EJB 工作原理

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()

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

2. Person的实现PersonServer类

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

    public PersonServer(String name, int age.html); {
        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.html)和Skeleton(Server端.html)程序。这是Stub的实现：

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.html);;
    }

    public int getAge(); throws Throwable {
        // pass method name to skeleton
        ObjectOutputStream outStream =
            new ObjectOutputStream(socket.getOutputStream(.html););;
        outStream.writeObject("age".html);;
        outStream.flush();;

        ObjectInputStream inStream =
            new ObjectInputStream(socket.getInputStream(.html););;
        return inStream.readInt();;
    }

    public String getName(); throws Throwable {
        // pass method name to skeleton
        ObjectOutputStream outStream =
            new ObjectOutputStream(socket.getOutputStream(.html););;
        outStream.writeObject("name".html);;
        outStream.flush();;

        ObjectInputStream inStream =
            new ObjectInputStream(socket.getInputStream(.html););;
        return (String.html);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

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.html); {
        // get reference of object server
        this.myServer = server;
    }

    public void run(); {
        try {
            // new socket at port 9000
            ServerSocket serverSocket = new ServerSocket(9000.html);;
            // accept stub's request
            Socket socket = serverSocket.accept();;

            while (socket != null.html); {
                // get stub's request
                ObjectInputStream inStream =
                    new ObjectInputStream(socket.getInputStream(.html););;
                String method = (String.html);inStream.readObject();;

                // check method name
                if (method.equals("age".html);); {
                    // execute object server's business method
                    int age = myServer.getAge();;
                    ObjectOutputStream outStream =
                        new ObjectOutputStream(socket.getOutputStream(.html););;

                    // return result to stub
                    outStream.writeInt(age.html);;
                    outStream.flush();;
                }

                if(method.equals("name".html);); {
                    // execute object server's business method
                    String name = myServer.getName();;
                    ObjectOutputStream outStream =
                        new ObjectOutputStream(socket.getOutputStream(.html););;

                    // return result to stub
                    outStream.writeObject(name.html);;
                    outStream.flush();;
                }
            }
        } catch(Throwable t.html); {
            t.printStackTrace();;
            System.exit(0.html);;
        }
    }

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

        Person\_Skeleton skel = new Person\_Skeleton(person.html);;
        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

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