JUnit Tutorial: Setting Up, Writing, and Running Java Unit Tests

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JUnit Tutorial: Setting Up, Writing, and Running Java Unit Tests

Want to learn more about how to successfully run a Java unit tests? Check out this tutorial to learn how to set up, write, and run and JUnit test.

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Today, we're going back to unit testing basics with a short JUnit tutorial about how to set up, write, and run your JUnit tests. What is a JUnit, and how do you get started? You can get started by understanding the basics and scaling your unit testing practice like a pro.

What Is Unit Testing?

But, before we go too much further into JUnits, let's talk a little bit about unit testing and regression testing and why they matter in general. We'll get into some good examples as we go on.

Unit testing is a form of white-box testing, in which test cases are based on an internal structure. The tester chooses inputs to explore particular paths and determines the appropriate output. The purpose of unit testing is to examine the individual components or piece of methods/classes to verify functionality, ensuring the behavior is as expected.

The exact scope of a “unit” is often left to interpretation, but a nice rule of thumb is for a unit to contain the least amount of code that performs a standalone task (e.g. a single method or class). There is good reason that we limit scope when unit testing — if we construct a test that incorporates multiple aspects of a project, we have shifted focus from functionality of a single method to interaction between different portions of the code. If the test fails, and we don't know why it failed, we are left wondering whether the point of failure was within the method we were interested in or in the dependencies associated with that method.

Regression Testing

Complementing unit testing, regression testing makes certain that the latest fix, enhancement, or patch did not break existing functionality, by testing the changes you've made to your code. Changes to code are inevitable, whether they are modifications of existing code, or adding packages for new functionality; your code will certainly change. It is in this change that lies the most danger, so, with that in mind, regression testing is a must.

What Is JUnit?

JUnit is a unit testing framework for the Java programming language that plays a big role in regression testing. As an open-source framework, it is used to write and run repeatable automated tests.

As with anything else, the JUnit framework has evolved over time. The major change to make note of is the introduction of annotations that came along with the release of JUnit 4, which provided an increase in organization and readability of JUnits. The rest of this blog post will be written from usages of Junit 4 and 5.

How to Set Up JUnit

So, let's dive right in.

The more common IDEs, such as Eclipse and IntelliJ, will already have JUnit functionality installed by default. If one is not using an IDE and, perhaps, relying solely on a build system, such as Maven or Gradle, the installation of Junit 4/5 is handled via the pom.xml or build.gradle, respectively. It is important to note that Junit 5 was split into three modules, one of those being a vintage module that supports annotation/syntax of Junit 4 and 3 (although, usage of Junit 3 is frankly dismal and typically seen only in much older projects).

JUnit 4

To add JUnit 4 to your Maven, build the following to the pom.xml. Be mindful of your version:


For Gradle, add the following to the build.gradle:

apply plugin: 'java'

dependencies {
testCompile 'junit:junit:4.12'

JUnit 5

Adding JUnit 5 is a bit different. Because of the modular fashion of JUnit 5, a BOM is used to import all aspects. If only particular classes are needed, individual groups or artifacts can be specified.

To add JUnit 5 to Maven, add the following to pom.xml:


For Gradle, add the following to the build.gradle:

apply plugin: 'java'

dependencies {
implementation 'org.junit:junit-bom:5.2.0'

Although not typically needed, the raw jar file, which allows one to use the JUnit framework, is also accessible, if needed to manually put on the class path. Git houses the code for JUnit. JUnit 4 has the jar available to download directly. Junit 5 (as I am writing this) does not currently have the jar file premade, but one can easily compile the code and generate the jars.

Writing Unit Tests: The Anatomy of a JUnit

Now that we have talked a little about unit testing and set-up itself, let's move on to actual construction and execution of these tests. To best illustrate the creation of JUnits, we want to start with something basic. In the example image below, we have a simple method (left) that converts Fahrenheit to Celsius, and the JUnit (right) associated with our method. I've numbered our JUnit and will discuss these sections in detail below.


Sections 1 and 2

These are imports for the JUnit libraries needed to leverage the testing framework. The imported libraries can be specified down to a particular functionality of JUnit, but are commonly imported with asterisks to have access to all functionality.

Section 3

This has the start of our test class. The important thing to take note of here is the naming convention for the class, which follows ClassNameTest.

Section 4

Here, we see our first JUnit-specific syntax, an annotation. Annotations are extremely important when creating JUnits. This is how JUnit knows what to do with the processing section of code. In our example case, we have an @Test annotation, which tells JUnit the public void method to that it is attached to can be run as a test case.

