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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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/* *******************************************************************
 * 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 v 2.0
 * which accompanies this distribution and is available at
 * https://www.eclipse.org/org/documents/epl-2.0/EPL-2.0.txt
 *
 * Contributors:
 *     Matthew Webster, Adrian Colyer,
 *     Martin Lippert     initial implementation
 *     Andy Clement
 *     Abraham Nevado
 * ******************************************************************/

package org.aspectj.weaver.loadtime;

import java.io.File;
import java.io.IOException;
import java.net.MalformedURLException;
import java.net.URL;
import java.security.CodeSource;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.StringTokenizer;

import org.aspectj.bridge.AbortException;
import org.aspectj.weaver.bcel.ExtensibleURLClassLoader;
import org.aspectj.weaver.tools.Trace;
import org.aspectj.weaver.tools.TraceFactory;
import org.aspectj.weaver.tools.WeavingAdaptor;
import org.aspectj.weaver.tools.WeavingClassLoader;

public class WeavingURLClassLoader extends ExtensibleURLClassLoader implements WeavingClassLoader {

	public static final String WEAVING_CLASS_PATH = "aj.class.path";
	public static final String WEAVING_ASPECT_PATH = "aj.aspect.path";

	private URL[] aspectURLs;
	private WeavingAdaptor adaptor;
	private boolean initializingAdaptor;
	private Map generatedClasses = new HashMap(); /* String -> byte[] */

	private static Trace trace = TraceFactory.getTraceFactory().getTrace(WeavingURLClassLoader.class);

	/*
	 * This constructor is needed when using "-Djava.system.class.loader".
	 */
	public WeavingURLClassLoader(ClassLoader parent) {
		this(getURLs(getClassPath()), getURLs(getAspectPath()), parent);
		// System.out.println("? WeavingURLClassLoader.WeavingURLClassLoader()");
	}

	public WeavingURLClassLoader(URL[] urls, ClassLoader parent) {
		super(urls, parent);
		if (trace.isTraceEnabled()) {
			trace.enter("", this, new Object[] { urls, parent });
		}
		// System.out.println("WeavingURLClassLoader.WeavingURLClassLoader()");
		if (trace.isTraceEnabled()) {
			trace.exit("");
		}
	}

	public WeavingURLClassLoader(URL[] classURLs, URL[] aspectURLs, ClassLoader parent) {
		super(classURLs, parent);
//		 System.out.println(">WIB WeavingURLClassLoader.WeavingURLClassLoader() classURLs=" + Arrays.asList(classURLs));
//		 System.out.println(">WIB WeavingURLClassLoader.WeavingURLClassLoader() aspectURLs=" + Arrays.asList(aspectURLs));
		this.aspectURLs = aspectURLs;

		/*
		 * If either we nor our parent is using an ASPECT_PATH use a new-style adaptor
		 */
		if (this.aspectURLs.length > 0 || getParent() instanceof WeavingClassLoader) {
			try {
				adaptor = new WeavingAdaptor(this);
			} catch (ExceptionInInitializerError ex) {
				ex.printStackTrace(System.out);
				throw ex;
			}
		}
		// System.out.println("< WeavingURLClassLoader.WeavingURLClassLoader() adaptor=" + adaptor);
	}

	private static String getAspectPath() {
		return System.getProperty(WEAVING_ASPECT_PATH, "");
	}

	private static String getClassPath() {
		return System.getProperty(WEAVING_CLASS_PATH, "");
	}

	private static URL[] getURLs(String path) {
		List urlList = new ArrayList<>();
		for (StringTokenizer t = new StringTokenizer(path, File.pathSeparator); t.hasMoreTokens();) {
			File f = new File(t.nextToken().trim());
			try {
				if (f.exists()) {
					URL url = f.toURI().toURL();
						urlList.add(url);
					}
			} catch (MalformedURLException ignored) {}
		}

