import java.util.*;
import java.util.concurrent.*;
// Noninstantiable class for service registration and access
public class Services {
   private Services() {}

   private static final Map<String, Provider> providers =
      new ConcurrentHashMap<String, Provider>();
   public static final String DEFAULT_PROVIDER_NAME = "Identity";

   // Provider registration API
   public static void registerDefaultProvider(Provider p) {
      registerProvider(DEFAULT_PROVIDER_NAME, p);
   }
   public static void registerProvider(String name, Provider p) {
      providers.put(name, p);
   }
   // Service access API
   public static Service newInstance() {
      return newInstance(DEFAULT_PROVIDER_NAME);
   }
   public static Service newInstance(String name) {
      Provider p = providers.get(name);
      if (p == null) {
         throw new IllegalArgumentException(
           "No Provider registered with name " + name);
      }
      return p.newService();
   }
}