Add unit test to the modules: 
dubbo-registry/dubbo-registry-multicast
   dubbo-registry/dubbo-registry-redis
  dubbo-registry/dubbo-registry-zookeeper 
import org.junit.Test;
import java.net.MulticastSocket;
import java.util.List;
import java.util.Map;
import java.util.Set;
import java.util.concurrent.atomic.AtomicReference;
import static org.hamcrest.CoreMatchers.is;
import static org.junit.Assert.assertThat;
import static org.junit.Assert.assertTrue;
import static org.junit.Assert.assertEquals;
public class MulticastRegistryTest {
    private String service = "com.alibaba.dubbo.test.injvmServie";
    private URL registryUrl = URL.valueOf("multicast://239.255.255.255/");
    private URL serviceUrl = URL.valueOf("dubbo://" + NetUtils.getLocalHost() + "/" + service
            + "?methods=test1,test2");
    private URL adminUrl = URL.valueOf("dubbo://" + NetUtils.getLocalHost() + "/*");
    private URL consumerUrl = URL.valueOf("subscribe://" + NetUtils.getLocalHost() + "/" + service + "?arg1=1&arg2=2");
    private MulticastRegistry registry = new MulticastRegistry(registryUrl);
    /**
     * @throws java.lang.Exception
     */
    @Before
    public void setUp() throws Exception {
        registry.register(serviceUrl);
    }
    @Test(expected = IllegalArgumentException.class)
    public void testUrlError() {
        URL errorUrl = URL.valueOf("multicast://mullticast/");
        new MulticastRegistry(errorUrl);
    }