OkHttp MockWebServer fails to accept connections when restarted

I'm using the OkHttp MockWebServer to mock my server responses for unit tests.

It works great for the first test, but on the 2nd test my client fails with:

Failed to connect to localhost/0:0:0:0:0:0:0:1:63631

This happens even if the 2nd test is exactly the same as the 1st one.
Here's what I'm doing:

@RunWith(RobolectricTestRunner.class)
@Config(shadows = MyClassTest.MyNetworkSecurityPolicy.class,
        manifest = "src/main/AndroidManifest.xml",
        constants = BuildConfig.class,
        sdk = 16)
public class MyClassTest {
    private MockWebServer mockServer;
    private MyServerApi serverApi;

    @Before
    public void setUp() throws Exception {
        System.out.println("\ntest start");
        this.mockServer = new MockWebServer();
        this.mockServer.start();

        this.serverApi = new MyServerApi(this.mockServer.url("/").toString());
    }

    @Test
    public void testOne() throws Exception {
        final String responseBody = // read response from file
        this.mockServer.enqueue(new MockResponse().setResponseCode(200).setBody(responseBody));

        final Waiter waiter = new Waiter();
        this.serverApi.getData("some_id", new Callback<MyResponseData> {
            @Override
            public void onResponse(final Call<MyResponseData> call, final Response<MyResponseData> response) {
                waiter.assertEquals("some_value", response.body().getValue());
                waiter.resume();
            }

            @Override
            public void onFailure(final Call<T> call, final Throwable error) {
                waiter.fail(error);
            }
        });

        waiter.await();

        final RecordedRequest recordedRequest = this.mockServer.takeRequest();
        assertEquals("GET", recordedRequest.getMethod());
    }

    @Test
    public void testTwo() throws Exception {
        final String responseBody = // read response from file
        this.mockServer.enqueue(new MockResponse().setResponseCode(200).setBody(responseBody));

        final Waiter waiter = new Waiter();
        this.serverApi.getData("some_id", new Callback<MyResponseData> {
            @Override
            public void onResponse(final Call<MyResponseData> call, final Response<MyResponseData> response) {
                waiter.assertEquals("some_value", response.body().getValue());
                waiter.resume();
            }

            @Override
            public void onFailure(final Call<T> call, final Throwable error) {
                waiter.fail(error);
            }
        });

        waiter.await();

        final RecordedRequest recordedRequest = this.mockServer.takeRequest();
        assertEquals("GET", recordedRequest.getMethod());
    }

    @After
    public void tearDown() throws Exception {
        System.out.println("test end\n");
        this.mockServer.shutdown();
    }

    @Implements(NetworkSecurityPolicy.class)
    public static class MyNetworkSecurityPolicy {
        @Implementation
        public static NetworkSecurityPolicy getInstance() {
            try {
                Class<?> shadow = MyNetworkSecurityPolicy.class.forName("android.security.NetworkSecurityPolicy");
                return (NetworkSecurityPolicy) shadow.newInstance();
            } catch (Exception e) {
                throw new AssertionError();
            }
        }

        @Implementation
        public boolean isCleartextTrafficPermitted() {
            return true;
        }
    }
}

The first test passes as it should, but the second one fails with the message I wrote above.
The output in the console is:

test start
okhttp3.mockwebserver.MockWebServer$3 execute
INFO: MockWebServer[63631] starting to accept connections
WARNING: no system properties value for gsm.sim.operator.alpha
okhttp3.mockwebserver.MockWebServer$4 processOneRequest
INFO: MockWebServer[63631] received request: GET REQUEST_PATH HTTP/1.1 and responded: HTTP/1.1 200 OK
okhttp3.mockwebserver.MockWebServer$3 acceptConnections
test end

INFO: MockWebServer[63631] done accepting connections: Socket closed

test start
okhttp3.mockwebserver.MockWebServer$3 execute
INFO: MockWebServer[63649] starting to accept connections
okhttp3.mockwebserver.MockWebServer$3 acceptConnections
INFO: MockWebServer[63649] done accepting connections: Socket closed
on error: Failed to connect to localhost/0:0:0:0:0:0:0:1:63631
test end

(The Waiter object thing is from the concurrentunit lib)

Any idea why this happens?

Your second request is using the URL of the first MockWebServer instance. (Each instance has a distinct URL.)