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The AspectJ weaver introduces advices to java classes
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/* *******************************************************************
 * Copyright (c) 2004 IBM Corporation
 * All rights reserved. 
 * This program and the accompanying materials are made available 
 * under the terms of the Eclipse Public License v1.0 
 * which accompanies this distribution and is available at 
 * http://www.eclipse.org/legal/epl-v10.html 
 *  
 * Contributors: 
 *     Matthew Webster, Adrian Colyer, 
 *     Martin Lippert     initial implementation 
 *     Andy Clement
 *     Roy Varghese - Bug 473555
 * ******************************************************************/

package org.aspectj.weaver.bcel;

import java.io.IOException;
import java.net.URL;
import java.net.URLClassLoader;
import java.security.CodeSource;

import org.aspectj.util.FileUtil;
import org.aspectj.weaver.BCException;
import org.aspectj.weaver.UnresolvedType;

public abstract class ExtensibleURLClassLoader extends URLClassLoader {

	private ClassPathManager classPath;

	public ExtensibleURLClassLoader(URL[] urls, ClassLoader parent) {
		super(urls, parent);

		// System.err.println("? ExtensibleURLClassLoader.() path=" + WeavingAdaptor.makeClasspath(urls));
		try {
			classPath = new ClassPathManager(FileUtil.makeClasspath(urls), null);
		} catch (ExceptionInInitializerError ex) {
			ex.printStackTrace(System.out);
			throw ex;
		}
	}

	protected void addURL(URL url) {
		super.addURL(url); // amc - this call was missing and is needed in
		// WeavingURLClassLoader chains
		classPath.addPath(url.getPath(), null);
	}

	protected Class findClass(String name) throws ClassNotFoundException {
		// System.err.println("? ExtensibleURLClassLoader.findClass(" + name + ")");
		try {
			byte[] bytes = getBytes(name);
			if (bytes != null) {
				return defineClass(name, bytes);
			} else {
				throw new ClassNotFoundException(name);
			}
		} catch (IOException ex) {
			throw new ClassNotFoundException(name);
		}
	}

	protected Class defineClass(String name, byte[] b, CodeSource cs) throws IOException {
		// System.err.println("? ExtensibleURLClassLoader.defineClass(" + name + ",[" + b.length + "])");
		return defineClass(name, b, 0, b.length, cs);
	}

	protected byte[] getBytes(String name) throws IOException {
		byte[] b = null;
		UnresolvedType unresolvedType = null;
		try {
			unresolvedType = UnresolvedType.forName(name);
		} catch (BCException bce) {
			if (bce.getMessage().indexOf("nameToSignature") == -1) {
				bce.printStackTrace(System.err);
			}
			return null;
		}
		ClassPathManager.ClassFile classFile = classPath.find(unresolvedType);
		if (classFile != null) {
			try {
				b = FileUtil.readAsByteArray(classFile.getInputStream());
			} finally {
				classFile.close();
			}
		}
		return b;
	}

	private Class defineClass(String name, byte[] bytes /* ClassPathManager.ClassFile classFile */) throws IOException {
		String packageName = getPackageName(name);
		if (packageName != null) {
			Package pakkage = getPackage(packageName);
			if (pakkage == null) {
				definePackage(packageName, null, null, null, null, null, null, null);
			}
		}

		return defineClass(name, bytes, null);
	}

	private String getPackageName(String className) {
		int offset = className.lastIndexOf('.');
		return (offset == -1) ? null : className.substring(0, offset);
	}
	
	@Override
	public void close() throws IOException {
		super.close();
		classPath.closeArchives();
	}

}