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The AspectJ weaver applies aspects to Java classes. It can be used as a Java agent in order to apply load-time weaving (LTW) during class-loading and also contains the AspectJ runtime classes.
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package org.aspectj.apache.bcel.classfile;

/* ====================================================================
 * The Apache Software License, Version 1.1
 *
 * Copyright (c) 2001 The Apache Software Foundation.  All rights
 * reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 *
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 *
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in
 *    the documentation and/or other materials provided with the
 *    distribution.
 *
 * 3. The end-user documentation included with the redistribution,
 *    if any, must include the following acknowledgment:
 *       "This product includes software developed by the
 *        Apache Software Foundation (https://www.apache.org/)."
 *    Alternately, this acknowledgment may appear in the software itself,
 *    if and wherever such third-party acknowledgments normally appear.
 *
 * 4. The names "Apache" and "Apache Software Foundation" and
 *    "Apache BCEL" must not be used to endorse or promote products
 *    derived from this software without prior written permission. For
 *    written permission, please contact [email protected].
 *
 * 5. Products derived from this software may not be called "Apache",
 *    "Apache BCEL", nor may "Apache" appear in their name, without
 *    prior written permission of the Apache Software Foundation.
 *
 * THIS SOFTWARE IS PROVIDED ``AS IS'' AND ANY EXPRESSED OR IMPLIED
 * WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
 * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
 * DISCLAIMED.  IN NO EVENT SHALL THE APACHE SOFTWARE FOUNDATION OR
 * ITS CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
 * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
 * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF
 * USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
 * ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
 * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
 * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 * ====================================================================
 *
 * This software consists of voluntary contributions made by many
 * individuals on behalf of the Apache Software Foundation.  For more
 * information on the Apache Software Foundation, please see
 * .
 */

import java.io.DataInputStream;
import java.io.DataOutputStream;
import java.io.IOException;

import org.aspectj.apache.bcel.Constants;

/**
 * This class represents a reference to an unknown (i.e., application-specific) attribute of a class. It is instantiated from the
 * Attribute.readAttribute() method. Applications that need to read in application-specific attributes should create an AttributeReader implementation and attach it via Attribute.addAttributeReader.
 *
 *
 * @version $Id: Unknown.java,v 1.6 2009/09/15 19:40:12 aclement Exp $
 * @see org.aspectj.apache.bcel.classfile.Attribute
 * @author M. Dahm
 */
public final class Unknown extends Attribute {
	private byte[] bytes;
	private String name;

	// evil static - removed by Andy C - no apparent users (4 Mar 06)
	// private static HashMap unknown_attributes = new HashMap();

	/**
	 * @return array of unknown attributes, but just one for each kind.
	 */
	// static Unknown[] getUnknownAttributes() {
	// Unknown[] unknowns = new Unknown[unknown_attributes.size()];
	// Iterator entries = unknown_attributes.values().iterator();
	//
	// for(int i=0; entries.hasNext(); i++)
	// unknowns[i] = (Unknown)entries.next();
	//
	// unknown_attributes.clear();
	// return unknowns;
	// }
	/**
	 * Initialize from another object. Note that both objects use the same references (shallow copy). Use clone() for a physical
	 * copy.
	 */
	public Unknown(Unknown c) {
		this(c.getNameIndex(), c.getLength(), c.getBytes(), c.getConstantPool());
	}

	/**
	 * Create a non-standard attribute.
	 *
	 * @param name_index Index in constant pool
	 * @param length Content length in bytes
	 * @param bytes Attribute contents
	 * @param constant_pool Array of constants
	 */
	public Unknown(int name_index, int length, byte[] bytes, ConstantPool constant_pool) {
		super(Constants.ATTR_UNKNOWN, name_index, length, constant_pool);
		this.bytes = bytes;

		name = ((ConstantUtf8) constant_pool.getConstant(name_index, Constants.CONSTANT_Utf8)).getValue();
		// unknown_attributes.put(name, this);
	}

	/**
	 * Construct object from file stream.
	 *
	 * @param name_index Index in constant pool
	 * @param length Content length in bytes
	 * @param file Input stream
	 * @param constant_pool Array of constants
	 * @throws IOException
	 */
	Unknown(int name_index, int length, DataInputStream file, ConstantPool constant_pool) throws IOException {
		this(name_index, length, (byte[]) null, constant_pool);

		if (length > 0) {
			bytes = new byte[length];
			file.readFully(bytes);
		}
	}

	/**
	 * Called by objects that are traversing the nodes of the tree implicitely defined by the contents of a Java class. I.e., the
	 * hierarchy of methods, fields, attributes, etc. spawns a tree of objects.
	 *
	 * @param v Visitor object
	 */
	@Override
	public void accept(ClassVisitor v) {
		v.visitUnknown(this);
	}

	/**
	 * Dump unknown bytes to file stream.
	 *
	 * @param file Output file stream
	 * @throws IOException
	 */
	@Override
	public final void dump(DataOutputStream file) throws IOException {
		super.dump(file);
		if (length > 0)
			file.write(bytes, 0, length);
	}

	/**
	 * @return data bytes.
	 */
	public final byte[] getBytes() {
		return bytes;
	}

	/**
	 * @return name of attribute.
	 */
	@Override
	public String getName() {
		return name;
	}

	/**
	 * @param bytes.
	 */
	public final void setBytes(byte[] bytes) {
		this.bytes = bytes;
	}

	/**
	 * @return String representation.
	 */
	@Override
	public final String toString() {
		if (length == 0 || bytes == null)
			return "(Unknown attribute " + name + ")";

		String hex;
		if (length > 10) {
			byte[] tmp = new byte[10];
			System.arraycopy(bytes, 0, tmp, 0, 10);
			hex = Utility.toHexString(tmp) + "... (truncated)";
		} else
			hex = Utility.toHexString(bytes);

		return "(Unknown attribute " + name + ": " + hex + ")";
	}

	/**
	 * @return deep copy of this attribute
	 */
	// @Override
	// public Attribute copy(ConstantPool constant_pool) {
	// Unknown c = (Unknown) clone();
	//
	// if (bytes != null)
	// c.bytes = bytes.clone();
	//
	// c.cpool = constant_pool;
	// return c;
	// }
}