JUnit is a unit testing framework for the Java programming language. JUnit has been important in the development of test-driven development, and is one of a family of unit testing frameworks which is collectively known as xUnit that originated with SUnit.

JUnit is linked as a JAR at compile-time. The latest version of the framework, JUnit 5, resides under package org.junit.jupiter. Previous versions JUnit 4 and JUnit 3 were under packages org.junit and junit.framework, respectively.

A research survey performed in 2013 across 10,000 Java projects hosted on GitHub found that JUnit (in a tie with slf4j-api) was the most commonly included external library. Each library was used by 30.7% of projects.^[3]

Example of a JUnit test fixture

A JUnit test fixture is a Java object. Test methods must be annotated by the @Test annotation. If the situation requires it,^[4] it is also possible to define a method to execute before (or after) each (or all) of the test methods with the @BeforeEach (or @AfterEach) and @BeforeAll (or @AfterAll) annotations.^[5]

import org.junit.jupiter.api.*;

public class FoobarTest {
    @BeforeAll
    public static void setUpClass() throws Exception {
        // Code executed before the first test method
    }

    @BeforeEach
    public void setUp() throws Exception {
        // Code executed before each test
    }
 
    @Test
    public void oneThing() {
        // Code that tests one thing
    }

    @Test
    public void anotherThing() {
        // Code that tests another thing
    }

    @Test
    public void somethingElse() {
        // Code that tests something else
    }

    @AfterEach
    public void tearDown() throws Exception {
        // Code executed after each test 
    }
 
    @AfterAll
    public static void tearDownClass() throws Exception {
        // Code executed after the last test method 
    }
}

Previous versions of JUnit

According to Martin Fowler, one of the early adopters of JUnit:^[6]

JUnit was born on a flight from Zurich to the 1997 OOPSLA in Atlanta. Kent was flying with Erich Gamma, and what else were two geeks to do on a long flight but program? The first version of JUnit was built there, pair programmed, and done test first (a pleasing form of meta-circular geekery).

As a side effect of its wide use, previous versions of JUnit remain popular, with JUnit 4 having over 100,000 usages by other software components on the Maven Central repository.^[7]

In JUnit 4, the annotations for test execution callbacks were @BeforeClass, @Before, @After, and @AfterClass, as opposed to JUnit 5's @BeforeAll, @BeforeEach, @AfterEach, and @AfterAll.^[5]

In JUnit 3, test fixtures had to inherit from junit.framework.TestCase.^[8] Additionally, test methods had to be prefixed with 'test'.^[9]