JUnit is one of available Java libraries we can use to test our application code.  Let's get started by buliding simple project using Maven.

Generating Project With JUnit Support

Go generate simple Java artifact using Maven, which can be done quite easilly using command line.

mvn archetype:generate -DgroupId={GROUP_ID} -DartifactId={ARTIFACT_ID} -DarchetypeArtifactId=maven-archetype-quickstart -DinteractiveMode=false

Just execute upper line in your console, giving it some {GROUP_ID} and {ARTIFACT_ID}. Make sure you haven Maven installed and on your path, so that you can run it as described. This is not related to JUnit testing, just give it any values, I will use com.mydomainas {GROUP_ID}, and junit-demoas {ARTIFACT_ID}. That should generate new java project ready to demo JUnit tests. You can import generated application into IDE of your choice.

To confirm we have all we need, open generated pom.xmlfile in the root of your project and verify there's dependency to JUnit library in there (I'm using 4.12 version which is the latest stable version of JUnit at the time of writing this article).

<dependency>
  <groupId>junit</groupId>
  <artifactId>junit</artifactId>
  <version>4.12</version>
  <scope>test</scope>
</dependency>

Once we confirm there's given dependency, we will proceed with some theory alongside example code. We will create simple calculator class which can do some basic operations as adding, subtracting, multiplying, dividing numbers. We want to create JUnit tests to make sure the calculator class is working as expected and has production ready quality.

Creating System Under Test (SUT) and Test case

JUnit testing takes care of testing our application at low level, meaning we want to make sure our classe methods are tested against different conditions, and they provide expected results in such a cases. Our methods we want to test can, during application runtime, receive different values for input parameters. We want to make sure our method is tested against these. Of course - not against all possible values, since that's not possible, but against some meaningful values, as well as some edge cases where our methods can receive, let's say  null as parameter, or some other value that could potentially crush our application. Let's start writing com.mydomain.Calculator.javaand JUnit tests for it, com.mydomain.CalculatorTest.java. Test classes are also known as Test cases.

It's a common practice to name test class after class we're testing (sometimes referred to as System Under Test - SUT), by appending Test suffix to it. In our case, our SUT is Calculator.java, so our test class should be CalculatorTest.java. Also, common practice is that test class should reside in same package as SUT one (in our case, both are in com.mydomainpackage, but test class is in src/test/javatree, and SUT is in src/main/java. These are maven defaults which state that application code should go to X, whereas test code should be inside src/testpackages. Our Calculator class should look like:

package com.mydomain;

public class Calculator {

    public long add(long first, long second) {
        return first + second;
    }

    public long subtract(long first, long second) {
        return first - second;
    }

    public long multiply(long first, long second) {
        return first * second;
    }

    public long divide(long first, long second) {
        return first / second;
    }
}

Without writing JUnit tests for given piece of code, we can only HOPE our code will work as expected. We should be able to exercise this piece of code before deploying it to production so that we can be confedent code is working as expected.

Our application should have layout such as given below (target directory is incomplete here, but that's irrelevant):

├── pom.xml
├── src
│   ├── main
│   │   └── java
│   │       └── com
│   │           └── mydomain
│   │               ├── App.java
│   │               └── Calculator.java
│   └── test
│       └── java
│           └── com
│               └── mydomain
│                   ├── AppTest.java
│                   └── CalculatorTest.java
└── target

Let's now write some JUnit tests in order to exercise our application. Our CalculatorTestclass could look like:

package com.mydomain;

import org.junit.Test;

import static org.hamcrest.CoreMatchers.is;
import static org.junit.Assert.assertThat;

public class CalculatorTest {
    @Test
    public void twoAndThreeIsFive() throws Exception {
        final long result = new Calculator().add(2, 3);
        assertThat(result, is(5L));
    }

    @Test
    public void twoAndZeroIsTwo() throws Exception {
        final long result = new Calculator().add(2, 0);
        assertThat(result, is(2L));
    }

    @Test
    public void twoAndMinusTwoIsZero() throws Exception {
        final long result = new Calculator().add(2, -2);
        assertThat(result, is(0L));
    }

