类加载处理

package com.atom.util;

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

/**
 * <p>
 * <a href="ClassLoaderUtils.java.html"><i>View Source</i></a>
 * </p>
 *
 * @author KHT
 * @version $Id: ClassLoaderUtils.java 9326 2008-10-07 03:44:39Z bai $
 */
public class ClassLoaderUtils {

    /**
     * Load a given resource.
     * <p/>
     * This method will try to load the resource using the following methods (in order):
     * <ul>
     * <li>From {@link Thread#getContextClassLoader() Thread.currentThread().getContextClassLoader()}
     * <li>From {@link Class#getClassLoader() ClassLoaderUtil.class.getClassLoader()}
     * <li>From the {@link Class#getClassLoader() callingClass.getClassLoader() }
     * </ul>
     *
     * @param resourceName The name of the resource to load
     * @param callingClass The Class object of the calling object
     */
    @SuppressWarnings("unchecked")
	public static URL getResource(String resourceName, Class callingClass) {
        URL url = null;

        url = Thread.currentThread().getContextClassLoader().getResource(resourceName);

        if (url == null) {
            url = ClassLoaderUtils.class.getClassLoader().getResource(resourceName);
        }

        if (url == null) {
            url = callingClass.getClassLoader().getResource(resourceName);
        }

        return url;
    }

    /**
     * This is a convenience method to load a resource as a stream.
     * <p/>
     * The algorithm used to find the resource is given in getResource()
     *
     * @param resourceName The name of the resource to load
     * @param callingClass The Class object of the calling object
     */
    @SuppressWarnings("unchecked")
	public static InputStream getResourceAsStream(String resourceName, Class callingClass) {
        URL url = getResource(resourceName, callingClass);

        try {
            return (url != null) ? url.openStream() : null;
        } catch (IOException e) {
            return null;
        }
    }

    /**
     * Load a class with a given name.
     * <p/>
     * It will try to load the class in the following order:
     * <ul>
     * <li>From {@link Thread#getContextClassLoader() Thread.currentThread().getContextClassLoader()}
     * <li>Using the basic {@link Class#forName(java.lang.String) }
     * <li>From {@link Class#getClassLoader() ClassLoaderUtil.class.getClassLoader()}
     * <li>From the {@link Class#getClassLoader() callingClass.getClassLoader() }
     * </ul>
     *
     * @param className    The name of the class to load
     * @param callingClass The Class object of the calling object
     * @throws ClassNotFoundException If the class cannot be found anywhere.
     */
    @SuppressWarnings("unchecked")
	public static Class loadClass(String className, Class callingClass) throws ClassNotFoundException {
        try {
            return Thread.currentThread().getContextClassLoader().loadClass(className);
        } catch (ClassNotFoundException e) {
            try {
                return Class.forName(className);
            } catch (ClassNotFoundException ex) {
                try {
                    return ClassLoaderUtils.class.getClassLoader().loadClass(className);
                } catch (ClassNotFoundException exc) {
                    return callingClass.getClassLoader().loadClass(className);
                }
            }
        }
    }

    /**
     * Prints the current classloader hierarchy - useful for debugging.
     */
    public static void printClassLoader() {
        System.out.println("ClassLoaderUtils.printClassLoader");
        printClassLoader(Thread.currentThread().getContextClassLoader());
    }

    /**
     * Prints the classloader hierarchy from a given classloader - useful for debugging.
     */
    public static void printClassLoader(ClassLoader cl) {
        System.out.println("ClassLoaderUtils.printClassLoader(cl = " + cl + ")");

        if (cl != null) {
            printClassLoader(cl.getParent());
        }
    }
}