Where are the three test containers coming from?

Question

I put together a demo JUnit5 project to try out the framework. The project consists of Gradle (4.4), Java (8) and Kotlin (1.2.0) with 4 test cases. I have the following Gradle build script (I've removed most of the boilerplate to keep only the important stuff):

apply plugin: 'java'
apply plugin: 'kotlin'
apply plugin: 'org.junit.platform.gradle.plugin'

repositories {
    mavenCentral()
}

buildscript {
    ext.kotlin_version = '1.2.0'
    repositories {
        mavenCentral()
    }
    dependencies {
        classpath 'org.junit.platform:junit-platform-gradle-plugin:1.0.2'
        classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
    }
}

configurations {
    all {
        exclude group: 'junit', module: 'junit'
    }
}

project.ext {
    junitPlatformVersion = '1.0.2'
    junitJupiterVersion = '5.0.2'
}

dependencies {
    compile "org.jetbrains.kotlin:kotlin-stdlib-jdk8:$kotlin_version"
    testCompile "org.jetbrains.kotlin:kotlin-reflect:$kotlin_version"
    testCompile "org.junit.jupiter:junit-jupiter-api:${junitJupiterVersion}"
    testRuntime "org.junit.jupiter:junit-jupiter-engine:${junitJupiterVersion}"
}

junitPlatform {
    platformVersion '1.0.2'
    filters {
        engines {
            include 'junit-jupiter'
        }
    }
}

I also have a KotlinTest.kt and a JavaTest.java that have the equivalent test cases:

@Test
fun junit5TestPasses() {
    assertTrue(true)
}

@Test
fun junit5TestFails() {
    assertTrue(false)
}

When I run my tests with gradlew junitPlatformTest, I correctly see 2 tests passing and 2 tests failing. However, I also see "3 containers found". My question is why are there 3 containers found? What are they? I can't seem to find a direct answer about test containers in the JUnit5 user guide that pertains to this scenario.

Answer 1

3 containers = JUnit Jupiter Engine + KotlinTest.class + JavaTest.class

An engine, an implementation of TestEngine, is also considered being a container. Next level is the class containing @Test-annotated methods. Look at the example copied from the user-guide:

├─ JUnit Vintage
│  └─ example.JUnit4Tests
│     └─ standardJUnit4Test ✔
└─ JUnit Jupiter
   ├─ StandardTests
   │  ├─ succeedingTest() ✔
   │  └─ skippedTest() ↷ for demonstration purposes
   └─ A special test case
      ├─ Custom test name containing spaces ✔
      ├─ ╯°□°）╯ ✔
      └─ 😱 ✔

Test run finished after 64 ms
[         5 containers found      ]
[         0 containers skipped    ]
[         5 containers started    ]
[         0 containers aborted    ]
[         5 containers successful ]
[         0 containers failed     ]
[         6 tests found           ]
[         1 tests skipped         ]
[         5 tests started         ]
[         0 tests aborted         ]
[         5 tests successful      ]
[         0 tests failed          ]

Here you see five containers, namely:

1. JUnit Vintage engine
2. example.JUnit4Tests class
3. JUnit Jupiter engine
4. StandardTests class
5. A special test case class

All six leaves are tests.

To see a similar tree rendered for your test plan run, add details 'tree' to the Gradle junitPlatform task.