public class HangingMockWebServerTest {
    @Test
    public void foo() throws IOException {
        MockWebServer server = new MockWebServer();
        server.start();
        server.enqueue(new MockResponse());
        String url = server.url("foo").toString();

        HttpURLConnection connection = (HttpURLConnection) new URL(url).openConnection();
        // relevant line 1:
        connection.setRequestProperty("Expect", "100-continue");
        connection.setDoOutput(true);
        // relevant line 2:
        connection.setChunkedStreamingMode(0); //or any other number
        OutputStream output = connection.getOutputStream(); // takes 5 seconds!
        try (PrintWriter writer = new PrintWriter(output)) {
            writer.write("Super!");
        }
    }
}