stub/skeleton 极简理解

墨承泽

2023-12-01

IPC, RPC, Stub与Skeleton相关 2012-09-06 15:30:52

分类： Java

理解跨进程通信，重点是理解stub与skeleton的关系（一）Java api中有关RMI部分见： http://download.oracle.com/javase/6/docs/api/index.html （二） java RMI规范部分见：http://download.oracle.com/javase/6/docs/platform/rmi/spec/rmiTOC.html
这里有部分中文翻译: http://blog.chaoskey.com/2008/01/14/80 3.1 Stubs and Skeletons 5 Server Interfaces 8 Stub/Skeleton Interfaces （三）实例(转)：见：http://ajava.org/course/java/16865.html

RMI工作原理 (自定义实现一个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。

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

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

2. Person的实现PersonServer类

PersonServer代码:

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:

import java.io.ObjectOutputStream;
import java.io.ObjectInputStream;
import java.net.Socket;
public class Person_Stub implements Person {
private Socket socket;
public Person_Stub() throws Throwable {
// connect to skeleton
socket = new Socket( "computer_name" , 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:

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:

[java] view plain copy

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

stub/skeleton 极简理解

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