MainActivityTest.java

@RunWith(AndroidJUnit4.class)
public class MainActivityTest {

    @Rule
    public ActivityTestRule<MainActivity> activityRule = 
      new ActivityTestRule<>(MainActivity.class,false,false);

    private MockWebServer webServer;

    @Before
    public void setup() throws Exception {

        webServer = new MockWebServer();
        webServer.start(8080);
    }

    @After
    public void tearDown() throws Exception {
        webServer.shutdown();
    }