不停止JVM动态更新Java类

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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：

1. 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
2. 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
3. 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]

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