public interface DataLayer {
    Object readData(int id);
}

public class SqlLayer implements DataLayer {
    @Override
    public Object readData(int id) {
        // Do the job
    }
}

public class Service {
    private final DataLayer dataLayer;

    public Service(DataLayer dataLayer) {
        this.dataLayer = dataLayer;
    }
}


// Use a SqlLayer as singleton
DataLayer dataLayer = new SqlLayer();
Service serviceA = new Service(dataLayer);
Service serviceB = new Service(dataLayer);

// ... or use multiple one
Service serviceA = new Service(new SqlLayer());
Service serviceB = new Service(new SqlLayer());