Kotlin: manage unit test resources in Kotlin, such as starting/stopping a database connection or an embedded elasticsearch server

Kotlin Programming

Question or issue of Kotlin Programming:

In my Kotlin JUnit tests, I want to start/stop embedded servers and use them within my tests.

I tried using the JUnit @Before annotation on a method in my test class and it works fine, but it isn’t the right behaviour since it runs every test case instead of just once.

Therefore I want to use the @BeforeClass annotation on a method, but adding it to a method results in an error saying it must be on a static method. Kotlin doesn’t appear to have static methods. And then the same applies for static variables, because I need to keep a reference to the embedded server around for use in the test cases.

So how do I create this embedded database just once for all of my test cases?

class MyTest {
    @Before fun setup() {
       // works in that it opens the database connection, but is wrong 
       // since this is per test case instead of being shared for all

    @BeforeClass fun setupClass() {
       // what I want to do instead, but results in error because 
       // this isn't a static method, and static keyword doesn't exist

    var referenceToServer: ServerType // wrong because is not static either


Note: this question is intentionally written and answered by the author (Self-Answered Questions), so that the answers to commonly asked Kotlin topics are present in SO.

How to solve this issue?

Solution no. 1:

Your unit test class usually needs a few things to manage a shared resource for a group of test methods. And in Kotlin you can use @BeforeClass and @AfterClass not in the test class, but rather within its companion object along with the @JvmStatic annotation.

The structure of a test class would look like:

class MyTestClass {
    companion object {
        init {
           // things that may need to be setup before companion class member variables are instantiated

        // variables you initialize for the class just once:
        val someClassVar = initializer() 

        // variables you initialize for the class later in the @BeforeClass method:
        lateinit var someClassLateVar: SomeResource 

        @BeforeClass @JvmStatic fun setup() {
           // things to execute once and keep around for the class

        @AfterClass @JvmStatic fun teardown() {
           // clean up after this class, leave nothing dirty behind

    // variables you initialize per instance of the test class:
    val someInstanceVar = initializer() 

    // variables you initialize per test case later in your @Before methods:
    var lateinit someInstanceLateZVar: MyType 

    @Before fun prepareTest() { 
        // things to do before each test

    @After fun cleanupTest() {
        // things to do after each test

    @Test fun testSomething() {
        // an actual test case

    @Test fun testSomethingElse() {
        // another test case

    // ...more test cases

Given the above, you should read about:

  • companion objects – similar to the Class object in Java, but a singleton per class that is not static
  • @JvmStatic – an annotation that turns a companion object method into a static method on the outer class for Java interop
  • lateinit – allows a var property to be initialized later when you have a well defined lifecycle
  • Delegates.notNull() – can be used instead of lateinit for a property that should be set at least once before being read.

Here are fuller examples of test classes for Kotlin that manage embedded resources.

The first is copied and modified from Solr-Undertow tests, and before the test cases are run, configures and starts a Solr-Undertow server. After the tests run, it cleans up any temporary files created by the tests. It also ensures environment variables and system properties are correct before the tests are run. Between test cases it unloads any temporary loaded Solr cores. The test:

class TestServerWithPlugin {
    companion object {
        val workingDir = Paths.get("test-data/solr-standalone").toAbsolutePath()
        val coreWithPluginDir = workingDir.resolve("plugin-test/collection1")

        lateinit var server: Server

        @BeforeClass @JvmStatic fun setup() {
            assertTrue(coreWithPluginDir.exists(), "test core w/plugin does not exist $coreWithPluginDir")

            // make sure no system properties are set that could interfere with test

            val config = mapOf(...) 
            val configLoader = ServerConfigFromOverridesAndReference(workingDir, config) verifiedBy { loader ->


            server = Server(configLoader)
            val (serverStarted, message) = server.run()
            if (!serverStarted) {
                fail("Server not started: '$message'")

        @AfterClass @JvmStatic fun teardown() {

        private fun cleanSysProps() { ... }

        private fun cleanFiles() {
            // don't leave any test files behind

    val adminClient: SolrClient = HttpSolrClient("http://localhost:8983/solr/")

    @Before fun prepareTest() {
        // anything before each test?

