package com.aragost.javahg.internals;

 import com.google.common.io.Closeables;

 import java.io.BufferedReader;

 import java.io.InputStreamReader;

 import java.net.URL;

 /**

  * The ServiceLoader searches for Service implementations in META-INF/services.

  * This class emulates the behavior of ServiceLoader introduced in Java 1.6.

  *

  * @author Sebastian Sdorra

  */

 public final class ServiceLoader

 {

   private static final String SERVICE_PREFIX = "META-INF/services/";

   /**

    * This method is shorthand for {@link #loadService(java.lang.ClassLoader, java.lang.Class)}

    * with the context classloader.

    * 

    * @param serviceClass classloader used to load the resource from META-INF/services

    * 

    * @return service implementation or null

    * 

    * @throws ServiceException 

    */

   public static <S> S loadService( Class<S> serviceClass ) throws ServiceException {

       ClassLoader classLoader = Thread.currentThread().getContextClassLoader();

       if ( classLoader == null ){

           classLoader = Utils.class.getClassLoader();

       }

       return loadService(classLoader, serviceClass);

   }

   /**

    * Returns a implementation of the given service or null if no service could 

    * be found.

    * 

    * @param classLoader classloader used to load the resource from META-INF/services

    * @param serviceClass service class to load

    * 

    * @return service implementation or null

    * @throws ServiceException 

    */

   public static <S> S loadService(ClassLoader classLoader, Class<S> serviceClass ) throws ServiceException{

       S service = null;

       String name = serviceClass.getName();

       URL resource = classLoader.getResource(SERVICE_PREFIX.concat(name));

       if ( resource != null ){

           BufferedReader reader = null;

           try {

               reader = new BufferedReader(new InputStreamReader(resource.openStream(), "UTF-8"));

               String line = reader.readLine();

               while (line != null){

                   int comment = line.indexOf("#");

                   if ( comment >= 0 ){

                       line = line.substring(0, comment);

                   }

                   line = line.trim();

                   if ( line.length() > 0 ){

                       service = (S) Class.forName(line).newInstance();

                       break;

                   }

                   line = reader.readLine();

               }

           } catch ( Exception ex ){

               throw new ServiceException(

                 "could not load service for class ".concat(name), ex);

           } finally {

               Closeables.closeQuietly(reader);

           }

       }

       return service;

   }

 }