There are many other annotations, but the more common are @Before (which runs some statement/precondition before  @Test , public void), @After  (which runs some statement after  @Test , public void, e.g. resetting variables, deleting temporary files, variables, etc.), and @Ignore   (which ignores some statement during test execution — note that @BeforeClass   and @AfterClass are used for running statements before and after all test cases, public static void, respectively).

Section 5

The take away here is again naming the convention. Note the structure, test  MethodName .

Section 6

Here, we construct a new instance of our class object. This is necessary so that we can call on the method we are testing. Without this object instance, we cannot test the method.

Section 7

Variables associated with the method need to be established, so, here, we declare variables corresponding to our method. These should be given meaningful values (note: if a parameter is an object, one can instantiate it or mock it), so that our test has meaning.

Section 8

This variable declaration could be argued as optional, but it's worthwhile for the sake of organization and readability. We assign the results of our method being tested to this variable, using it as needed for assertions and such.

Section 9

The assert methods (which are part of the org.junit.Assert class) are used in determining pass/fail status of test cases. Only failed assertions are recorded. Like annotations, there are many assert options. In our example JUnit above, we use  assertEquals (expected, actual, delta). This takes in the expected outcome, which the user defines, the actual, which is the result of the method being called, and the delta, which allows for implementing an allowed deviation between expected and actual values. The purpose of an assertion is validation. Although not required to run your JUnit, failing to add assertions arguably defeats the purpose of your test. Without assertions, you have no verification and — at most — a smoke test, which gives feedback only when test errors out.

How to Run a JUnit

Choose your own adventure! Here, we will look at three ways to run JUnits: straight from the command line, from the IDE (Eclipse and IntelliJ), and using build systems (Maven and Gradle).

How to Run a JUnit from the Command Line

To run a JUnit directly from the command line, you need a few things: JDK on your path, raw Junit jar file, and the test cases. The command is as follows (this example is for Junit 4):

java -cp /path/to/junit.jar org.junit.runner.JUnitCore <test class name>

NOTE: It is unlikely, in a professional setting, that one would be running a test manually from the command line, without some build system, but the ability is there.

How to Run a JUnit From the IDE


To run Eclipse from your Package Explorer, locate your JUnit test in whichever folder you have designated it to. Right-click, and move down to Run As > JUnit Test. This will execute your test and open a new JUnit window, if it is not already open.

Run a JUnit from Eclipse


Running a test in IntelliJ is very similar to Eclipse. From the project window, locate test, right-click, and select Run ‘testName.' Like Eclipse, a JUnit window will open with the results of the test.

Run a JUnit from IntelliJ

How to Run a JUnit Using Build Systems


Maven has made running tests simple. Ensure you are in the proper location from your command line, and the project pom.xml is properly configured. Then, you can run the following to execute your JUnits:

To run the entire test suite:

mvn test

To run a single/specific test(s):

mvn -Dtest=TestName test


Gradle, like Maven, has made running tests simple.

To run the entire test suite:

gradlew test

To run a single/specific test(s):

gradlew -Dtest.single=testName test

(NOTE: Maven and Gradle are their own monster — what is shown here is minimal at covering the basics.)

Continuing With Unit Testing

Our example ran through a very simple snippet of code, and, of course, this is just the start of unit testing. More complex methods call databases or other methods, but to reassure functionality, we need isolation, which we achieve through mocking. Mocking helps us isolate units of code to focus our validation (frameworks commonly used for mocking are Mockito and PowerMock). To read more about mocking, read my colleague's post: How to automate a Java unit test, including mocking and assertions.

So, if unit testing is so amazing, why isn't everyone awesome at it? Well, unit testing just isn't easy. It takes a lot of development skill and time. Developers tend not to enjoy it very much, and it takes commitment and time to maintain test suites.

But, the benefits of unit testing are clear:

  • Isolation and focused testing
  • Assuring behavior of individual methods or pieces of methods
  • Helps to insure addition or modification of code does not break application
  • Boundary analysis — checking for invalid/bad input
  • Test every aspect of the method (coverage)
  • Coverage

So, it's helpful to deploy powerful unit testing tools, like Parasoft Jtest, that can remedy much of the pain associated with JUnits and save developers valuable time. Parasoft Jtest's Unit Test Assistant takes on the headache of time, reducing JUnit test generation time by guiding the user through test creation with intuitive one-click actions. Maintainability is addressed with automatic recommendations that identify things like unstable test environments (and other risks), assertion recommendations, and more.

To find out more on how you can ease the pain and elevate your unit testing, download a free trial of the Parasoft Jtest Unit Test Assistant. It's free, easy to set up, and you can follow all of the tutorials here to efficiently get started.

dependency, gradle, ide, java, junit, maven, regression testing, tutorial

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