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The Java Persistence API (JPA) : a standard interface-based Java model abstraction of persistence, developed by the JCP.

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/*
 * Copyright (c) 2008, 2009, 2011 Oracle, Inc. 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.
 */
package javax.persistence.spi;

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

/**
 * 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.
 */
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.
     * @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;
}