不停止JVM动态更新Java类，到底有什么用处？

对于大多数WEB程序这个功能并没有什么意义，这里假设在桌面系统以及服务器的情况：

桌面软件，当用户在线升级程序后，需要将最新的Java类动态提取到当前的classloader以供用户使用最新状态的程序

桌面软件，当需要各种插件时，需要从设定文件中提取信息，并在启动时提取。而这种提取需要在程序启动之后，例如Eclipse

Servlet容器，动态提取最新状态的jar包、Servlet类、JSP类以及设定文件等

不停止JVM动态更新Java类功能，显然在很多地方是必不可少的。以下笔者将以代码的形势演示如何实现此功能。

代码：

package org.logical_paradox.reloader;

import java.io.File;

import java.lang.reflect.Method;

import java.net.URL;

import java.net.URLClassLoader;

/**

* @author satoshi akabane

* @version $Revision$

*/

public class Reloader {

public static final String REPOSITORY = "./lib/";

private Reloader() {

}

public static final void main(String[] args) throws Exception {

Reloader reloader = new Reloader();

ClassLoader current = Thread.currentThread().getContextClassLoader();

// v1

log("HelloWorldLogic_v1 is now loading.");

ClassLoader loader = reloader.load(REPOSITORY + "HelloWorldLogic_v1.jar");

Thread.currentThread().setContextClassLoader(loader); // switch context class loader

Object objv1 = loader.loadClass("HelloWorldLogic").newInstance();

Method method = objv1.getClass().getMethod("getHello", new Class[0]);

// getHello()

Object msg = method.invoke(objv1, new Object[0]);

log("answer from v1 is [" + msg.toString() + "]");

// v2

loader = null;

log("HelloWorldLogic_v2 is now loading.");

loader = reloader.load(REPOSITORY + "HelloWorldLogic_v2.jar");

Thread.currentThread().setContextClassLoader(loader); // switch context class loader

Object objv2 = loader.loadClass("HelloWorldLogic").newInstance();

method = objv2.getClass().getMethod("getHello", new Class[0]);

// getHello()

msg = method.invoke(objv2, new Object[0]);

log("answer from v2 is [" + msg.toString() + "]");

Thread.currentThread().setContextClassLoader(current);

}

private static final void log(String msg) {

System.out.println("log: " + msg);

}

private URLClassLoader load(String jarname) throws Exception {

final String base = (new File(jarname).getCanonicalFile().toURL()).toString();

URL url = new URL("jar:" + base + "!/");

URLClassLoader loader = new URLClassLoader(new URL[]{url});

return loader;

}

}

执行结果：

[java] log: HelloWorldLogic_v1 is now loading.

[java] log: answer from v1 is [version 1]

[java] log: HelloWorldLogic_v2 is now loading.

[java] log: answer from v2 is [version 2]