MockWebServer Connection refused: no further information

Using MockWebServer with the following dependencies:

testImplementation("com.squareup.okhttp3:mockwebserver")

And the following test class:

class MockWebServerTest() {
    private val mockWebServer = MockWebServer()
    private val client = HttpClient.newBuilder().build()

    @BeforeClass
    fun setup() {
        mockWebServer.start(8081)
        mockWebServer.dispatcher = object : Dispatcher() {
            override fun dispatch(request: RecordedRequest): MockResponse {
                when(request.path){
                    ("success") ->{
                        return MockResponse()
                            .setResponseCode(200)
                            .setBody("success")}
                    ("error") ->{
                        return MockResponse()
                                .setResponseCode(500)
                                .setBody("error")}
                    else -> {
                        return MockResponse()
                                .setResponseCode(200)
                    }
                }
            }
        }
    }

    @AfterClass
    fun teardown() {
        mockWebServer.shutdown()
    }
    @Test
    fun givenMockServerRuns_itShouldRespond(){
        val request = HttpRequest.newBuilder()
                .uri(URI.create("http://localhost:8081/success"))
                .setHeader("Content-Type", "text/html")
                .GET()
                .build()
        val response = client.send(request, HttpResponse.BodyHandlers.ofString())
        assertThat(response.statusCode()).isEqualTo(200)
        assertThat(response.body()).isEqualTo("success")
    }
}

I always get Connection refused: no further information. I found some answers already on stackoverflow as for example this answer but I don't now what this solution there means, I assume it is only relevant for android.

I was not able to make the answer given by Jongho Jeon working. The setup method actually did not execute at all.

To get it working I finally did this:

@TestInstance(TestInstance.Lifecycle.PER_CLASS)
class MockWebServerTest {

    private val client = HttpClient.newBuilder().build()
    private val mockWebServer = MockWebServer()

    @BeforeAll
    fun setup() {
        mockWebServer.start(8081)
        mockWebServer.dispatcher = object : Dispatcher() {
            override fun dispatch(request: RecordedRequest): MockResponse {
                return when(request.path){
                    ("/success") ->{
                        MockResponse()
                                .setResponseCode(200)
                                .setBody("success")
                    }
                    ("/error") ->{
                        MockResponse()
                                .setResponseCode(500)
                                .setBody("error")
                    }
                    else -> {
                        MockResponse()
                                .setResponseCode(200)
                    }
                }
            }
        }
    }

    @AfterAll
    fun teardown() {
        mockWebServer.shutdown()
    }

    @Test
    fun givenMockServerRuns_itShouldRespond(){
        val request = HttpRequest.newBuilder()
                .uri(URI.create("http://localhost:8081/success"))
                .setHeader("Content-Type", "text/html")
                .GET()
                .build()
        val response = client.send(request, HttpResponse.BodyHandlers.ofString())
        assertThat(response.statusCode()).isEqualTo(200)
        assertThat(response.body()).isEqualTo("success")
    }