5.3.6. RMI

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5.3.6.1. 简介

RMI(Remote Method Invocation,远程方法调用)能够让在客户端Java虚拟机上的对象像调用本地对象一样调用服务端Java虚拟机中的对象上的方法。其中RMI标准实现是Java RMI,之外还有Weblogic RMI、Spring RMI等不同的实现。

RMI中比较重要的两个概念是Stub和Skeleton,Stub和Skeleton对同一套接口进行实现,其中Stub由Client端调用,并不进行真正的实现,而是和Server端通信。Skeleton是Server端,监听来自Stub的连接,根据Stub发送的数据进行真正的操作。

5.3.6.2. 调用步骤

  • 客户调用客户端辅助对象Stub上的方法
  • 客户端辅助对象Stub打包调用信息(变量,方法名),通过网络发送给服务端辅助对象Skeleton
  • 服务端辅助对象Skeleton将客户端辅助对象发送来的信息解包,找出真正被调用的方法以及该方法所在对象
  • 调用真正服务对象上的真正方法,并将结果返回给服务端辅助对象Skeleton
  • 服务端辅助对象将结果打包,发送给客户端辅助对象Stub
  • 客户端辅助对象将返回值解包,返回给调用者
  • 客户获得返回值

5.3.6.3. 样例

一份代码样例如下(来自《Enterprise JavaBeans》):

5.3.6.3.1. Person接口定义

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

5.3.6.3.2. 使用PersonServer实现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;
    }
}

5.3.6.3.3. 使用Person_Stub实现Person

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();
    }
}

5.3.6.3.4. Skeleton的实现

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();
    }
}

5.3.6.3.5. Client实现

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();
        }
    }
}

5.3.6.4. T3协议

T3协议是用于在WebLogic服务器和其他类型的Java程序之间传输信息的协议,是Weblogic对RMI规范的实现。简单来说,可以把T3视为暴露JDNI给用户调用的接口。

5.3.6.5. JRMP

Java远程方法协议(Java Remote Method Protocol,JRMP)是特定于Java技术的、用于查找和引用远程对象的协议。这是运行在Java远程方法调用(RMI)之下、TCP/IP之上的线路层协议。

JRMP是一个Java特有的、适用于Java之间远程调用的基于流的协议,要求客户端和服务器上都使用Java对象。

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