin, which is responsible for running tests during the build process. This plugin is configured to use version 3.0.0-M5.
	<!-- JUnit dependency -->

	<!-- Mockito dependency -->

		<!-- Maven Surefire Plugin for running tests -->

After making these changes, you can run your tests using Maven commands such as:

  • mvn clean test: Cleans the project and runs tests.
  • mvn test: Runs tests without cleaning.

2.2 Java Class

Suppose you have a simple Java class with a main() method:

public class MyApp {

    public static void main(String[] args) {
        if (args.length > 0) {
            String input = args[0];
            String result = processInput(input);
            System.out.println("Result: " + result);
        } else {
            System.out.println("No input provided.");

    public static String processInput(String input) {
        // Some processing logic, for example, converting input to uppercase
        return input.toUpperCase();

2.3 JUnit Test

Now, let’s create a JUnit test for the processInput() method and simulate the main() method behavior:

import org.junit.Test;
import org.junit.runner.RunWith;
import org.mockito.InjectMocks;
import org.mockito.Mock;
import org.mockito.junit.MockitoJUnitRunner;

import static org.junit.Assert.assertEquals;
import static org.mockito.Mockito.when;

public class MyAppTest {

    private String mockedInput;

    private MyApp myApp;

    public void testProcessInput() {
        // Arrange
        // Act
        String result = myApp.processInput(mockedInput);
        // Assert
        assertEquals("MOCKED_INPUT", result);

    public void testMainMethodWithInput() {
        // Arrange
        String[] args = {"input"};
        // Mock the System.out stream for testing output
        System.setOut(new PrintStream(outputStream));
        // Act
        // Assert
        assertEquals("Result: INPUT\n", outputStream.toString());

    public void testMainMethodWithoutInput() {
        // Arrange
        String[] args = {};
        // Mock the System.out stream for testing output
        System.setOut(new PrintStream(outputStream));
        // Act
        // Assert
        assertEquals("No input provided.\n", outputStream.toString());

In this example:

  • MyAppTest uses JUnit and Mockito to test the processInput() method by mocking the input.
  • testMainMethodWithInput and testMainMethodWithoutInput simulate the behavior of the main() method by redirecting System.out and capturing the output for assertion.

Keep in mind that testing the main() method directly might not always be the most practical approach. It’s often more effective to focus on testing the methods or components that the main() method calls.

3. Conclusion

In conclusion, testing the main() method in Java poses unique challenges due to its role as the entry point for the application and its often direct interaction with the environment. While direct testing of the main() method might not always be the most practical or valuable approach, focusing on testing the underlying logic and components it calls is essential for ensuring code reliability and maintainability. Refactoring the code for better testability by separating concerns, extracting logic into modular methods or classes, and embracing good design practices can significantly enhance the ease of testing. Unit testing frameworks like JUnit and mocking frameworks like Mockito provide powerful tools for creating robust and automated test suites. The demonstrated example illustrated how to approach testing Java code, incorporating JUnit and Mockito dependencies in a Maven project. By writing unit tests for individual methods and components, developers can ensure that their code behaves as expected, adheres to specifications, and handles various scenarios gracefully. Additionally, the use of parameterized tests, mocking external dependencies and considering integration tests further enriches the testing strategy. Redirecting standard input/output streams and providing command-line arguments programmatically enables comprehensive testing of the main() method’s behavior. Ultimately, effective testing practices contribute to early defect detection, reduced debugging efforts, and improved overall code quality.


An experience full-stack engineer well versed with Core Java, Spring/Springboot, MVC, Security, AOP, Frontend (Angular & React), and cloud technologies (such as AWS, GCP, Jenkins, Docker, K8).
Notify of

This site uses Akismet to reduce spam. Learn how your comment data is processed.

Inline Feedbacks
View all comments
Back to top button