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Unit Testing with JUnit - Tutorial

Lars Vogel

Version 2.4


Revision History
Revision 0.1 - 2.4 03.09.2007 - 28.10.2013 Lars
created, bugfixes and enhancements


This tutorial explains unit testing with JUnit 4.x. It explains the creation of JUnit tests and how to run them in Eclipse or via own code.

Table of Contents

1. Types of tests
1.1. Unit tests and unit testing
1.2. Test fixture
1.3. Functional and integration tests
1.4. Performance tests
2. Test organization
2.1. Test organization for Java projects
2.2. What should you test?
2.3. Introducing tests in legacy code
3. Using JUnit
3.1. Unit testing with JUnit
3.2. Available JUnit annotations
3.3. Assert statements
3.4. Create a JUnit test suite
3.5. Run your test outside Eclipse
4. Installation of JUnit
4.1. Using JUnit integrated into Eclipse
4.2. Downloading the JUnit library
5. Eclipse support for JUnit
5.1. Creating JUnit tests
5.2. Running JUnit tests
5.3. JUnit static imports
5.4. Wizard for creating test suites
5.5. Testing exception
6. Exercise: Using JUnit
6.1. Project preparation
6.2. Create a Java class
6.3. Create a JUnit test
6.4. Run your test in Eclipse
7. Advanced JUnit options
7.1. Parameterized test
7.2. Rules
8. Mocking
9. Support free vogella tutorials
9.1. Thank you
9.2. Questions and Discussion
10. Links and Literature
10.1. JUnit Resources
10.2. vogella Resources

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1. Types of tests

1.1. Unit tests and unit testing

A unit test is a piece of code written by a developer that executes a specific functionality in the code to be tested. The percentage of code which is tested by unit tests is typically called test coverage.

A unit test targets a small unit of code, e.g., a method or a class, (local tests).

Unit tests ensure that code works as intended. They are also very helpful to ensure that the code still works as intended in case you need to modify code for fixing a bug or extending functionality. Having a high test coverage of your code allows you to continue developing features without having to perform lots of manual tests.

1.2. Test fixture

The test fixture is a fixed state of the software under test used as a baseline for running tests.

1.3. Functional and integration tests

An integration test has the target to test the behavior of a component or the integration between a set of components. The term functional test is sometimes used as synonym for integration test.

These kind of tests allow you to translate your user stories into a test suite, i.e., the test would resemble a expected user interaction with the application.

1.4. Performance tests

Performance tests are used to benchmark software components in a repeatable way.

2. Test organization

2.1. Test organization for Java projects

Typically unit tests are created in a separate project or separate source folder to avoid that the normal code and the test code is mixed.

2.2. What should you test?

What should be tested is a hot topic for discussion. Some developers believe every statement in your code should be tested.

In general it is save to ignore trivial code as ,for example, getter and setter methods which simply assign values to fields. Writing tests for these statements is time consuming and pointless, as you would be testing the Java virtual machine. The JVM itself already has test cases for this and you are safe to assume that field assignment works in Java if you are developing end user applications.

You should write software tests in any case for the critical and complex parts of your application. A solid test suite also protects you against regression in existing code if you introduce new features.

2.3. Introducing tests in legacy code

If you start developing tests for an existing code base without any tests, it is good practice to start writing tests for the parts of the application in which most errors happened in the past. This way you can focus on the critical parts of your application.

3. Using JUnit

3.1. Unit testing with JUnit

JUnit in version 4.x is a test framework which uses annotations to identify methods that specify a test. Typically these test methods are contained in a class which is only used for testing. It is typically called a Test class.

The following code shows a JUnit test method . It can be created via FileNewJUnitJUnit Test case.

public void testMultiply() {

   // MyClass is tested
   MyClass tester = new MyClass();
   // check if multiply(10,5) returns 50
   assertEquals("10 x 5 must be 50", 50, tester.multiply(10, 5));

JUnit assumes that all test methods can be executed in an arbitrary order. Therefore tests should not depend on other tests.

To write a test with JUnit you annotate a method with the @org.junit.Test annotation and use a method provided by JUnit to check the expected result of the code execution versus the actual result.

You can use the Eclipse user interface to run the test, via right-click on the test class and selecting Run Run AsJUnit Test. Outside of Eclipse you can use the org.junit.runner.JUnitCore class to run the test. You can also mark a test method in a class and select to run the test. Only the selected method will be executed.

