# JUnit Platform, Jupiter and Build Setup — JUnit 5 & Mockito

Source: https://www.skillbyai.com/en/java-testing/f-setup

> Set up JUnit with Maven or Gradle and understand the JUnit architecture.

## How the pieces fit

Modern JUnit is split into modules. The **JUnit Platform** launches tests and provides the API that build tools and IDEs use. **JUnit Jupiter** is the programming model you write tests with (`@Test`, `@BeforeEach`, assertions, extensions), introduced in JUnit 5 and continued in **JUnit 6** (released in 2025, requiring Java 17) with the same package names. **JUnit Vintage** runs old JUnit 3 and 4 tests on the platform during migrations. Add the **`junit-jupiter`** dependency (ideally through the `junit-bom` to keep versions aligned) with **test** scope. **Maven Surefire** (3.x) and **Gradle** (`useJUnitPlatform()`) run tests on the platform automatically. Tests live in `src/test/java`, mirroring the package of the class under test, and test classes conventionally end in `Test`. Add **Mockito** (`mockito-core` and `mockito-junit-jupiter`) and an assertion library such as **AssertJ**. Spring Boot projects get all of these through **`spring-boot-starter-test`**.

## Maven and Gradle test dependencies

Use a BOM so all JUnit modules share one version.

```xml
<!-- pom.xml (versions are examples; use the latest stable releases) -->
<dependencyManagement>
  <dependencies>
    <dependency>
      <groupId>org.junit</groupId>
      <artifactId>junit-bom</artifactId>
      <version>5.12.2</version>
      <type>pom</type>
      <scope>import</scope>
    </dependency>
  </dependencies>
</dependencyManagement>
<dependencies>
  <dependency>
    <groupId>org.junit.jupiter</groupId>
    <artifactId>junit-jupiter</artifactId>
    <scope>test</scope>
  </dependency>
  <dependency>
    <groupId>org.mockito</groupId>
    <artifactId>mockito-junit-jupiter</artifactId>
    <version>5.17.0</version>
    <scope>test</scope>
  </dependency>
  <dependency>
    <groupId>org.assertj</groupId>
    <artifactId>assertj-core</artifactId>
    <version>3.27.3</version>
    <scope>test</scope>
  </dependency>
</dependencies>

<!-- Gradle (Kotlin DSL):
dependencies {
    testImplementation(platform("org.junit:junit-bom:5.12.2"))
    testImplementation("org.junit.jupiter:junit-jupiter")
    testRuntimeOnly("org.junit.platform:junit-platform-launcher")
    testImplementation("org.mockito:mockito-junit-jupiter:5.17.0")
}
tasks.test { useJUnitPlatform() }
-->
```

## A stage, a script and an old script reader

The Platform is the theatre stage that any play can use, Jupiter is the modern script format, and Vintage is a reader who can still perform old scripts while you rewrite them.

**Quiz:** Which JUnit module provides the annotations and assertions you write modern tests with?

- [ ] JUnit Platform
- [ ] JUnit Vintage
- [ ] Maven Surefire
- [x] JUnit Jupiter

*Answer:* JUnit Jupiter. Jupiter is the programming and extension model; the Platform launches tests.