		URL[] urls = new URL[urlList.size()];
		urlList.toArray(urls);
		return urls;
	}

	protected void addURL(URL url) {
		if (adaptor == null) {
			createAdaptor();
		}
		adaptor.addURL(url);
		super.addURL(url);
	}

	/**
	 * Override to weave class using WeavingAdaptor
	 */
	protected Class defineClass(String name, byte[] b, CodeSource cs) throws IOException {
		if (trace.isTraceEnabled()) {
			trace.enter("defineClass", this, new Object[] { name, b, cs });
		}
		// System.err.println("? WeavingURLClassLoader.defineClass(" + name + ", [" + b.length + "])");
		byte orig[] = b;
		/* Avoid recursion during adaptor initialization */
		if (!initializingAdaptor) {

			/* Need to defer creation because of possible recursion during constructor execution */
			if (adaptor == null && !initializingAdaptor) {
				createAdaptor();
			}

			try {
				b = adaptor.weaveClass(name, b, false);
				if (b == null)
					b = orig;
			} catch (AbortException ex) {
				trace.error("defineClass", ex);
				throw ex;
			} catch (Throwable th) {
				trace.error("defineClass", th);
			}
		}
		Class clazz;

		// On error, define the original form of the class and log the issue
		try {
			clazz= super.defineClass(name, b, cs);
		} catch (Throwable th) {
			trace.error("Weaving class problem. Original class has been returned. Error cause: " + th, th);
			clazz= super.defineClass(name, orig, cs);
		}
		if (trace.isTraceEnabled()) {
			trace.exit("defineClass", clazz);
		}
		return clazz;
	}

	private void createAdaptor() {
		DefaultWeavingContext weavingContext = new DefaultWeavingContext(this) {

			/* Ensures consistent LTW messages for testing */
			public String getClassLoaderName() {
				ClassLoader loader = getClassLoader();
				return loader.getClass().getName();
			}

		};

		ClassLoaderWeavingAdaptor clwAdaptor = new ClassLoaderWeavingAdaptor();
		initializingAdaptor = true;
		clwAdaptor.initialize(this, weavingContext);
		initializingAdaptor = false;
		adaptor = clwAdaptor;
	}

	/**
	 * Override to find classes generated by WeavingAdaptor
	 */
	protected byte[] getBytes(String name) throws IOException {
		byte[] bytes = super.getBytes(name);

		if (bytes == null) {
			// return adaptor.findClass(name);
			return (byte[]) generatedClasses.remove(name);
		}

		return bytes;
	}

	/**
	 * Implement method from WeavingClassLoader
	 */
	public URL[] getAspectURLs() {
		return aspectURLs;
	}

	public void acceptClass (String name, byte[] classBytes, byte[] weavedBytes) {
		generatedClasses.put(name, weavedBytes);
	}

	// protected synchronized Class loadClass(String name, boolean resolve) throws ClassNotFoundException {
	// System.err.println("> WeavingURLClassLoader.loadClass() name=" + name);
	// Class clazz= super.loadClass(name, resolve);
	// System.err.println("< WeavingURLClassLoader.loadClass() clazz=" + clazz + ", loader=" + clazz.getClassLoader());
	// return clazz;
	// }

	// private interface ClassPreProcessorAdaptor extends ClassPreProcessor {
	// public void addURL(URL url);
	// }
	//
	// private class WeavingAdaptorPreProcessor implements ClassPreProcessorAdaptor {
	//
	// private WeavingAdaptor adaptor;
	//
	// public WeavingAdaptorPreProcessor (WeavingClassLoader wcl) {
	// adaptor = new WeavingAdaptor(wcl);
	// }
	//
	// public void initialize() {
	// }
	//
	// public byte[] preProcess(String className, byte[] bytes, ClassLoader classLoader) {
	// return adaptor.weaveClass(className,bytes);
	// }
	//
	// public void addURL(URL url) {
	//
	// }
	// }

}