RMI的原理与java代码实现

RMI 概述

RMI(Remote Method Invocation)是一种执行远程调用的Java API。RMI的目的是使在不同计算机上运行的对象(Object)之间的调用行为类似于本地调用。

在RMI机制中，服务器(Server)应实现一定的功能并将其注册到注册表(Registry)中，客户端(Client)应在注册表的命名空间(Naming space)中获取对象

RMI 细节 

当机器A上的代码想要调用驻留在机器B上的远程对象时，始终有两个中间对象实际处理通信：存根(stub object)和骨架(skeleton object)。

存根(stub)对象位于机器A中，必须：

构建一个信息块(information block)，该信息块包括

要使用的远程对象的标识符(identifier)，

描述要调用的方法的操作号(operation number)

骨架对象位于机器B中，其任务是

要解组(unmarshal)参数，

要在服务器上的真实对象上调用所需的方法，

要捕获服务器上调用的返回值或异常(exception)，

要将包含封送格式(marshalled form)的值的返回给客户机上的存根。

RMI简单实验

一台电脑调用不同电脑上的‘获取随机数方法’，并把获得的随机数加和。

效果：

 机器间连接：

Client中Getregistry() 使用给定的localhost、端口号(19999)在本地创建一个存根对象(stub object)作为注册表远程对象的代理

Server中用 createRegistry() 绑定相同的端口。用Rebind() 使用名称绑定自己。这样就可以在RMI注册表中注册远程对象。

实验拓扑结构： 

JAVA代码 

HelloClient.java

import java.rmi.*;
import java.rmi.registry.*;
import java.net.InetAddress;

public class HelloClient {

    public static void main(String[] args) {
		int num1 = (int)(Math.random()*100);
		int result = 0;
        String text = "rmi work";
        HelloInterface rmi = null;

        try {
            Registry registry = LocateRegistry.getRegistry("192.168.128.21", 19999);
            rmi = (HelloInterface) registry.lookup("server"); 
            System.out.println("Connected to Server");
        } catch (Exception e) {
            e.printStackTrace();
        }
        try {
            InetAddress address = InetAddress.getLocalHost();
            rmi.sendAddress(address);
        } catch (Exception e) {
            System.out.println("ERROR: Could not create the registry.");
            e.printStackTrace();
        }
        if (rmi != null) {
            try {
                System.out.println(rmi.getMessage(text));
                result = rmi.sum(num1, rmi.getMessage(text));
				rmi.sendMessage(Integer.toString(num1));
                rmi.setSum(result);
				rmi.printSum();
				
            } catch (RemoteException e) {
                e.printStackTrace();
            }
            System.out.println("Finished");
        }
    }
}

HelloServer.java

import java.rmi.*;
import java.rmi.registry.*;
import java.rmi.server.*;
import java.net.InetAddress;

public class HelloServer implements HelloInterface {
    private int sum;
	@Override
    public int sum(int num1, int num2) throws RemoteException {
        return num1+num2;
    }
    @Override
    public String say() throws RemoteException {
        return "message";
    }
    @Override
    public void sendMessage(String s) throws RemoteException {
        System.out.println(s);
    }

    @Override
    public void sendAddress(InetAddress address) throws RemoteException {
        System.out.println(address);
    }

    @Override
    public int getMessage(String text) throws RemoteException {
        return sum;
    }
    @Override
    public void setSum(int result) throws RemoteException {
        sum = result;
    }
    @Override
    public void printSum() throws RemoteException {
        System.out.println("sum: " + Integer.toString(sum));
    }

    public static void main(String[] args) {
        int sum = 0;
        Registry reg = null;
        try {
            reg = LocateRegistry.createRegistry(19999);
            InetAddress address = InetAddress.getLocalHost();
            System.out.println("DCS Starter:" + address);
        } catch (Exception e) {
            System.out.println("ERROR: Could not create the registry.");
            e.printStackTrace();
        }

        HelloServer serverObject = new HelloServer();
        try {
            reg.rebind("server", (HelloInterface) UnicastRemoteObject.exportObject(serverObject, 0));
        } catch (Exception e) {
            System.out.println("ERROR: Failed to register the server object.");
            e.printStackTrace();
        }
    }
}

HelloInterface.java

import java.rmi.*;
import java.net.InetAddress;

public interface HelloInterface extends Remote {
	public String say() throws RemoteException;
	public void sendMessage(String text) throws RemoteException;
	public void setSum(int result) throws RemoteException;
	public int getMessage(String text) throws RemoteException;
	public int sum(int num1, int num2) throws RemoteException;
	public void sendAddress(InetAddress address) throws RemoteException;
	public void printSum() throws RemoteException;
	}