Maven Java Project Unit Testing

Maven's promise is that broken code does not get packaged. That promise depends on unit tests: small pieces of code that call your methods and check the results. If any test fails, mvn test and mvn package fail too. In this lesson we write one class and one test by hand, so you can see exactly where each file goes in the Maven structure and how Maven reacts when a test passes and when it fails.

Create a simple project in Eclipse

This time we skip the archetype. In Eclipse choose File > New > Maven Project, tick Create a simple project (skip archetype selection), click Next, and enter:

Click Finish. A simple project gives you all four standard folders and an almost empty POM:

JavaApp/
├── pom.xml
└── src/
    ├── main/java/          <- application code
    ├── main/resources/     <- application config
    ├── test/java/          <- unit tests
    └── test/resources/     <- test config

Any IDE works. In IntelliJ, a new Maven project gives you the same folders.

Write the class under test

Right click src/main/java, choose New > Package, name it com.telusko.sampleApp, then create a class Calc in it:

package com.telusko.sampleApp;

public class Calc {

    int divide(int num1, int num2) {
        return num1 / num2;
    }
}

Java package names are normally all lowercase (com.telusko.sampleapp). We keep sampleApp to match the video. It compiles either way.

Add JUnit 5 to the POM

The test will use JUnit 5 (JUnit Jupiter), the standard unit testing library for Java. It is not part of the JDK, so it is a dependency. On Maven Central (the video uses mvnrepository.com, category "Testing Frameworks & Tools"), find junit-jupiter-api and copy version 5.13.4. Add it inside <dependencies> with test scope, and set the Java version while you are in the POM:

<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">
  <modelVersion>4.0.0</modelVersion>
  <groupId>com.telusko</groupId>
  <artifactId>JavaApp</artifactId>
  <version>0.0.1-SNAPSHOT</version>

  <properties>
    <maven.compiler.release>17</maven.compiler.release>
    <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
  </properties>

  <dependencies>
    <dependency>
      <groupId>org.junit.jupiter</groupId>
      <artifactId>junit-jupiter-api</artifactId>
      <version>5.13.4</version>
      <scope>test</scope>
    </dependency>
  </dependencies>
</project>

Save, then right click the project and choose Maven > Update Project.... Under Maven Dependencies you now see junit-jupiter-api-5.13.4.jar plus its transitive dependencies (opentest4j, junit-platform-commons, apiguardian-api).

Write the test

Tests mirror the main code. Create the same package com.telusko.sampleApp under src/test/java, and in it a class CalcTest:

package com.telusko.sampleApp;

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

import org.junit.jupiter.api.Test;

public class CalcTest {

    @Test
    void testCalc() {
        Calc calc = new Calc();                   // the class under test
        int actualResult = calc.divide(10, 5);    // what the code really does
        int expectedResult = 2;                   // what we expect

        assertEquals(expectedResult, actualResult);
    }
}

Run the test

From Eclipse

Right click the project or the test class and choose Run As > Maven test. The console ends with:

Tests run: 1, Failures: 0, Errors: 0, Skipped: 0
[INFO] BUILD SUCCESS

You can also choose Run As > JUnit Test, which uses Eclipse's own runner and shows a green bar. Both are fine during development, but only the Maven run is what your CI server will do.

From the terminal

Open a terminal in the project folder (the one with pom.xml):

mvn clean
mvn test

test compiles the main code, compiles the tests and runs them. We do not need package here, because we only want to know whether the tests pass.

Make it fail on purpose

Change the expectation to a wrong value, save, and run mvn test again:

int expectedResult = 5;   // wrong on purpose

A failing test stops the build. Surefire reports which test failed, the expected and actual values, and where to find the detailed report.

Read the output from the Results section down:

Fix the expectation back to 2, run mvn clean test again, and you get Failures: 0 and BUILD SUCCESS.

Handy test commands

Try It Yourself

1. Create JavaApp with Calc and CalcTest, and get a passing mvn test from the terminal.
2. Break the test, read the failure line, open the .txt report in target/surefire-reports, then fix it.
3. Add a second test that checks dividing by zero throws an exception: assertThrows(ArithmeticException.class, () -> calc.divide(10, 0));
4. Rename CalcTest to CalcChecker and run mvn test. Notice that no test runs. Rename it back.
5. Run mvn package with the test still failing and confirm no JAR is created.

Official references

• Apache Maven: Surefire Plugin
• Apache Maven: Surefire with the JUnit Platform
• JUnit User Guide

Written By: Shramik Masti