Running test with MockWebServer always call the failure call back (Connection Exception)

Question

I'm running mock web server to test REST API calls, when activity launch and start the mockWebserver and execute the API call I'm getting the connection refused,using OKHTTP v-3.10.0 & Retrofit 2.3.0

ActivityTest Code:

 @Rule
    public ActivityTestRule<STBUpgradeActivity> mActivityRule = new ActivityTestRule<>(STBUpgradeActivity.class, false, false);

    private MockWebServer server;

    @Before
    public void setUp() throws Exception {

        super.setUp();
        server = new MockWebServer();
        server.start();
        injectInstrumentation(InstrumentationRegistry.getInstrumentation());
        String serverUrl = server.url("/").toString();
        EnvConfig.WEB_API_SIT = serverUrl;
    }

    @Test
    public void testPageContentIsShown() throws Exception {
        String fileName = "stb_upgrade_page_content_200.json";
        server.enqueue(new MockResponse()
                .setResponseCode(200)
                .setBody(RestServiceTestHelper.getStringFromFile(getInstrumentation().getContext(), fileName)));

        Intent intent = new Intent();
        mActivityRule.launchActivity(intent);

        onView(withId(R.id.button_upgrade)).check(matches(isDisplayed()));
        onView(withText("New residential subscribers and standard install only. If Sports is a part of your package")).check(matches(isDisplayed()));
    }

======= RestAPI client

 private Retrofit getClient() { 
        retrofit = new Retrofit.Builder()
                .baseUrl(WEB_API_HOST)
                .addConverterFactory(GsonConverterFactory.create())
                .client(getHttpClient())
                .build();

        return retrofit;
    }

Answer 1

Change the setUp code to

 @Before
    public void setUp() throws Exception {

        super.setUp();
        server = new MockWebServer();
        server.start(8080);
        injectInstrumentation(InstrumentationRegistry.getInstrumentation());
    }

set the Retroft Base URL to

private Retrofit getClient() {
        retrofit = new Retrofit.Builder()
                .baseUrl("http://localhost:8080/")
                .addConverterFactory(GsonConverterFactory.create())
                .client(getHttpClient())
                .build();

        return retrofit;
    }