Load classes : Class Loader « Reflection « Java Tutorial






/**
 * EasyBeans
 * Copyright (C) 2006 Bull S.A.S.
 * Contact: easybeans@ow2.org
 *
 * This library is free software; you can redistribute it and/or
 * modify it under the terms of the GNU Lesser General Public
 * License as published by the Free Software Foundation; either
 * version 2.1 of the License, or any later version.
 *
 * This library is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public
 * License along with this library; if not, write to the Free Software
 * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307
 * USA
 *
 * --------------------------------------------------------------------------
 * $Id: ClassUtils.java 3783 2008-07-30 13:44:06Z benoitf $
 * --------------------------------------------------------------------------
 */


/**
 * The <code>ClassUtils</code> class is the central point used to load classes.
 * @author Guillaume Sauthier
 */
public final class ClassUtils {

    /**
     * Default constructor.
     */
    private ClassUtils() {}

    /**
     * Look up the class in the Tread Context ClassLoader and in the "current" ClassLoader.
     * @param className The class name to load
     * @return the corresponding Class instance
     * @throws ClassNotFoundException if the Class was not found.
     */
    public static Class forName(final String className) throws ClassNotFoundException {
        // Load classes from different classloaders :
        // 1. Thread Context ClassLoader
        // 2. ClassUtils ClassLoader

        ClassLoader tccl = Thread.currentThread().getContextClassLoader();
        Class cls = null;

        try {
            // Try with TCCL
            cls = Class.forName(className, true, tccl);
        } catch (ClassNotFoundException cnfe) {

            // Try now with the classloader used to load ClassUtils
            ClassLoader current = ClassUtils.class.getClassLoader();
            try {
                cls = Class.forName(className, true, current);
            } catch (ClassNotFoundException cnfe2) {
                // If this is still unknown, throw an Exception
                throw cnfe2;
            }
        }

        return cls;
    }

    /**
     * Look up the class in the Tread Context ClassLoader and in the "current" ClassLoader.
     * @param className The class name to load
     * @param clazz a class used to get classloader
     * @return the corresponding Class instance
     * @throws ClassNotFoundException if the Class was not found.
     */
    public static Class forName(final String className, final Class clazz) throws ClassNotFoundException {
        // Load classes from different classloaders :
        // 1. Thread Context ClassLoader
        // 2. ClassUtils ClassLoader

        ClassLoader tccl = Thread.currentThread().getContextClassLoader();
        Class cls = null;

        try {
            // Try with TCCL
            cls = Class.forName(className, true, tccl);
        } catch (ClassNotFoundException cnfe) {

            // Try now with the classloader used to load ClassUtils
            ClassLoader current = clazz.getClassLoader();
            if (current != null) {
                try {
                    cls = Class.forName(className, true, current);
                } catch (ClassNotFoundException cnfe2) {
                    // If this is still unknown, throw an Exception
                    throw new ClassNotFoundException("Class Not found in current ThreadClassLoader '" + tccl + "' and in '" + current + "' classloaders.", cnfe2);
                }
            } else {
                // rethrow exception
                throw cnfe;
            }
        }

        return cls;
    }


}








7.7.Class Loader
7.7.1.URL class loader
7.7.2.extends URLClassLoader
7.7.3.Load classes
7.7.4.how to use reflection to print the names and values of all nonstatic fields of an object
7.7.5.Runs a jar application from any url
7.7.6.BufferedReader reflection
7.7.7.Get the class By way of an object
7.7.8.Get the class By way of a string
7.7.9.Get the class By way of .class
7.7.10.Catch InvocationTargetException
7.7.11.Determining from Where a Class Was Loaded
7.7.12.Dynamically Reloading a Modified Class
7.7.13.Creating an Object Using a Constructor Object
7.7.14.Create an object from a string
7.7.15.Using the forName() method
7.7.16.Context ClassLoader
7.7.17.A tree structure that maps inheritance hierarchies of classes
7.7.18.Analyze ClassLoader hierarchy for any given object or class loader
7.7.19.Instantiate unknown class at runtime and call the object's methods
7.7.20.Load Class