    @Test
    public void threeMinusTwoIsOne() throws Exception {
        final long result = new Calculator().subtract(3, 2);
        assertThat(result, is(1L));
    }

    @Test
    public void threeMinusThreeIsZero() throws Exception {
        final long result = new Calculator().subtract(3, 3);
        assertThat(result, is(0L));
    }

    @Test
    public void threeMinusMinusThreeIsSix() throws Exception {
        final long result = new Calculator().subtract(3, -3);
        assertThat(result, is(6L));
    }

    @Test
    public void threeXThreeIsNine() throws Exception {
        final long result = new Calculator().multiply(3, 3);
        assertThat(result, is(9L));
    }

    @Test
    public void threeXZeroIsZero() throws Exception {
        final long result = new Calculator().multiply(3, 0);
        assertThat(result, is(0L));
    }

    @Test
    public void threeXMinusThreeIsMinusNine() throws Exception {
        final long result = new Calculator().multiply(3, -3);
        assertThat(result, is(-9L));
    }

}

Idea is to exercise our public methods present in Calculatorclass. We should give it various inputs, and assert that results are as expected. In order to create JUnit test, we should just mark our method with @TestJUnit annotation (org.junit.Test). When we execute this set of tests we wrote, JUnit will executed all methods marked with @Test annotation.

In each test method we have two phases:

1. Call SUT method for particular input (parameters)
2. Assert that result matches our expectancy (like we assert that 3 x 3 should be 9)

Running Tests and Verifying Results

The easy way to run all the tests is to package our application, using Maven command in our comand line:

mvn clean package

In console output, after our application code was compiled, we can see that JUnit tests are executed:

-------------------------------------------------------
 T E S T S
-------------------------------------------------------
Running com.mydomain.AppTest
Tests run: 1, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 0.013 sec - in com.mydomain.AppTest
Running com.mydomain.CalculatorTest
Tests run: 9, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 0.004 sec - in com.mydomain.CalculatorTest

Results :
Tests run: 10, Failures: 0, Errors: 0, Skipped: 0

We can see that there's total of 9 tests being run in CalculatorTestand that there were no failures (Failures: 0 in log output). If some tests were failing, we would be presented with an error message, and Maven build would have failed. Let's change implementation of threeMinusMinusThreeIsSixmethod to wrongly assert that 3 multiplied by -3 should be 2.

@Test
public void threeMinusMinusThreeIsSix() throws Exception {
    final long result = new Calculator().subtract(3, -3);
    assertThat(result, is(2L));
}

When repeating upper maven command to package app, we will get output:

-------------------------------------------------------
 T E S T S
-------------------------------------------------------
Running com.mydomain.AppTest
Tests run: 1, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 0.013 sec - in com.mydomain.AppTest
Running com.mydomain.CalculatorTest
Tests run: 9, Failures: 1, Errors: 0, Skipped: 0, Time elapsed: 0.027 sec <<< FAILURE! - in com.mydomain.CalculatorTest
threeMinusMinusThreeIsSix(com.mydomain.CalculatorTest) Time elapsed: 0.005 sec <<< FAILURE!
java.lang.AssertionError: 
Expected: is <2L>
 but: was <6L>

....

Tests run: 10, Failures: 1, Errors: 0, Skipped: 0

[INFO] ------------------------------------------------------------------------
[INFO] BUILD FAILURE

We clearly see that build failed, and what's causing build to fail. It's quite handy we can quickly execute tests and easilly see if something is failing and why. Test can fail if SUT has issues, or JUnit tests are not properly written (such as we did above - just for demo purpose).

Bare in mind that JUnit test should have quality standards same as application code, so make sure you apply all the good practices you already apply to application code. Also, JUnit tests should be independent of each other, and must be able to run in any order given. They should also be quick to execute and easy to verify results.

Key Takeaways

• JUnit is library for automated unit testing
• Idea is to excercise the application code for various cases before application goes live, thus preventing issues that might happen during application runtime.
• Each JUnit test should be independent of other tests, and should be able to run fast, so that we can execute them frequently.

The next post will be related to extending JUnit test using Mockito library.

Hope you liked the post and feel free to download application sources at GitHub.

Stay tuned and please - don't forget to subscribe in case you're eager to find out what's coming next in upcoming posts.