http://www.javassist.org/: Javassist (JAVA programming ASSISTant) makes Java bytecode manipulation simple. It is a class library for editing bytecodes in Java. (Shigeru Chiba, www.javassist.org) 
/*
 * Javassist, a Java-bytecode translator toolkit.
 * Copyright (C) 1999- Shigeru Chiba. All Rights Reserved.
 *
 * The contents of this file are subject to the Mozilla Public License Version
 * 1.1 (the "License"); you may not use this file except in compliance with
 * the License.  Alternatively, the contents of this file may be used under
 * the terms of the GNU Lesser General Public License Version 2.1 or later,
 * or the Apache License Version 2.0.
 *
 * Software distributed under the License is distributed on an "AS IS" basis,
 * WITHOUT WARRANTY OF ANY KIND, either express or implied. See the License
 * for the specific language governing rights and limitations under the
 * License.
 */

package javassist;

import java.io.InputStream;
import java.net.URL;


ClassPath is an interface implemented by objects
representing a class search path.
ClassPool uses those objects for reading class files.

The users can define a class implementing this interface so that
a class file is obtained from a non-standard source.

See Also:
/**
 * <code>ClassPath</code> is an interface implemented by objects
 * representing a class search path.
 * <code>ClassPool</code> uses those objects for reading class files.
 *
 * <p>The users can define a class implementing this interface so that
 * a class file is obtained from a non-standard source.
 *
 * @see ClassPool#insertClassPath(ClassPath)
 * @see ClassPool#appendClassPath(ClassPath)
 * @see ClassPool#removeClassPath(ClassPath)
 */
public interface ClassPath {
    
Opens a class file.
This method may be called just to examine whether the class file
exists as well as to read the contents of the file.

This method can return null if the specified class file is not
found.  If null is returned, the next search path is examined.
However, if an error happens, this method must throw an exception
so that the search will be terminated.

This method should not modify the contents of the class file.

Params:
  • classname –         a fully-qualified class name
See Also:
Returns:         the input stream for reading a class file
/**
     * Opens a class file.
     * This method may be called just to examine whether the class file
     * exists as well as to read the contents of the file.
     *
     * <p>This method can return null if the specified class file is not
     * found.  If null is returned, the next search path is examined.
     * However, if an error happens, this method must throw an exception
     * so that the search will be terminated.
     *
     * <p>This method should not modify the contents of the class file.
     *
     * @param classname         a fully-qualified class name
     * @return          the input stream for reading a class file
     * @see javassist.Translator
     */
    InputStream openClassfile(String classname) throws NotFoundException;

    
Returns the uniform resource locator (URL) of the class file
with the specified name.

Params:
  • classname –         a fully-qualified class name.
Returns:null if the specified class file could not be found.
/**
     * Returns the uniform resource locator (URL) of the class file
     * with the specified name.
     *
     * @param classname         a fully-qualified class name.
     * @return null if the specified class file could not be found.
     */
    URL find(String classname);
}