RMI代码动态下载

我看网上讲RMI应用时都是些比较老的文章，从前部署一个RMI应用有6个步骤:
(1)定义和实现远端接口中的参数
(2)定义和实现远端接口
(3)编写服务端代码
(4)编写客户端代码
(5)生成stub和skeltion ,并将stub打包到客户端jar中，将skeltion打包到服务端jar中
(6)启动rmiregistry , 并将服务注册到rmiregistry中，然后运行代码。
上边的步骤涉及到了好几个长串复杂的命令。。。
但在JDK5中使用了动态代理技术，实现了动态生成stub和skeltion类，从而省却了相当多的繁琐工作。(我这还是后来知道，我是说我这么简单就实现了RMI，跟网上说的咋区别那大。。。)

如此一来RMI成了相当轻量级的技术了，但还是会遇到繁琐的事情，比如代码动态下载，动态下载允许RMI应用动态从http后者ftp下载类定义的字节码（运行时加入参数-Djava.rmi.server.codebase=http://***/*.jar），就像是把远程的类加入本地的classpath一样，相当方便的技术。问题就是如果我们只是学习或做个简单的应用，为了部署一个RMI还要启用一个http或ftp服务，多么麻烦，谁都想只用启动程序就可以了，尤其是在调试的时候。当然如果只是在本地调试的话，可以用file://代替http，可是如果是个简单的应用必须用到http服务，但又不想兴师动众的启动一个http服务器的话JDK6的新特性之一http轻量级服务就派上用场了。
RMI代码动态下载其实就用到了HTTP协议的下载功能，所以，可以把这部分工作给一个简单的通用类就行了。如下：
//RmiHttpServer.java import java.io.File; import java.io.FileInputStream; import java.io.IOException; import java.io.OutputStream; import java.net.InetSocketAddress; import java.util.concurrent.Executors; import com.sun.net.httpserver.HttpExchange; import com.sun.net.httpserver.HttpHandler; import com.sun.net.httpserver.HttpServer; import com.sun.net.httpserver.spi.HttpServerProvider; public class RmiHttpServer { private int port; private HttpServer httpServer; private boolean isRunning = false; private String folder = ""; private String context = "/"; public RmiHttpServer() { this.port = 80; } public RmiHttpServer(int port) { this.port = port; } public RmiHttpServer(String context, String folder, int port) { this.folder = folder; this.context = context; this.port = port; } public synchronized void service() { if (isRunning) { System.out.println("HTTP服务已启动..."); } else { HttpServerProvider httpServerProvider = HttpServerProvider .provider(); InetSocketAddress addr = new InetSocketAddress(port); try { httpServer = httpServerProvider.createHttpServer(addr, 1); httpServer.createContext(context, new RmiHttpHandler(folder)); httpServer.setExecutor(Executors.newCachedThreadPool()); httpServer.start(); System.out.println("HTTP服务已启动..."); isRunning = true; } catch (IOException e) { e.printStackTrace(); System.out.println("HTTP服务启动失败"); } } } public synchronized void close() { isRunning = false; httpServer.stop(0); System.out.println("HTTP服务关闭"); } } class RmiHttpHandler implements HttpHandler { public static int BUFFER_LENGTH = 1024; private String folder; public RmiHttpHandler(String folder) { this.folder = folder; } public void handle(HttpExchange httpExchange) throws IOException { String response; OutputStream out = httpExchange.getResponseBody(); String path; if (folder.equals("")) path = httpExchange.getRequestURI().getPath().substring(1); else path = folder + httpExchange.getRequestURI().getPath(); //System.out.println(path); File file = new File(path); if (file.exists() && file.canRead()) { httpExchange.sendResponseHeaders(200, file.length()); FileInputStream in = new FileInputStream(file); byte[] buffer = new byte[BUFFER_LENGTH]; int count = 0; for (int i = 0; count != -1; i++) { count = in.read(buffer); if (count != -1) out.write(buffer, 0, count); } in.close(); } else { response = "error"; httpExchange.sendResponseHeaders(404, response.length()); out.write(response.getBytes()); } out.close(); } }
下面是个简单的例子来说明代码的动态下载和上面类的使用方法。
服务端建立文件夹server。
首先是要动态下载的接口
//User.java import java.io.Serializable; public interface User extends Serializable{ public String getName(); }
然后是服务接口
//Hello.java import java.rmi.Remote; import java.rmi.RemoteException; public interface Hello extends Remote { public User hello(String name) throws RemoteException; }

两个接口的实现
//UserImpl.java public class UserImpl implements User { private static final long serialVersionUID = -5401902546629154615L; private String name; public UserImpl(String n){ name = n; } public String getName() { return name+" 你好！我是aa"; } } //HelloImpl.java import java.rmi.RemoteException; import java.rmi.server.UnicastRemoteObject; public class HelloImpl extends UnicastRemoteObject implements Hello { private static final long serialVersionUID = -4981732846986548202L; public HelloImpl() throws RemoteException { super(); } public User hello(String name) throws RemoteException { System.out.println("服务端负责接口的实例化"); return new UserImpl(name); } }
启动服务
//Server.java import java.rmi.Naming; import java.rmi.RMISecurityManager; import java.rmi.RemoteException; import java.rmi.registry.LocateRegistry; public class Server { public static void main(String argv[]) throws RemoteException { System.setProperty("java.security.policy", "rmi.policy");//加载安全配置 LocateRegistry.createRegistry(1099); if (System.getSecurityManager() == null) { System.setSecurityManager(new RMISecurityManager()); } try { Hello hello = new HelloImpl(); Naming.rebind("//127.0.0.1/HelloServer", hello);//绑定 } catch (Exception e) { e.printStackTrace(); } new RmiHttpServer(80).service();//启动http服务，负责将当前目录的字节码动态传输给客户端 } }

配置文件rmi.policy
grant  {
permission java.security.AllPermission;
};

下面就是客户端，建立文件夹client
我们只需要把客户端需要的接口User.class，Hello.class拷贝到classpath（即client目录下）就可以了，由于用到了代码动态下载，所以得获得许可，我们还需把rmi.policy也拷贝到环境中。
//Client.java import java.rmi.*; public class Client { public static void main(String[] args) throws Exception { System.setProperty("java.security.policy", "rmi.policy"); System.setProperty("java.rmi.server.codebase","http://127.0.0.1/");//指名需要下载的类的路径 //注意，如果没有上面一行，会出现java.lang.ClassNotFoundException: UserImpl错误 //因为UserImpl并不在本地环境中 System.setSecurityManager(new RMISecurityManager()); Hello hello = (Hello) Naming.lookup("//127.0.0.1/HelloServer"); //通过RMI名称查找远程对象 System.out.println(hello.hello("syc").getName());//执行下载下来的类的getName方法 } }

 

接下来执行Server，和Client，就可以看到结果了，如果我们改变UserImpl的实现，再次执行Client时，就会发现变化。