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 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
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 * This code 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
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
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 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
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package java.lang.instrument;

import java.lang.reflect.Module;
import java.security.AccessController;
import java.security.PrivilegedAction;
import java.security.ProtectionDomain;

/*
 * Copyright 2003 Wily Technology, Inc.
 */

/**
 * A transformer of class files. An agent registers an implementation of this
 * interface using the {@link Instrumentation#addTransformer addTransformer}
 * method so that the transformer's {@link
 * ClassFileTransformer#transform(Module,ClassLoader,String,Class,ProtectionDomain,byte[])
 * transform} method is invoked when classes are loaded,
 * {@link Instrumentation#redefineClasses redefined}, or
 * {@link Instrumentation#retransformClasses retransformed}. The implementation
 * should override one of the {@code transform} methods defined here.
 * Transformers are invoked before the class is defined by the Java virtual
 * machine.
 *
 * <P>
 * There are two kinds of transformers, determined by the <code>canRetransform</code>
 * parameter of
 * {@link java.lang.instrument.Instrumentation#addTransformer(ClassFileTransformer,boolean)}:
 *  <ul>
 *    <li><i>retransformation capable</i> transformers that were added with
 *        <code>canRetransform</code> as true
 *    </li>
 *    <li><i>retransformation incapable</i> transformers that were added with
 *        <code>canRetransform</code> as false or where added with
 *        {@link java.lang.instrument.Instrumentation#addTransformer(ClassFileTransformer)}
 *    </li>
 *  </ul>
 *
 * <P>
 * Once a transformer has been registered with
 * {@link java.lang.instrument.Instrumentation#addTransformer(ClassFileTransformer,boolean)
 * addTransformer},
 * the transformer will be called for every new class definition and every class redefinition.
 * Retransformation capable transformers will also be called on every class retransformation.
 * The request for a new class definition is made with
 * {@link java.lang.ClassLoader#defineClass ClassLoader.defineClass}
 * or its native equivalents.
 * The request for a class redefinition is made with
 * {@link java.lang.instrument.Instrumentation#redefineClasses Instrumentation.redefineClasses}
 * or its native equivalents.
 * The request for a class retransformation is made with
 * {@link java.lang.instrument.Instrumentation#retransformClasses Instrumentation.retransformClasses}
 * or its native equivalents.
 * The transformer is called during the processing of the request, before the class file bytes
 * have been verified or applied.
 * When there are multiple transformers, transformations are composed by chaining the
 * <code>transform</code> calls.
 * That is, the byte array returned by one call to <code>transform</code> becomes the input
 * (via the <code>classfileBuffer</code> parameter) to the next call.
 *
 * <P>
 * Transformations are applied in the following order:
 *  <ul>
 *    <li>Retransformation incapable transformers
 *    </li>
 *    <li>Retransformation incapable native transformers
 *    </li>
 *    <li>Retransformation capable transformers
 *    </li>
 *    <li>Retransformation capable native transformers
 *    </li>
 *  </ul>
 *
 * <P>
 * For retransformations, the retransformation incapable transformers are not
 * called, instead the result of the previous transformation is reused.
 * In all other cases, this method is called.
 * Within each of these groupings, transformers are called in the order registered.
 * Native transformers are provided by the <code>ClassFileLoadHook</code> event
 * in the Java Virtual Machine Tool Interface).
 *
 * <P>
 * The input (via the <code>classfileBuffer</code> parameter) to the first
 * transformer is:
 *  <ul>
 *    <li>for new class definition,
 *        the bytes passed to <code>ClassLoader.defineClass</code>
 *    </li>
 *    <li>for class redefinition,
 *        <code>definitions.getDefinitionClassFile()</code> where
 *        <code>definitions</code> is the parameter to
 *        {@link java.lang.instrument.Instrumentation#redefineClasses
 *         Instrumentation.redefineClasses}
 *    </li>
 *    <li>for class retransformation,
 *         the bytes passed to the new class definition or, if redefined,
 *         the last redefinition, with all transformations made by retransformation
 *         incapable transformers reapplied automatically and unaltered;
 *         for details see
 *         {@link java.lang.instrument.Instrumentation#retransformClasses
 *          Instrumentation.retransformClasses}
 *    </li>
 *  </ul>
 *
 * <P>
 * If the implementing method determines that no transformations are needed,
 * it should return <code>null</code>.
 * Otherwise, it should create a new <code>byte[]</code> array,
 * copy the input <code>classfileBuffer</code> into it,
 * along with all desired transformations, and return the new array.
 * The input <code>classfileBuffer</code> must not be modified.
 *
 * <P>
 * In the retransform and redefine cases,
 * the transformer must support the redefinition semantics:
 * if a class that the transformer changed during initial definition is later
 * retransformed or redefined, the
 * transformer must insure that the second class output class file is a legal
 * redefinition of the first output class file.
 *
 * <P>
 * If the transformer throws an exception (which it doesn't catch),
 * subsequent transformers will still be called and the load, redefine
 * or retransform will still be attempted.
 * Thus, throwing an exception has the same effect as returning <code>null</code>.
 * To prevent unexpected behavior when unchecked exceptions are generated
 * in transformer code, a transformer can catch <code>Throwable</code>.
 * If the transformer believes the <code>classFileBuffer</code> does not
 * represent a validly formatted class file, it should throw
 * an <code>IllegalClassFormatException</code>;
 * while this has the same effect as returning null. it facilitates the
 * logging or debugging of format corruptions.
 *
 * <P>
 * Note the term <i>class file</i> is used as defined in section 3.1 of
 * <cite>The Java&trade; Virtual Machine Specification</cite>, to mean a
 * sequence of bytes in class file format, whether or not they reside in a
 * file.
 *
 * @see     java.lang.instrument.Instrumentation
 * @since   1.5
 */

