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;
    }
  }