3.2. Available JUnit annotations

The following table gives an overview of the available annotations in JUnit 4.x.

Table 1. Annotations

Annotation Description

public void method()

The @Test annotation identifies a method as a test method.
@Test (expected = Exception.class) Fails if the method does not throw the named exception.
@Test(timeout=100) Fails if the method takes longer than 100 milliseconds.
public void method()
This method is executed before each test. It is used to prepare the test environment (e.g., read input data, initialize the class).
public void method()
This method is executed after each test. It is used to cleanup the test environment (e.g., delete temporary data, restore defaults). It can also save memory by cleaning up expensive memory structures.
public static void method()
This method is executed once, before the start of all tests. It is used to perform time intensive activities, for example, to connect to a database. Methods marked with this annotation need to be defined as static to work with JUnit.
public static void method()
This method is executed once, after all tests have been finished. It is used to perform clean-up activities, for example, to disconnect from a database. Methods annotated with this annotation need to be defined as static to work with JUnit.
@Ignore Ignores the test method. This is useful when the underlying code has been changed and the test case has not yet been adapted. Or if the execution time of this test is too long to be included.

3.3. Assert statements

JUnit provides static methods in the Assert class to test for certain conditions. These assertion methods typically start with assert and allow you to specify the error message, the expected and the actual result. An assertion method compares the actual value returned by a test to the expected value, and throws an AssertionException if the comparison test fails.

The following table gives an overview of these methods. Parameters in [] brackets are optional.

Table 2. Test methods

Statement Description
fail(String) Let the method fail. Might be used to check that a certain part of the code is not reached or to have a failing test before the test code is implemented. The String parameter is optional.
assertTrue([message], boolean condition) Checks that the boolean condition is true.
assertFalse([message], boolean condition) Checks that the boolean condition is false.
assertEquals([String message], expected, actual) Tests that two values are the same. Note: for arrays the reference is checked not the content of the arrays.
assertEquals([String message], expected, actual, tolerance) Test that float or double values match. The tolerance is the number of decimals which must be the same.
assertNull([message], object) Checks that the object is null.
assertNotNull([message], object) Checks that the object is not null.
assertSame([String], expected, actual) Checks that both variables refer to the same object.
assertNotSame([String], expected, actual) Checks that both variables refer to different objects.


You should provide meaningful messages in assertions so that it is easier for the developer to identify the problem. This helps in fixing the issue, especially if someone looks at the problem, who did not write the code under test or the test code.

3.4. Create a JUnit test suite

If you have several test classes, you can combine them into a test suite. Running a test suite will execute all test classes in that suite in the specified order.

The following example code shows a test suite which defines that two test classes should be executed. If you want to add another test class you can add it to @Suite.SuiteClasses statement.

package com.vogella.junit.first;

import org.junit.runner.RunWith;
import org.junit.runners.Suite;
import org.junit.runners.Suite.SuiteClasses;

@SuiteClasses({ MyClassTest.class, MySecondClassTest.class })
public class AllTests {


3.5. Run your test outside Eclipse

Eclipse provides support for running your test interactively in the Eclipse IDE. You can also run your JUnit tests outside Eclipse via standard Java code. The org.junit.runner.JUnitCore class provides the runClasses() method which allows you to run one or several tests classes. As a return parameter you receive an object of the type org.junit.runner.Result. This object can be used to retrieve information about the tests.

In your test folder create a new class MyTestRunner with the following code. This class will execute your test class and write potential failures to the console.

package de.vogella.junit.first;

import org.junit.runner.JUnitCore;
import org.junit.runner.Result;
import org.junit.runner.notification.Failure;

public class MyTestRunner {
  public static void main(String[] args) {
    Result result = JUnitCore.runClasses(MyClassTest.class);
    for (Failure failure : result.getFailures()) {

To run your JUnit tests outside Eclipse you need to add the JUnit library jar to the classpath of your program. Typically build frameworks like Apache Ant or Apache Maven are used to execute tests automatically on a regular basis.

4. Installation of JUnit

4.1. Using JUnit integrated into Eclipse

Eclipse allows you to use the version of JUnit which is integrated in Eclipse. If you use Eclipse, no additional setup is required. In this case you can skip the following section.

