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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
 * .
 *
 * Extended by Adrian Colyer, June 2005 to support unpacking of Signature
 * attribute
 */

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

import org.aspectj.apache.bcel.Constants;

/**
 * This class is derived from Attribute and represents a reference to a GJ
 * attribute.
 *
 * @version $Id: Signature.java,v 1.11 2009/09/15 19:40:12 aclement Exp $
 * @author M. Dahm
 * @see Attribute
 */
public final class Signature extends Attribute {
	private int signature_index;

	/**
	 * Initialize from another object. Note that both objects use the same references (shallow copy). Use clone() for a physical
	 * copy.
	 */
	public Signature(Signature c) {
		this(c.getNameIndex(), c.getLength(), c.getSignatureIndex(), c.getConstantPool());
	}

	/**
	 * Construct object from file stream.
	 *
	 * @param name_index Index in constant pool to CONSTANT_Utf8
	 * @param length Content length in bytes
	 * @param file Input stream
	 * @param constant_pool Array of constants
	 * @throws IOException
	 */
	Signature(int name_index, int length, DataInputStream file, ConstantPool constant_pool) throws IOException {
		this(name_index, length, file.readUnsignedShort(), constant_pool);
	}

	/**
	 * @param name_index Index in constant pool to CONSTANT_Utf8
	 * @param length Content length in bytes
	 * @param constant_pool Array of constants
	 * @param Signature_index Index in constant pool to CONSTANT_Utf8
	 */
	public Signature(int name_index, int length, int signature_index, ConstantPool constant_pool) {
		super(Constants.ATTR_SIGNATURE, name_index, length, constant_pool);
		this.signature_index = signature_index;
	}

	/**
	 * 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) {
		System.err.println("Visiting non-standard Signature object");
		v.visitSignature(this);
	}

	/**
	 * Dump source file attribute to file stream in binary format.
	 *
	 * @param file Output file stream
	 * @throws IOException
	 */
	@Override
	public final void dump(DataOutputStream file) throws IOException {
		super.dump(file);
		file.writeShort(signature_index);
	}

	/**
	 * @return Index in constant pool of source file name.
	 */
	public final int getSignatureIndex() {
		return signature_index;
	}

	/**
	 * @param Signature_index.
	 */
	public final void setSignatureIndex(int signature_index) {
		this.signature_index = signature_index;
	}

	/**
	 * @return GJ signature.
	 */
	public final String getSignature() {
		ConstantUtf8 c = (ConstantUtf8) cpool.getConstant(signature_index, Constants.CONSTANT_Utf8);
		return c.getValue();
	}

	/**
	 * Extends ByteArrayInputStream to make 'unreading' chars possible.
	 */
	private static final class MyByteArrayInputStream extends ByteArrayInputStream {
		MyByteArrayInputStream(String data) {
			super(data.getBytes());
		}

		final int mark() {
			return pos;
		}

		final String getData() {
			return new String(buf);
		}

		final void reset(int p) {
			pos = p;
		}

		final void unread() {
			if (pos > 0)
				pos--;
		}
	}

	private static boolean identStart(int ch) {
		return ch == 'T' || ch == 'L';
	}

	private static final void matchIdent(MyByteArrayInputStream in, StringBuilder buf) {
		int ch;

		if ((ch = in.read()) == -1)
			throw new RuntimeException("Illegal signature: " + in.getData() + " no ident, reaching EOF");

		// System.out.println("return from ident:" + (char)ch);

		if (!identStart(ch)) {
			StringBuilder buf2 = new StringBuilder();

			int count = 1;
			while (Character.isJavaIdentifierPart((char) ch)) {
				buf2.append((char) ch);
				count++;
				ch = in.read();
			}

			if (ch == ':') { // Ok, formal parameter
				in.skip("Ljava/lang/Object".length());
				buf.append(buf2);

				ch = in.read();
				in.unread();
				// System.out.println("so far:" + buf2 + ":next:" +(char)ch);
			} else {
				for (int i = 0; i < count; i++)
					in.unread();
			}

			return;
		}

		StringBuilder buf2 = new StringBuilder();
		ch = in.read();

		do {
			buf2.append((char) ch);
			ch = in.read();
			// System.out.println("within ident:"+ (char)ch);

		} while ((ch != -1) && (Character.isJavaIdentifierPart((char) ch) || (ch == '/')));

		buf.append(buf2.toString().replace('/', '.'));

		// System.out.println("regular return ident:"+ (char)ch + ":" + buf2);

		if (ch != -1)
			in.unread();
	}

	private static final void matchGJIdent(MyByteArrayInputStream in, StringBuilder buf) {
		int ch;

		matchIdent(in, buf);

		ch = in.read();
		if ((ch == '<') || ch == '(') { // Parameterized or method
			// System.out.println("Enter <");
			buf.append((char) ch);
			matchGJIdent(in, buf);

			while (((ch = in.read()) != '>') && (ch != ')')) { // List of parameters
				if (ch == -1)
					throw new RuntimeException("Illegal signature: " + in.getData() + " reaching EOF");

				// System.out.println("Still no >");
				buf.append(", ");
				in.unread();
				matchGJIdent(in, buf); // Recursive call
			}

			// System.out.println("Exit >");

			buf.append((char) ch);
		} else
			in.unread();

		ch = in.read();
		if (identStart(ch)) {
			in.unread();
			matchGJIdent(in, buf);
		} else if (ch == ')') {
			in.unread();
			return;
		} else if (ch != ';')
			throw new RuntimeException("Illegal signature: " + in.getData() + " read " + (char) ch);
	}

	public static String translate(String s) {
		// System.out.println("Sig:" + s);
		StringBuilder buf = new StringBuilder();

		matchGJIdent(new MyByteArrayInputStream(s), buf);

		return buf.toString();
	}

	public static final boolean isFormalParameterList(String s) {
		return s.startsWith("<") && (s.indexOf(':') > 0);
	}

	public static final boolean isActualParameterList(String s) {
		return s.startsWith("L") && s.endsWith(">;");
	}

	/**
	 * @return String representation
	 */
	@Override
	public final String toString() {
		String s = getSignature();

		return "Signature(" + s + ")";
	}

	// /**
	// * @return deep copy of this attribute
	// */
	// @Override
	// public Attribute copy(ConstantPool constant_pool) {
	// return (Signature) clone();
	// }

}