    @After fun cleanupTest() {
        // make sure test cores do not bleed over between test cases

    private fun unloadCoreIfExists(name: String) { ... }

    fun testServerLoadsPlugin() {
        println("Loading core 'withplugin' from dir ${coreWithPluginDir.toString()}")
        val response = CoreAdminRequest.createCore("tempCollection1", coreWithPluginDir.toString(), adminClient)
        assertEquals(0, response.status)

    // ... other test cases

And another starting AWS DynamoDB local as an embedded database (copied and modified slightly from Running AWS DynamoDB-local embedded). This test must hack the java.library.path before anything else happens or local DynamoDB (using sqlite with binary libraries) won’t run. Then it starts a server to share for all test classes, and cleans up temporary data between tests. The test:

class TestAccountManager {
    companion object {
        init {
            // we need to control the "java.library.path" or sqlite cannot find its libraries
            val dynLibPath = File("./src/test/dynlib/").absoluteFile
            System.setProperty("java.library.path", dynLibPath.toString());

            // TEST HACK: if we kill this value in the System classloader, it will be
            // recreated on next access allowing java.library.path to be reset
            val fieldSysPath = ClassLoader::class.java.getDeclaredField("sys_paths")
            fieldSysPath.set(null, null)

            // ensure logging always goes through Slf4j
            System.setProperty("org.eclipse.jetty.util.log.class", "org.eclipse.jetty.util.log.Slf4jLog")

        private val localDbPort = 19444

        private lateinit var localDb: DynamoDBProxyServer
        private lateinit var dbClient: AmazonDynamoDBClient
        private lateinit var dynamo: DynamoDB

        @BeforeClass @JvmStatic fun setup() {
            // do not use ServerRunner, it is evil and doesn't set the port correctly, also
            // it resets logging to be off.
            localDb = DynamoDBProxyServer(localDbPort, LocalDynamoDBServerHandler(
                    LocalDynamoDBRequestHandler(0, true, null, true, true), null)

            // fake credentials are required even though ignored
            val auth = BasicAWSCredentials("fakeKey", "fakeSecret")
            dbClient = AmazonDynamoDBClient(auth) initializedWith {
                signerRegionOverride = "us-east-1"
            dynamo = DynamoDB(dbClient)

            // create the tables once

            // for debugging reference
            dynamo.listTables().forEach { table ->

        @AfterClass @JvmStatic fun teardown() {

    val jsonMapper = jacksonObjectMapper()
    val dynamoMapper: DynamoDBMapper = DynamoDBMapper(dbClient)

    @Before fun prepareTest() {
        // insert commonly used test data

    @After fun cleanupTest() {
        // delete anything that shouldn't survive any test case

    private inline fun  deleteAllInTable() { ... }

    @Test fun testAccountJsonRoundTrip() {
        val acct = Account("123",  ...)

        val item = dynamo.getTable("Accounts").getItem("id", "123")
        val acctReadJson = jsonMapper.readValue(item.toJSON())
        assertEquals(acct, acctReadJson)

    // ...more test cases


NOTE: some parts of the examples are abbreviated with ...

Solution no. 2:

Managing resources with before/after callbacks in tests, obviously, has it’s pros:

  • Tests are “atomic”. A test executes as a whole things with all the callbacks One won’t forget to fire up a dependency service before the tests and shut it down after it’s done. If done properly, executions callbacks will work on any environment.
  • Tests are self-contained. There is no external data or setup phases, everything is contained within a few test classes.

It has some cons too. One important of them is that it pollutes the code and makes the code violate single responsibility principle. Tests now not only test something, but perform a heavyweight initialization and resource management. It can be ok in some cases (like configuring an ObjectMapper), but modifying java.library.path or spawning another processes (or in-process embedded databases) are not so innocent.

Why not treat those services as dependencies for your test eligible for “injection”, like described by 12factor.net.

This way you start and initialize dependency services somewhere outside of the test code.

Nowadays virtualization and containers are almost everywhere and most developers’ machines are able to run Docker. And most of the application have a dockerized version: Elasticsearch, DynamoDB, PostgreSQL and so on. Docker is a perfect solution for external services that your tests need.

  • It can be a script that runs is run manually by a developer every time she wants to execute tests.
  • It can be a task run by build tool (e.g. Gradle has awesome dependsOn and finalizedBy DSL for defining dependencies). A task, of course, can execute the same script that developer executes manually using shell-outs / process execs.
  • It can be a task run by IDE before test execution. Again, it can use the same script.
  • Most CI / CD providers have a notion of “service” — an external dependency (process) that runs in parallel to your build and can be accessed via it’s usual SDK / connector / API: Gitlab, Travis, Bitbucket, AppVeyor, Semaphore, …

This approach:

  • Frees your test code from initialization logic. Your tests will only test and do nothing more.
  • Decouples code and data. Adding a new test case can now be done by adding new data into dependency services with it’s native toolset. I.e. for SQL databases you’ll use SQL, for Amazon DynamoDB you’ll use CLI to create tables and put items.
  • Is closer to a production code, where you obviously do not start those services when your “main” application starts.

Of course, it has it’s flaws (basically, the statements I’ve started from):

  • Tests are not more “atomic”. Dependency service must be started somehow prior test execution. The way it is started may be different in different environments: developer’s machine or CI, IDE or build tool CLI.
  • Tests are not self-contained. Now your seed data may be even packed inside an image, so changing it may require rebuilding a different project.

Hope this helps!