Mock final classes and its method to throw exceptions

Question

I have a final class which tries to connect to a provided URL using openConnection() . I mocked it to have it return an UnknownHostException because I know that the provided URL is unknown and to also reduce the time of the unit test (> 0.01 sec is too long for this one test).

Class to be tested:

public final class Server {
    private Server() {}

    public static void getServerStuff(final URL url) throws IOException {
        try {
            final URLConn urlConn;
            // immediately throw exception without trying to make a connection
            urlConn = url.openConnection(); 
        }
        catch (Exception e) {
            // not relevant
        }
    }
}

Unit test

public class ServerTest {

    public class UrlWrapper {

        Url url;

        public UrlWrapper(String spec) throws MalformedURLException {
        url = new URL(spec);
        }

        public URLConnection openConnection() throws IOException {
          return url.openConnection();
        }
    }

    public void testUnknownHostExceptionIsThrown() throws IOException {

        UrlWrapper mockUrl = mock(UrlWrapper.class);
        URLConnection mockUrlConn = mock(URLConnection.class);

        when(mockUrl.openConnection()).thenThrow(UnknownHostException.class);

        final String server = "blabla";
        final URL url = new URL("http://" + server);
        mockUrl.url = url;

        try {
            Server.getServerStuff(mockUrl.url);
        }
        catch (IOException e) {
            // do stuff with Assert
        }
    }
}

I am required to use mockito instead of powermockito which can mock final classes. My main problem is that I do not know how to tell the unit test to use my mocked openConnection() . It should still test my getServerStuff() method but throw the exception without actually trying to connect.

What do I need to change to make it work?

EDIT: I do not think it is a duplicate of the referenced question because I know how to mock a final class (using a wrapper, eg). My problem is the next step, meaning how to use my mocked method. My unit test will go into the to-be tested method and use the openConnection() from standard lib but I want it to use my mocked one to reduce time needed for this unit test to complete.

Answer 1

What is purpose of UrlWrapper when you are passing the wrapped URL object to Server ?

I am going on the assumption that you can edit Server .

Personally I would create a new interface that gets passed to your Server#getServerStuff(..) method. You can then mock the interface to provide the mocked behaviour that you desire.

public interface ServerRemote {
    public InputStream getInput() throws IOException
}

public class URLServerRemote implements ServerRemote {

    private URL url;

    public URLServerRemote(URL url) {
        this.url = url;
    }

    public InputStream getInputStream() throws IOException {
        return url.openConnection().getInputStream();
    }
}

public final class Server {
    private Server() {}

    public static void getServerStuff(final ServerRemote remote) throws IOException {
        try {
            final InputStream input;
            // immediately throw exception without trying to make a connection
            input = remote.getInputStream();
        }
        catch (Exception e) {
            // not relevant
        }
    }
}

...

public void testUnknownHostExceptionIsThrown() throws IOException {

    ServerRemote mockServerRemote = mock(ServerRemote.class);
    when(mockServerRemote.getInputStream()).thenThrow(UnknownHostException.class);

    try {
        Server.getServerStuff(mockServerRemote);
    }
    catch (IOException e) {
        // do stuff with Assert
    }
}

...

If you can not change your Server class then you are going to be stuck unless you use PowerMock.