//******************************************************************************
//
// File:    BackendClassLoader.java
// Package: edu.rit.pj.cluster
// Unit:    Class edu.rit.pj.cluster.BackendClassLoader
//
// This Java source file is copyright (C) 2012 by Alan Kaminsky. All rights
// reserved. For further information, contact the author, Alan Kaminsky, at
// ark@cs.rit.edu.
//
// This Java source file is part of the Parallel Java Library ("PJ"). PJ is free
// software; you can redistribute it and/or modify it under the terms of the GNU
// General Public License as published by the Free Software Foundation; either
// version 3 of the License, or (at your option) any later version.
//
// PJ is distributed in the hope that it will be useful, but WITHOUT ANY
// WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR
// A PARTICULAR PURPOSE. See the GNU General Public License for more details.
//
// Linking this library statically or dynamically with other modules is making a
// combined work based on this library. Thus, the terms and conditions of the
// GNU General Public License cover the whole combination.
//
// As a special exception, the copyright holders of this library give you
// permission to link this library with independent modules to produce an
// executable, regardless of the license terms of these independent modules, and
// to copy and distribute the resulting executable under terms of your choice,
// provided that you also meet, for each linked independent module, the terms
// and conditions of the license of that module. An independent module is a
// module which is not derived from or based on this library. If you modify this
// library, you may extend this exception to your version of the library, but
// you are not obligated to do so. If you do not wish to do so, delete this
// exception statement from your version.
//
// A copy of the GNU General Public License is provided in the file gpl.txt. You
// may also obtain a copy of the GNU General Public License on the World Wide
// Web at http://www.gnu.org/licenses/gpl.html.
//
//******************************************************************************

package edu.rit.pj.cluster;

import java.io.BufferedOutputStream;
import java.io.File;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.OutputStream;
import java.net.URL;
import java.util.Collections;
import java.util.HashMap;
import java.util.Map;

/**
 * Class BackendClassLoader provides a class loader for a job backend process in
 * the PJ cluster middleware. If a backend class loader is requested to load a
 * class and the parent class loaders cannot do so, the backend class loader
 * sends a request for the class file to the job frontend process, waits for the
 * job frontend process to send the class file, and loads the class.
 *
 * @author  Alan Kaminsky
 * @version 15-Jun-2012
 */
public class BackendClassLoader
	extends ClassLoader
	{

// Hidden data members.

	private JobBackendRef myJobBackend;
	private JobFrontendRef myJobFrontend;
	private ResourceCache myCache;

	// Map from resource name to resource URL for non-class-file resources.
	private Map<String,URL> myResourceURLMap =
		Collections.synchronizedMap (new HashMap<String,URL>());

// Exported constructors.

	/**
	 * Construct a new backend class loader. The parent class loader is the one
	 * returned by <TT>ClassLoader.getSystemClassLoader()</TT>. Class files will
	 * be requested from <TT>theJobFrontend</TT>. Class files will be stored in
	 * <TT>theCache</TT>.
	 *
	 * @param  theJobBackend   Reference to job backend.
	 * @param  theJobFrontend  Reference to job frontend.
	 * @param  theCache        Resource cache.
	 */
	public BackendClassLoader
		(JobBackendRef theJobBackend,
		 JobFrontendRef theJobFrontend,
		 ResourceCache theCache)
		{
		super();
		myJobBackend = theJobBackend;
		myJobFrontend = theJobFrontend;
		myCache = theCache;
		}

	/**
	 * Construct a new backend class loader. The parent class loader is
	 * <TT>parent</TT>. Class files will be requested from
	 * <TT>theJobFrontend</TT>. Class files will be stored in <TT>theCache</TT>.
	 *
	 * @param  parent          Parent class loader.
	 * @param  theJobBackend   Reference to job backend.
	 * @param  theJobFrontend  Reference to job frontend.
	 * @param  theCache        Resource cache.
	 */
	public BackendClassLoader
		(ClassLoader parent,
		 JobBackendRef theJobBackend,
		 JobFrontendRef theJobFrontend,
		 ResourceCache theCache)
		{
		super (parent);
		myJobBackend = theJobBackend;
		myJobFrontend = theJobFrontend;
		myCache = theCache;
		}

// Hidden operations.

	/**
	 * Find the class with the given name.
	 *
	 * @param  className  Fully-qualified class name.
	 *
	 * @return  Class object.
	 *
	 * @exception  ClassNotFoundException
	 *     Thrown if the class could not be found.
	 */
	protected Class<?> findClass
		(String className)
		throws ClassNotFoundException
		{
		try
			{
			// Convert class name to resource name.
			String resourceName = className.replace ('.', '/') + ".class";

			// If the resource is not in the cache, ask the Job Frontend for it.
			if (! myCache.contains (resourceName))
				{
				myJobFrontend.requestResource (myJobBackend, resourceName);
				}

			// Wait until the resource shows up in the cache.
			byte[] content = myCache.get (resourceName);
			if (content == null)
				{
				throw new ClassNotFoundException
					("Class " + className + " not found");
				}

			// Load the class.
			return defineClass (className, content, 0, content.length);
			}

		catch (IOException exc)
			{
			throw new ClassNotFoundException
				("Class " + className + " not found due to I/O error",
				 exc);
			}

		catch (InterruptedException exc)
			{
			throw new ClassNotFoundException
				("Class " + className + " not found because thread interrupted",
				 exc);
			}
		}

	/**
	 * Find the resource with the given name.
	 *
	 * @param  name  Resource name.
	 *
	 * @return  URL for reading the resource, or null if the resource could not
	 *          be found.
	 */
	protected URL findResource
		(String name)
		{
		try
			{
			URL url = myResourceURLMap.get (name);

			if (url == null)
				{
				// If the resource is not in the cache, ask the Job Frontend for
				// it.
				if (! myCache.contains (name))
					{
					myJobFrontend.requestResource (myJobBackend, name);
					}

				// Wait until the resource shows up in the cache.
				byte[] content = myCache.get (name);
				if (content == null) return null;

				// Store the resource contents in a temporary file, which will
				// be deleted when the JVM exits.
				String fname = new File (name) .getName();
				int i = fname.lastIndexOf ('.');
				String fprefix = i == -1 ? fname : fname.substring (0, i);
				String fsuffix = i == -1 ? null : fname.substring (i);
				File file = File.createTempFile (fprefix+"_tmp", fsuffix);
				OutputStream out =
					new BufferedOutputStream
						(new FileOutputStream (file));
				out.write (content);
				out.close();
				file.deleteOnExit();

				// Map resource name to temporary file URL.
				url = file.toURI().toURL();
				myResourceURLMap.put (name, url);
				}

			return url;
			}

		catch (IOException exc)
			{
			return null;
			}

		catch (InterruptedException exc)
			{
			return null;
			}
		}

	}