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EJB Pre之一----------RMI工作原理

发表于: 2006-09-08   作者:crownquartz   来源:转载   浏览:
摘要: 首先,我想先说说RMI的工作原理,因为EJB毕竟是基于RMI的嘛。废话就不多讲了,RMI的本质就是实现在不同JVM之间的调用,工作原理图如下: 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
首先,我想先说说RMI的工作原理,因为EJB毕竟是基于RMI的嘛。废话就不多讲了,RMI的本质就是实现在不同JVM之间的调用,工作原理图如下:

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()

java代码: 


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




2. Person的实现PersonServer类
java代码: 


public class PersonServer implements Person {
    int age;
    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:
java代码: 


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

public class Person_Stub implements Person {
    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. Skeleton实现
java代码: 


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

public class Person_Skeleton extends Thread {
    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
java代码: 


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。

EJB Pre之一----------RMI工作原理

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