/*
 * JBoss, Home of Professional Open Source
 * Copyright 2008 Red Hat Inc. and/or its affiliates and other contributors
 * by the @authors tag. See the copyright.txt in the distribution for a
 * full listing of individual contributors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 * http://www.apache.org/licenses/LICENSE-2.0
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,  
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package org.jboss.arquillian.container.test.spi.util;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.lang.reflect.Constructor;
import java.lang.reflect.InvocationTargetException;
import java.net.URL;
import java.util.Collections;
import java.util.Enumeration;
import java.util.Iterator;
import java.util.LinkedHashSet;
import java.util.Set;

/**
 * This class handles looking up service providers on the class path. It
 * implements the <a href="http://java.sun.com/javase/6/docs/technotes/guides/jar/jar.html#Service%20Provider"
 * >Service Provider section of the JAR File Specification</a>.
 * 
 * The Service Provider programmatic lookup was not specified prior to Java 6 so
 * this interface allows use of the specification prior to Java 6.
 * 
 * The API is copied from <a
 * href="http://java.sun.com/javase/6/docs/api/java/util/ServiceLoader.html"
 * >java.util.ServiceLoader</a>
 * 
 * @author Pete Muir
 * @author <a href="mailto:dev@avalon.apache.org">Avalon Development Team</a>
 */
public class ServiceLoader<S> implements Iterable<S>
{
   
   private static final String SERVICES = "META-INF/services";

   /**
    * Creates a new service loader for the given service type, using the current
    * thread's context class loader.
    * 
    * An invocation of this convenience method of the form
    * 
    * {@code ServiceLoader.load(service)</code>}
    * 
    * is equivalent to
    * 
    * <code>ServiceLoader.load(service,
    *                   Thread.currentThread().getContextClassLoader())</code>
    * 
    * @param service The interface or abstract class representing the service
    * @return A new service loader
    */
   public static <S> ServiceLoader<S> load(Class<S> service)
   {
      return load(service, Thread.currentThread().getContextClassLoader());
   }

   /**
    * Creates a new service loader for the given service type and class loader.
    * 
    * @param service The interface or abstract class representing the service
    * @param loader The class loader to be used to load provider-configuration
    *           files and provider classes, or null if the system class loader
    *           (or, failing that, the bootstrap class loader) is to be used
    * @return A new service loader
    */
   public static <S> ServiceLoader<S> load(Class<S> service, ClassLoader loader)
   {
      if (loader == null)
      {
         loader = service.getClassLoader();
      }
      return new ServiceLoader<S>(service, loader);
   }

   /**
    * Creates a new service loader for the given service type, using the
    * extension class loader.
    * 
    * This convenience method simply locates the extension class loader, call it
    * extClassLoader, and then returns
    * 
    * <code>ServiceLoader.load(service, extClassLoader)</code>
    * 
    * If the extension class loader cannot be found then the system class loader
    * is used; if there is no system class loader then the bootstrap class
    * loader is used.
    * 
    * This method is intended for use when only installed providers are desired.
    * The resulting service will only find and load providers that have been
    * installed into the current Java virtual machine; providers on the
    * application's class path will be ignored.
    * 
    * @param service The interface or abstract class representing the service
    * @return A new service loader
    */
   public static <S> ServiceLoader<S> loadInstalled(Class<S> service)
   {
      throw new UnsupportedOperationException();
   }
   
   private final String serviceFile;
   private Class<S> expectedType;
   private final ClassLoader loader;
   
   private Set<S> providers;

   private ServiceLoader(Class<S> service, ClassLoader loader)
   {
      this.loader = loader;
      this.serviceFile = SERVICES + "/" + service.getName();
      this.expectedType = service;
   }
   
