自定义类加载器，实现动态更新DEMO

import java.io.File;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.io.InputStream;
import java.util.HashSet;

class CustomCL extends ClassLoader {

	private String basedir; // 需要该类加载器直接加载的类文件的基目录

	private HashSet dynaclazns; // 需要由该类加载器直接加载的类名

	public CustomCL(String basedir, String[] clazns) {

		super(null); // 指定父类加载器为 null

		this.basedir = basedir;

		dynaclazns = new HashSet();

		loadClassByMe(clazns);

	}

	private void loadClassByMe(String[] clazns) {

		for (int i = 0; i < clazns.length; i++) {

			loadDirectly(clazns[i]);

			dynaclazns.add(clazns[i]);

		}

	}

	private Class loadDirectly(String name) {

		Class cls = null;

		StringBuffer sb = new StringBuffer(basedir);

		String classname = name.replace('.', File.separatorChar) + ".class";

		sb.append(File.separator + classname);

		File classF = new File(sb.toString());

		try {
			cls = instantiateClass(name, new FileInputStream(classF),

			classF.length());
		} catch (FileNotFoundException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}

		return cls;

	}

	private Class instantiateClass(String name, InputStream fin, long len) {

		byte[] raw = new byte[(int) len];

		try {
			fin.read(raw);
			fin.close();
		} catch (IOException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}

		return defineClass(name, raw, 0, raw.length);

	}

	protected Class loadClass(String name, boolean resolve) throws ClassNotFoundException {

		Class cls = null;

		cls = findLoadedClass(name);

		if (!this.dynaclazns.contains(name) && cls == null)

			cls = getSystemClassLoader().loadClass(name);

		if (cls == null)

			throw new ClassNotFoundException(name);

		if (resolve)

			resolveClass(cls);

		return cls;

	}

}

import java.lang.reflect.Method;
import java.util.TimerTask;

public class TimeTask extends TimerTask {
	// 在这里写你要定时执行的方法
	public void run() {
		//System.out.println("Hello World!");
		try {

			// 每次都创建出一个新的类加载器
			String baseDir = Test.class.getResource("/").getPath();
			
			/*System.out.println(ClassLoader.getSystemResource(""));        
			System.out.println(Test.class.getResource(""));        
			System.out.println(Test.class.getResource("/")); //Class文件所在路径 
			System.out.println(new File("/").getAbsolutePath());        
			System.out.println(System.getProperty("user.dir"));*/

			CustomCL cl = new CustomCL(baseDir, new String[] { "com.strongit.test.Foo" });

			Class cls = cl.loadClass("com.strongit.test.Foo");

			Object foo = cls.newInstance();

			Method m = foo.getClass().getMethod("sayHello", new Class[] {});

			m.invoke(foo, new Object[] {});

		} catch (Exception ex) {

			ex.printStackTrace();

		}
	}
}

public class Foo {

	/**
	 * @param args
	 */
	public void sayHello() {
		// TODO Auto-generated method stub
		System.out.println("hello world! (version one)");
	}

}