4.2. Downloading the JUnit library

If you want to control the used JUnit library explicitly, download JUnit4.x.jar from the following JUnit website. The download contains the junit-4.*.jar which is the JUnit library. Add this library to your Java project and add it to the classpath. 

5. Eclipse support for JUnit

5.1. Creating JUnit tests

You can write the JUnit tests manually, but Eclipse supports the creation of JUnit tests via wizards.

For example, to create a JUnit test or a test class for an existing class, right-click on your new class, select this class in the Package Explorer view, right-click on it and select NewJUnit Test Case.

Alternatively you can also use the JUnit wizards available under FileNew Other...JavaJUnit.

5.2. Running JUnit tests

To run a test, select the class which contains the tests, right-click on it and select Run-asJUnit Test. This starts JUnit and executes all test methods in this class.

Eclipse provides the Alt+Shift+X, ,T shortcut to run the test in the selected class. If you position the cursor on one method name, this shortcut runs only the selected test method.

To see the result of an JUnit test, Eclipse uses the JUnit view which shows the results of the tests. You can also select individual unit tests in this view , right-click on them and select Run to execute them again.

JUnit view

By default this view shows all tests. You can also configure, that it only shows failing tests.

JUnit view

You can also define that the view is only activated if you have a failing test.

JUnit view


Eclipse creates run configurations for tests. You can see and modify these via the RunRun Configurations... menu.

5.3. JUnit static imports

JUnit uses static methods and Eclipse cannot always create the corresponding static import statements automatically.

You can make the JUnit test methods available via the Content Assists. Content Assists is a functionality in Eclipse which allows the developer to get context sensitive code completion in an editor upon user request.

Open the Preferences via WindowPreferences and select JavaEditorContent AssistFavorites.

Use the New Type button to add the org.junit.Assert type. This makes, for example, the assertTrue, assertFalse and assertEquals methods directly available in the Content Assists.

Adding static imports to the preferences

You can now use Content Assists (shortcut: Ctrl+Space) to add the method and the import.

5.4. Wizard for creating test suites

To create a test suite in Eclipse, you select the test classes which should be included into this in the Package Explorer view, right-click on them and select New Other...JUnitJUnit Test Suite.

Create a test suite

5.5. Testing exception

The @Test (expected = Exception.class) annotation is limited as it can only test for one exception. To test exceptions, you can use the following test pattern.

try {
} catch (Exception e) {
   // expected
   // could also check for message of exception, etc.

6. Exercise: Using JUnit

6.1. Project preparation

Create a new project called com.vogella.junit.first.

Create a new source folder test. For this right-click on your project, select Properties and choose JavaBuild Path. Select the Source tab.

Create new source folder for the tests

Press the Add Folder button. Afterwards, press the Create New Folder button. Create the test folder.

The result is depicted in the following screenshot.

Creating a new folder

Alternatively, you can add a new source folder by right-clicking on a project and selecting New Source Folder.

6.2. Create a Java class

In the src folder, create the com.vogella.junit.first package and the following class.

package com.vogella.junit.first;

public class MyClass {
  public int multiply(int x, int y) {
    // the following is just an example
    if (x > 999) {
      throw new IllegalArgumentException("X should be less than 1000");
    return x / y;

6.3. Create a JUnit test

Right-click on your new class in the Package Explorer view and select NewJUnit Test Case.

In the following wizard ensure that the New JUnit 4 test flag is selected and set the source folder to test, so that your test class gets created in this folder.

Create new test class

Press the Next button and select the methods that you want to test.

Selecting the methods to test

If the JUnit library is not part of the classpath of your project, Eclipse will prompt you to add it. Use this to add JUnit to your project.

Eclipse prompt for adding JUnit to the project classpath

Create a test with the following code.

package com.vogella.junit.first;

import static org.junit.Assert.assertEquals;

import org.junit.AfterClass;
import org.junit.BeforeClass;
import org.junit.Test;

public class MyClassTest {

  public static void testSetup() {

  public static void testCleanup() {
    // Teardown for data used by the unit tests

  @Test(expected = IllegalArgumentException.class)
  public void testExceptionIsThrown() {
    MyClass tester = new MyClass();
    tester.multiply(1000, 5);

  public void testMultiply() {
    MyClass tester = new MyClass();
    assertEquals("10 x 5 must be 50", 50, tester.multiply(10, 5));

6.4. Run your test in Eclipse

Right-click on your new test class and select Run-AsJUnit Test.

