自定义ClassLoader

import java.io.DataInputStream;
import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;
import java.util.Hashtable;

public class CustomClassLoader extends ClassLoader {
	/**
	 * 从来存放所加载的类
	 */
	private Hashtable<String, Class<?>> classes = new Hashtable<String, Class<?>>();

	/**
	 * 要加载文件的目录
	 */
	private String rootDir;

	/**
	 * 将此类加载器的父类设置为加载此类的类加载器
	 */
	public CustomClassLoader(String rootDir) {
		super(CustomClassLoader.class.getClassLoader());
		if (rootDir == null || rootDir.trim().equals("")) {
			throw new IllegalArgumentException("您说输入的文件目录为空");
		}
		this.rootDir = rootDir;
	}

	/**
	 * 根据类名加载类文件
	 */
	public Class<?> loadClass(String className) throws ClassNotFoundException {
		return findClass(className);
	}

	/**
	 * 根据类名查找类
	 */
	public Class<?> findClass(String className) {
		byte classByte[];
		Class<?> result = null;

		result = (Class<?>) classes.get(className);
		if (result == null) {
			try {//首先通过系统类加载器加载
				return findSystemClass(className);
			} catch (Exception e) {

			}
		}else{
			return result;
		}

		//通过自定义类加载器加载类文件
		try {
			String classPath = rootDir + "\\"
					+ className.replace('.', File.separatorChar) + ".class";
			classByte = loadClassData(classPath);
			result = defineClass(className, classByte, 0, classByte.length,
					null);
			if(result==null){
				throw new ClassNotFoundException();
			}
			classes.put(className, result);
			return result;
		} catch (Exception e) {
			e.printStackTrace();
			return null;
		}

	}

	// 负责加载class文件的数据信息
	private byte[] loadClassData(String className) throws IOException {
		File f;
		f = new File(className);
		int size = (int) f.length();
		byte buff[] = new byte[size];
		FileInputStream fis = new FileInputStream(f);
		DataInputStream dis = new DataInputStream(fis);
		dis.readFully(buff);
		dis.close();
		return buff;
	}
}