EJB的调用原理

前两天在这个版块的精华区里翻到了Robbin关于EJB的调用原理的分析，受益非浅，但感觉用纯文字来表达效果似乎不够直观，而且对RMI的阐述也略嫌少了些。这里我根据自己的一点体会，在Robbin帖子的基础上再来说说这个话题，供大家参考。

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()


代码

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


2. Person的实现PersonServer类


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：


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


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


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。

感谢您有耐心看到这里，关于RMI，我想说的就这么多了。但是好象还没写到EJB，本人就累了个半死，算了，我还是先去睡觉，明天再往下续吧。。。