https://github.com/javaee/jpa-spec: Java(TM) Persistence API 
Copyright (c) 2008 - 2013 Oracle Corporation. All rights reserved.
This program and the accompanying materials are made available under the
terms of the Eclipse Public License v1.0 and Eclipse Distribution License v. 1.0
which accompanies this distribution.
The Eclipse Public License is available at http://www.eclipse.org/legal/epl-v10.html
and the Eclipse Distribution License is available at
http://www.eclipse.org/org/documents/edl-v10.php.
Contributors:
    Linda DeMichiel - Java Persistence 2.1
    Linda DeMichiel - Java Persistence 2.0

/*******************************************************************************
 * Copyright (c) 2008 - 2013 Oracle Corporation. All rights reserved.
 *
 * This program and the accompanying materials are made available under the
 * terms of the Eclipse Public License v1.0 and Eclipse Distribution License v. 1.0
 * which accompanies this distribution.
 * The Eclipse Public License is available at http://www.eclipse.org/legal/epl-v10.html
 * and the Eclipse Distribution License is available at
 * http://www.eclipse.org/org/documents/edl-v10.php.
 *
 * Contributors:
 *     Linda DeMichiel - Java Persistence 2.1
 *     Linda DeMichiel - Java Persistence 2.0
 *
 ******************************************************************************/ 
package javax.persistence.spi;

import java.security.ProtectionDomain;
import java.lang.instrument.IllegalClassFormatException;


A persistence provider supplies an instance of this interface to the 
PersistenceUnitInfo.addTransformer method. The supplied transformer instance will get called to transform entity class files when they are loaded or redefined. The transformation occurs before the class is defined by the JVM. 
Since:Java Persistence 1.0
/**
 * A persistence provider supplies an instance of this 
 * interface to the {@link PersistenceUnitInfo#addTransformer 
 * PersistenceUnitInfo.addTransformer}
 * method. The supplied transformer instance will get 
 * called to transform entity class files when they are 
 * loaded or redefined. The transformation occurs before  
 * the class is defined by the JVM.
 *
 * @since Java Persistence 1.0
 */
public interface ClassTransformer {

    
Invoked when a class is being loaded or redefined.
The implementation of this method may transform the 
supplied class file and return a new replacement class 
file.

Params:
  • loader –  the defining loader of the class to be 
       transformed, may be null if the bootstrap loader
  • className –  the name of the class in the internal form 
       of fully qualified class and interface names
  • classBeingRedefined –  if this is a redefine, the 
       class being redefined, otherwise null
  • protectionDomain –  the protection domain of the 
       class being defined or redefined
  • classfileBuffer –  the input byte buffer in class 
       file format - must not be modified
Throws:
Returns:a well-formed class file buffer (the result of 
        the transform), or null if no transform is performed
/**
     * Invoked when a class is being loaded or redefined.
     * The implementation of this method may transform the 
     * supplied class file and return a new replacement class 
     * file.
     *
     * @param loader  the defining loader of the class to be 
     *        transformed, may be null if the bootstrap loader
     * @param className  the name of the class in the internal form 
     *        of fully qualified class and interface names 
     * @param classBeingRedefined  if this is a redefine, the 
     *        class being redefined, otherwise null
     * @param protectionDomain  the protection domain of the 
     *        class being defined or redefined
     * @param classfileBuffer  the input byte buffer in class 
     *        file format - must not be modified 
     * @return a well-formed class file buffer (the result of 
     *         the transform), or null if no transform is performed
     * @throws IllegalClassFormatException  if the input does 
     *         not represent a well-formed class file
     */
    byte[] transform(ClassLoader loader,
                     String className,
                     Class<?> classBeingRedefined,
                     ProtectionDomain protectionDomain, 
                     byte[] classfileBuffer) 
        throws IllegalClassFormatException;
}