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The core O/RM functionality as provided by Hibernate

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/*
 * Hibernate, Relational Persistence for Idiomatic Java
 *
 * Copyright (c) 2008-2013, Red Hat Inc. or third-party contributors as
 * indicated by the @author tags or express copyright attribution
 * statements applied by the authors.  All third-party contributions are
 * distributed under license by Red Hat Inc.
 *
 * This copyrighted material is made available to anyone wishing to use, modify,
 * copy, or redistribute it subject to the terms and conditions of the GNU
 * Lesser General Public License, as published by the Free Software Foundation.
 *
 * This program 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 Lesser General Public License
 * for more details.
 *
 * You should have received a copy of the GNU Lesser General Public License
 * along with this distribution; if not, write to:
 * Free Software Foundation, Inc.
 * 51 Franklin Street, Fifth Floor
 * Boston, MA  02110-1301  USA
 */
package org.hibernate.bytecode.internal.javassist;

import java.io.IOException;

import javassist.CannotCompileException;
import javassist.ClassPool;
import javassist.CtClass;
import javassist.NotFoundException;

import org.hibernate.HibernateException;

/**
 * A ClassLoader implementation applying Class transformations as they are being loaded.
 *
 * @author Steve Ebersole
 */
@SuppressWarnings("UnusedDeclaration")
public class TransformingClassLoader extends ClassLoader {
	private ClassLoader parent;
	private ClassPool classPool;

	TransformingClassLoader(ClassLoader parent, String[] classpaths) {
		this.parent = parent;
		this.classPool = new ClassPool( true );
		for ( String classpath : classpaths ) {
			try {
				classPool.appendClassPath( classpath );
			}
			catch (NotFoundException e) {
				throw new HibernateException(
						"Unable to resolve requested classpath for transformation [" +
								classpath + "] : " + e.getMessage()
				);
			}
		}
	}

	@Override
	protected Class findClass(String name) throws ClassNotFoundException {
		try {
			final CtClass cc = classPool.get( name );
			// todo : modify the class definition if not already transformed...
			final byte[] b = cc.toBytecode();
			return defineClass( name, b, 0, b.length );
		}
		catch (NotFoundException e) {
			throw new ClassNotFoundException();
		}
		catch (IOException e) {
			throw new ClassNotFoundException();
		}
		catch (CannotCompileException e) {
			throw new ClassNotFoundException();
		}
	}

	/**
	 * Used to release resources.  Call when done with the ClassLoader
	 */
	public void release() {
		classPool = null;
		parent = null;
	}
}