   /**
    * Clear this loader's provider cache so that all providers will be reloaded.
    * 
    * After invoking this method, subsequent invocations of the iterator method
    * will lazily look up and instantiate providers from scratch, just as is
    * done by a newly-created loader.
    * 
    * This method is intended for use in situations in which new providers can
    * be installed into a running Java virtual machine.
    */
   public void reload()
   {
      providers = new LinkedHashSet<S>();
      Enumeration<URL> enumeration = null;
      boolean errorOccurred = false;

      try
      {
         enumeration = loader.getResources(serviceFile);
      }
      catch (IOException ioe)
      {
         errorOccurred = true;
      }

      if (!errorOccurred)
      {
         while (enumeration.hasMoreElements())
         {
            try
            {
               final URL url = enumeration.nextElement();
               final InputStream is = url.openStream();
               final BufferedReader reader = new BufferedReader(new InputStreamReader(is, "UTF-8"));

               String line = reader.readLine();
               while (null != line)
               {
                  try
                  {
                     final int comment = line.indexOf('#');

                     if (comment > -1)
                     {
                        line = line.substring(0, comment);
                     }

                     line.trim();

                     if (line.length() > 0)
                     {
                        providers.add(createInstance(line));
                     }
                  }
                  catch (Exception e)
                  {
                     e.printStackTrace();
                     // TODO Don't use exceptions for flow control!
                     // try the next line
                  }

                  line = reader.readLine();
               }
               reader.close();
            }
            catch (Exception e)
            {
               // try the next file
            }
         }
      }
   }

   public S createInstance(String line) throws ClassNotFoundException, NoSuchMethodException, InvocationTargetException,
         NoClassDefFoundError, InstantiationException, IllegalAccessException
   {
      try
      {
         Class<?> clazz = loader.loadClass(line);
         Class<? extends S> serviceClass;
         try
         {
            serviceClass = clazz.asSubclass(expectedType);
         }
         catch (ClassCastException e)
         {
            throw new IllegalStateException("Service " + line + " does not implement expected type " + expectedType.getName());
         }
         Constructor<? extends S> constructor = serviceClass.getConstructor();
         if(!constructor.isAccessible()) {
            constructor.setAccessible(true);
         }
         return constructor.newInstance();
      }
      catch (NoClassDefFoundError e)
      {
         throw e;
      }
      catch (InstantiationException e)
      {
         throw e;
      }
      catch (IllegalAccessException e)
      {
         throw e;
      }
   }

   /**
    * Lazily loads the available providers of this loader's service.
    * 
    * The iterator returned by this method first yields all of the elements of
    * the provider cache, in instantiation order. It then lazily loads and
    * instantiates any remaining providers, adding each one to the cache in
    * turn.
    * 
    * To achieve laziness the actual work of parsing the available
    * provider-configuration files and instantiating providers must be done by
    * the iterator itself. Its hasNext and next methods can therefore throw a
    * ServiceConfigurationError if a provider-configuration file violates the
    * specified format, or if it names a provider class that cannot be found and
    * instantiated, or if the result of instantiating the class is not
    * assignable to the service type, or if any other kind of exception or error
    * is thrown as the next provider is located and instantiated. To write
    * robust code it is only necessary to catch ServiceConfigurationError when
    * using a service iterator.
    * 
    * If such an error is thrown then subsequent invocations of the iterator
    * will make a best effort to locate and instantiate the next available
    * provider, but in general such recovery cannot be guaranteed.
    * 
    * Design Note Throwing an error in these cases may seem extreme. The
    * rationale for this behavior is that a malformed provider-configuration
    * file, like a malformed class file, indicates a serious problem with the
    * way the Java virtual machine is configured or is being used. As such it is
    * preferable to throw an error rather than try to recover or, even worse,
    * fail silently.
    * 
    * The iterator returned by this method does not support removal. Invoking
    * its remove method will cause an UnsupportedOperationException to be
    * thrown.
    * 
    * @return An iterator that lazily loads providers for this loader's service
    */
   public Iterator<S> iterator()
   {
      if (providers == null)
      {
         reload();
      }
      return providers.iterator();
   }

   public Set<S> getProviders() {
      if (providers == null)
      {
         reload();
      }
      return Collections.unmodifiableSet(providers);
   }

   /**
    * Returns a string describing this service.
    * 
    * @return A descriptive string
    */
   @Override
   public String toString()
   {
      return "Services for " + serviceFile;
   }

}
