This is a quick write-up on migrating a Gradle-based Spring Boot app from JUnit 4 to the shiny new Junit 5. JUnit 4 tests continue to work with Junit 5 Test Engine abstraction, which provides support for tests written in different programming models. In this instance, JUnit 5 supports a Vintage Test Engine with the ability to run JUnit 4 tests.

Here is a sample project with JUnit 5 integrations already in place along with sample tests in JUnit 4 and JUnit 5.

Sample JUnit 4 Candidate Test

As a candidate project, I have a Spring Boot 2 app with tests written in Kotlin using JUnit 4 as the testing framework. This is how a sample test looks with all dependencies explicitly called out. It uses JUnit4's @RunWith annotation to load up the Spring Context:

import org.assertj.core.api.Assertions.assertThat
import org.junit.Test
import org.junit.runner.RunWith
import org.springframework.beans.factory.annotation.Autowired
import org.springframework.boot.test.autoconfigure.web.reactive.WebFluxTest
import org.springframework.test.context.junit4.SpringRunner
import org.springframework.test.web.reactive.server.WebTestClient
import java.nio.charset.StandardCharsets

@RunWith(SpringRunner::class)
@WebFluxTest(controllers = arrayOf(RouteConfig::class))
class SampleJunit4Test {

    @Autowired
    lateinit var webTestClient: WebTestClient

    @Test
    fun `get of hello URI should return Hello World!`() {
        webTestClient.get()
                .uri("/hello")
                .exchange()
                .expectStatus().isOk
                .expectBody()
                .consumeWith({ m ->
                    assertThat(String(m.responseBodyContent, StandardCharsets.UTF_8)).isEqualTo("Hello World!")
                })

    }

}

The JUnit 4 dependencies are pulled in transitively via the "spring-boot-starter-test" module:

testCompile('org.springframework.boot:spring-boot-starter-test')

Junit 5 Migration

The first step is to pull in the JUnit 5 dependencies along with the Gradle plugin, which enables running the tests.

Plugin:

buildscript {
    dependencies {
        ....
        classpath 'org.junit.platform:junit-platform-gradle-plugin:1.0.2'
    }
}
apply plugin: 'org.junit.platform.gradle.plugin'

Dependencies:

testCompile("org.junit.jupiter:junit-jupiter-api")
testRuntime("org.junit.jupiter:junit-jupiter-engine")
testRuntime("org.junit.vintage:junit-vintage-engine:4.12.2")

With these changes in place, all the JUnit 4 tests will continue to run both in the IDE and when the Gradle build is executed. At that point, the tests can be slowly migrated over.

The test I had shown before looks like this with JUnit 5 Jupiter, which provides the programming model for the tests:

import org.junit.jupiter.api.Assertions.assertEquals
import org.junit.jupiter.api.Test
import org.junit.jupiter.api.extension.ExtendWith
import org.springframework.beans.factory.annotation.Autowired
import org.springframework.boot.test.autoconfigure.web.reactive.WebFluxTest
import org.springframework.test.context.junit.jupiter.SpringExtension
import org.springframework.test.web.reactive.server.WebTestClient
import java.nio.charset.StandardCharsets

@ExtendWith(SpringExtension::class)
@WebFluxTest(controllers = arrayOf(RouteConfig::class))
class SampleJunit5Test {

    @Autowired
    lateinit var webTestClient: WebTestClient

    @Test
    fun `get of hello URI should return Hello World!`() {
        webTestClient.get()
                .uri("/hello")
                .exchange()
                .expectStatus().isOk
                .expectBody()
                .consumeWith({ m ->
                    assertEquals("Hello World!", String(m.responseBodyContent, StandardCharsets.UTF_8))
                })
    }

}

Note that now, instead of using JUnit 4 the @RunWith annotation, I am using the @ExtendWith annotation and providing SpringExtension as a parameter. It is responsible for loading up the Spring Context like before. The rest of the Spring annotations will continue to work with JUnit 5. This way, tests can be slowly moved over from JUnit 4 to JUnit 5.

Caveats

Not everything is smooth though. There are a few issues in migrating from JUnit 4 to JUnit 5. The biggest of them is likely the support for JUnit's @Rule and @ClassRule annotations. The JUnit 5 documentation does go into the details of how that can be mitigated.