public interface ClassFileTransformer {

    /**
     * Transforms the given class file and returns a new replacement class file.
     * This method is invoked when the {@link Module Module} bearing {@link
     * ClassFileTransformer#transform(Module,ClassLoader,String,Class,ProtectionDomain,byte[])
     * transform} is not overridden.
     *
     * @implSpec The default implementation returns null.
     *
     * @param loader                the defining loader of the class to be transformed,
     *                              may be {@code null} if the bootstrap loader
     * @param className             the name of the class in the internal form of fully
     *                              qualified class and interface names as defined in
     *                              <i>The Java Virtual Machine Specification</i>.
     *                              For example, <code>"java/util/List"</code>.
     * @param classBeingRedefined   if this is triggered by a redefine or retransform,
     *                              the class being redefined or retransformed;
     *                              if this is a class load, {@code 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
     *
     * @throws IllegalClassFormatException
     *         if the input does not represent a well-formed class file
     * @return a well-formed class file buffer (the result of the transform),
     *         or {@code null} if no transform is performed
     */
    default byte[]
    transform(  ClassLoader         loader,
                String              className,
                Class<?>            classBeingRedefined,
                ProtectionDomain    protectionDomain,
                byte[]              classfileBuffer)
        throws IllegalClassFormatException {
        return null;
    }


    /**
     * Transforms the given class file and returns a new replacement class file.
     *
     * @implSpec The default implementation of this method invokes the
     * {@link #transform(ClassLoader,String,Class,ProtectionDomain,byte[]) transform}
     * method.
     *
     * @param module                the module of the class to be transformed
     * @param loader                the defining loader of the class to be transformed,
     *                              may be {@code null} if the bootstrap loader
     * @param className             the name of the class in the internal form of fully
     *                              qualified class and interface names as defined in
     *                              <i>The Java Virtual Machine Specification</i>.
     *                              For example, <code>"java/util/List"</code>.
     * @param classBeingRedefined   if this is triggered by a redefine or retransform,
     *                              the class being redefined or retransformed;
     *                              if this is a class load, {@code 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
     *
     * @throws IllegalClassFormatException
     *         if the input does not represent a well-formed class file
     * @return a well-formed class file buffer (the result of the transform),
     *         or {@code null} if no transform is performed
     *
     * @since  9
     * @spec JPMS
     */
    default byte[]
    transform(  Module              module,
                ClassLoader         loader,
                String              className,
                Class<?>            classBeingRedefined,
                ProtectionDomain    protectionDomain,
                byte[]              classfileBuffer)
        throws IllegalClassFormatException {

        // invoke the legacy transform method
        return transform(loader,
                         className,
                         classBeingRedefined,
                         protectionDomain,
                         classfileBuffer);
    }
}