stub和skeleton

RMI的本质就是实现在不同JVM之间的调用,它的实现方法就是在两个JVM中各开一个Stub和Skeleton，二者通过socket通信来实现参数和返回值的传递。

     有关RMI的例子代码网上可以找到不少,但绝大部分都是通过extend the interface java.rmi.Remote实现，已经封装的很完善了，不免使人有雾里看花的感觉。下面的例子是我在《Enterprise JavaBeans》里看到的，虽然很粗糙，但很直观，利于很快了解它的工作原理。

Ⅰ：RMI工作原理(自定义实现一个Stub 和 Skeleton)

1. 定义一个Person的接口，其中有两个business method, getAge() 和getName()

Person代码:

 

package common;

public interface Person {
	public int getAge() throws Throwable;

	public String getName() throws Throwable;
}

 

 

 

2. Person的实现PersonServer类

PersonServer代码:

 

package server_test;

import common.Person;

public class PersonServer implements Person {
	private int age;
	private String name;

	public PersonServer(String name, int age) {
		this.age = age;
		this.name = name;
	}

	public int getAge() {
		return age;
	}

	public String getName() {
		return name;
	}
}

 

 

3. 好，我们现在要在Client机器上调用getAge()和getName()这两个business method，那么就得编写相应的Stub(Client端)和Skeleton(Server端)程序。这是Stub的实现：

Person_Stub代码:

//存根（stub）Person_Stub的实现:

 

 

package client_test;

import java.io.ObjectOutputStream;
import java.io.ObjectInputStream;
import java.net.Socket;

import common.Person;

public class Person_Stub implements Person {
	private Socket socket;

	public Person_Stub() throws Throwable {
		// connect to skeleton
		socket = new Socket("10.9.192.156", 9000);
	}

	public int getAge() throws Throwable {
		// pass method name to skeleton
		ObjectOutputStream outStream = new ObjectOutputStream(
				socket.getOutputStream());
		outStream.writeObject("age");
		outStream.flush();
		ObjectInputStream inStream = new ObjectInputStream(
				socket.getInputStream());
		return inStream.readInt();
	}

	public String getName() throws Throwable {
		// pass method name to skeleton
		ObjectOutputStream outStream = new ObjectOutputStream(
				socket.getOutputStream());
		outStream.writeObject("name");
		outStream.flush();
		ObjectInputStream inStream = new ObjectInputStream(
				socket.getInputStream());
		return (String) inStream.readObject();
	}
}

 

 

注意，Person_Stub和PersonServer一样，都implements Person。它们都实现了getAge()和getName()两个business method，不同的是PersonServer是真的实现，Person_Stub是建立socket连接，并向Skeleton发请求，然后通过Skeleton调用PersonServer的方法，最后接收返回的结果。

4. 骨架（Skeleton）的实现

Person_Skeleton代码:

 

package server_test;

import java.io.ObjectOutputStream;
import java.io.ObjectInputStream;
import java.net.Socket;
import java.net.ServerSocket;

public class Person_Skeleton extends Thread {
	private PersonServer myServer;

	public Person_Skeleton(PersonServer server) {
		// get reference of object server
		this.myServer = server;
	}

	public void run() {
		try {
			// new socket at port 9000
			ServerSocket serverSocket = new ServerSocket(9000);
			// accept stub's request
			Socket socket = serverSocket.accept();
			while (socket != null) {
				// get stub's request
				ObjectInputStream inStream = new ObjectInputStream(
						socket.getInputStream());
				String method = (String) inStream.readObject();
				// check method name
				if (method.equals("age")) {
					// execute object server's business method
					int age = myServer.getAge();
					ObjectOutputStream outStream = new ObjectOutputStream(
							socket.getOutputStream());
					// return result to stub
					outStream.writeInt(age);
					outStream.flush();
				}
				if (method.equals("name")) {
					// execute object server's business method
					String name = myServer.getName();
					ObjectOutputStream outStream = new ObjectOutputStream(
							socket.getOutputStream());
					// return result to stub
					outStream.writeObject(name);
					outStream.flush();
				}
			}
		} catch (Throwable t) {
			t.printStackTrace();
			System.exit(0);
		}
	}

	public static void main(String args[]) {
		// new object server
		PersonServer person = new PersonServer("Richard", 34);
		Person_Skeleton skel = new Person_Skeleton(person);
		skel.start();
	}
}

 

  

Skeleton类 extends from Thread，它长驻在后台运行，随时接收client发过来的request。并根据发送过来的key去调用相应的business method。

5. 最后一个，Client的实现

PersonClient 代码:

 

package client_test;

import common.Person;

public class PersonClient {      
    public static void main(String [] args) {      
        try {      
            Person person = new Person_Stub();      
            int age = person.getAge();      
            String name = person.getName();      
            System.out.println(name + " is " + age + " years old");      
       } catch(Throwable t) {      
            t.printStackTrace();      
        }      
    }      
}      

 

Client(PersonClient)的本质是，它要知道Person接口的定义，并实例一个Person_Stub，通过Stub来调用business method,至于Stub怎么去和Server沟通，Client就不用管了。

注意它的写法：

Person person = new Person_Stub();而不是Person_Stub person = new Person_Stub();为什么？因为要面向接口编程嘛，呵呵。

//RMI实质上就是生成2个类stub，skeleton来进行参数和返回值的传递，采用值传递方式

//类似于以前写的聊天室程序，被传递的对象应实现java.io.Serializable接口