Run JUnit test in Eclipse

The result of the tests will be displayed in the JUnit view . In our example one test should be succesful and one test should show an error. This error is indicated by a red bar.

Result of running a unit test

The test is failing, because our multiplier class is currently not working correctly. It does a division instead of multiplication. Fix the bug and re-run the test to get a green bar.

7. Advanced JUnit options

7.1. Parameterized test

JUnit allows you to use parameters in a tests class. This class can contain one test method and this method is executed with the different parameters provided.

You mark a test class as a parameterized test with the @RunWith(Parameterized.class) annotation.

Such a test class must contain a static method annotated with @Parameters that generates and returns a collection of arrays. Each item in this collection is used as parameter for the test method.

You also need to create a constructor in which you store the values for each test. The number of elements in each array provided by the method annotated with @Parameters must correspond to the number of parameters in the constructor of the class. The class is created for each parameter and the test values are passed via the constructor to the class.

The following code shows an example for a parameterized test. It assumes that you test the multiply() method of the MyClass class which was used in an example earlier.

package de.vogella.junit.first;

import static org.junit.Assert.assertEquals;

import java.util.Arrays;
import java.util.Collection;

import org.junit.Test;
import org.junit.runner.RunWith;
import org.junit.runners.Parameterized;
import org.junit.runners.Parameterized.Parameters;

public class MyParameterizedClassTest {

  private int multiplier;

  public MyParameterizedClassTest(int testParameter) {
    this.multiplier = testParameter;

  // creates the test data
  public static Collection<Object[]> data() {
    Object[][] data = new Object[][] { { 1 }, { 5 }, { 121 } };
    return Arrays.asList(data);

  public void testMultiplyException() {
    MyClass tester = new MyClass();
    assertEquals("Result", multiplier * multiplier,
        tester.multiply(multiplier, multiplier));


If you run this test class, the test method is executed with each defined parameter. In the above example the test method is executed three times.

7.2. Rules

Via the @Rule annotation you can create objects which can be used and configured in your test methods. This adds more flexibility to your tests. You could, for example, specify which exception message you expect during execution of your test code.

package de.vogella.junit.first;

import org.junit.Rule;
import org.junit.Test;
import org.junit.rules.ExpectedException;

public class RuleExceptionTesterExample {

  public ExpectedException exception = ExpectedException.none();

  public void throwsIllegalArgumentExceptionIfIconIsNull() {
    exception.expectMessage("Negative value not allowed");
    ClassToBeTested t = new ClassToBeTested();

JUnit already provides several useful implementations of rules. For example, the TemporaryFolder class allows to setup files and folders which are automatically removed after a test.

The following code shows an example for the usage of the TemporaryFolder implementation.

package de.vogella.junit.first;

import static org.junit.Assert.assertTrue;


import org.junit.Rule;
import org.junit.Test;
import org.junit.rules.TemporaryFolder;

public class RuleTester {

  public TemporaryFolder folder = new TemporaryFolder();

  public void testUsingTempFolder() throws IOException {
    File createdFolder = folder.newFolder("newfolder");
    File createdFile = folder.newFile("myfilefile.txt");

To write your custom rule, you need to implement the TestRule interface.

8. Mocking

Unit testing also makes use of object mocking. In this case the real object is exchanged by a replacement which has a predefined behavior for the test.

There are several frameworks available for mocking. To learn more about mock frameworks please see the Mockito tutorial and the EasyMock tutorial

9. Support free vogella tutorials

Maintaining high quality free online tutorials is a lot of work. Please support free tutorials by donating or by reporting typos and factual errors.

9.1. Thank you

Please consider a contribution if this article helped you.

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9.2. Questions and Discussion

If you find errors in this tutorial, please notify me (see the top of the page). Please note that due to the high volume of feedback I receive, I cannot answer questions to your implementation. Ensure you have read the vogella FAQ as I don't respond to questions already answered there.

10. Links and Literature

10.2. vogella Resources

vogella Training Android and Eclipse Training from the vogella team

Android Tutorial Introduction to Android Programming

GWT Tutorial Program in Java, compile to JavaScript and HTML

Eclipse RCP Tutorial Create native applications in Java

JUnit Tutorial Test your application

Git Tutorial Put all your files in a distributed version control system