下面的一個小工具類提供了對系統類載入器和擴充套件類載入器的動態控制能力.
可以在程式中加入classpath,當然也可以獲得類載入器載入的類列表,相信
java的動態能力!

package org.rut.core;

import java.io.File;
import java.io.PrintStream;
import java.lang.reflect.Field;
import java.lang.reflect.Method;
import java.net.MalformedURLException;
import java.net.URL;
import java.net.URLClassLoader;
import java.util.List;

import org.rut.RootException;

import sun.misc.Launcher;

/**
 * @author treeroot
 * @since 2006-2-12
 * @version 1.0
 */
public class ClassLoaderUtil {
    private static Field classes;

    private static Method addURL;
    static {
        try {
            classes = ClassLoader.class.getDeclaredField("classes");
            addURL = URLClassLoader.class.getDeclaredMethod("addURL",
                    new Class[] { URL.class });
        } catch (Exception e) {
            //won't happen ,but remain it
            throw new RootException(e);
        }
        classes.setAccessible(true);
        addURL.setAccessible(true);
    }

    private static URLClassLoader system = (URLClassLoader) getSystemClassLoader();

    private static URLClassLoader ext = (URLClassLoader) getExtClassLoader();

    public static ClassLoader getSystemClassLoader() {
        return ClassLoader.getSystemClassLoader();
    }

    public static ClassLoader getExtClassLoader() {
        return getSystemClassLoader().getParent();
    }

    /**
     * 獲得載入的類
     *
     * @return
     */
    public static List getClassesLoadedBySystemClassLoader() {
        return getClassesLoadedByClassLoader(getSystemClassLoader());
    }

    public static List getClassesLoadedByExtClassLoader() {
        return getClassesLoadedByClassLoader(getExtClassLoader());
    }

    public static List getClassesLoadedByClassLoader(ClassLoader cl) {
        try {
            return (List) classes.get(cl);
        } catch (Exception e) {
            throw new RootException(e);
        }
    }

    public static URL[] getBootstrapURLs() {
        return Launcher.getBootstrapClassPath().getURLs();
    }

    public static URL[] getSystemURLs() {
        return system.getURLs();
    }

    public static URL[] getExtURLs() {
        return ext.getURLs();
    }

    private static void list(PrintStream ps, URL[] classPath) {
        for (int i = 0; i < classPath.length; i++) {
            ps.println(classPath[i]);
        }
    }

    public static void listBootstrapClassPath() {
        listBootstrapClassPath(System.out);
    }

    public static void listBootstrapClassPath(PrintStream ps) {
        ps.println("BootstrapClassPath:");
        list(ps, getBootstrapClassPath());
    }

    public static void listSystemClassPath() {
        listSystemClassPath(System.out);
    }

    public static void listSystemClassPath(PrintStream ps) {
        ps.println("SystemClassPath:");
        list(ps, getSystemClassPath());
    }

    public static void listExtClassPath() {
        listExtClassPath(System.out);
    }

    public static void listExtClassPath(PrintStream ps) {
        ps.println("ExtClassPath:");
        list(ps, getExtClassPath());
    }

    public static URL[] getBootstrapClassPath() {
        return getBootstrapURLs();
    }

    public static URL[] getSystemClassPath() {
        return getSystemURLs();
    }

    public static URL[] getExtClassPath() {
        return getExtURLs();
    }

    public static void addURL2SystemClassLoader(URL url) {
        try {
            addURL.invoke(system, new Object[] { url });
        } catch (Exception e) {
            throw new RootException(e);
        }
    }

    public static void addURL2ExtClassLoader(URL url) {
        try {
            addURL.invoke(ext, new Object[] { url });
        } catch (Exception e) {
            throw new RootException(e);
        }
    }

    public static void addClassPath(String path) {
        addClassPath(new File(path));
    }

    public static void addExtClassPath(String path) {
        addExtClassPath(new File(path));
    }

    public static void addClassPath(File dirOrJar) {
        try {
            addURL2SystemClassLoader(dirOrJar.toURL());
        } catch (MalformedURLException e) {
            throw new RootException(e);
        }
    }

    public static void addExtClassPath(File dirOrJar) {
        try {
            addURL2ExtClassLoader(dirOrJar.toURL());
        } catch (MalformedURLException e) {
            throw new RootException(e);
        }
    }

}