前兩天在這個版塊的精華區里翻到了Robbin關于EJB的調用原理的分析，受益非淺，但感覺用純文字來表達效果似乎不夠直觀，而且對RMI的闡述也略嫌少了些。這里我根據自己的一點體會，在Robbin帖子的基礎上再來說說這個話題，供大家參考。 

首先，我想先說說RMI的工作原理，因為EJB畢竟是基于RMI的嘛。廢話就不多講了，RMI的本質就是實現在不同JVM之間的調用，工作原理圖如下： 

 

它的實現方法就是在兩個JVM中各開一個Stub和Skeleton，二者通過socket通信來實現參數和返回值的傳遞。 

有關RMI的例子代碼網上可以找到不少，但絕大部分都是通過extend the interface java.rmi.Remote實現，已經封裝的很完善了，不免使人有霧里看花的感覺。下面的例子是我在《Enterprise JavaBeans》里看到的，雖然很粗糙，但很直觀，利于很快了解它的工作原理。 

1. 定義一個Person的接口，其中有兩個business method, getAge() 和getName() 


Java代碼 
1.public interface Person {   
2.    public int getAge(); throws Throwable;   
3.    public String getName(); throws Throwable;   
4.}  
public interface Person {
    public int getAge(); throws Throwable;
    public String getName(); throws Throwable;
}


2. Person的實現PersonServer類 

Java代碼 
1.public class PersonServer implements Person {   
2.    int age;   
3.    String name;   
4.  
5.    public PersonServer(String name, int age); {   
6.        this.age = age;   
7.        this.name = name;   
8.    }   
9.  
10.    public int getAge(); {   
11.        return age;   
12.    }   
13.  
14.    public String getName(); {   
15.        return name;   
16.    }   
17.}  
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，我們現在要在Client機器上調用getAge()和getName()這兩個business method，那么就得編寫相應的Stub(Client端)和Skeleton(Server端)程序。這是Stub的實現： 

Java代碼 
1.import java.io.ObjectOutputStream;   
2.import java.io.ObjectInputStream;   
3.import java.net.Socket;   
4.  
5.public class Person_Stub implements Person {   
6.    Socket socket;   
7.  
8.    public Person_Stub(); throws Throwable {   
9.        // connect to skeleton   
10.        socket = new Socket("computer_name", 9000);;   
11.    }   
12.  
13.    public int getAge(); throws Throwable {   
14.        // pass method name to skeleton   
15.        ObjectOutputStream outStream =   
16.            new ObjectOutputStream(socket.getOutputStream(););;   
17.        outStream.writeObject("age");;   
18.        outStream.flush();;   
19.  
20.        ObjectInputStream inStream =   
21.            new ObjectInputStream(socket.getInputStream(););;   
22.        return inStream.readInt();;   
23.    }   
24.  
25.    public String getName(); throws Throwable {   
26.        // pass method name to skeleton   
27.        ObjectOutputStream outStream =   
28.            new ObjectOutputStream(socket.getOutputStream(););;   
29.        outStream.writeObject("name");;   
30.        outStream.flush();;   
31.  
32.        ObjectInputStream inStream =   
33.            new ObjectInputStream(socket.getInputStream(););;   
34.        return (String);inStream.readObject();;   
35.    }   
36.}  
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一樣，都implements Person。它們都實現了getAge()和getName()兩個business method，不同的是PersonServer是真的實現，Person_Stub是建立socket連接，并向Skeleton發請求，然后通過Skeleton調用PersonServer的方法，最后接收返回的結果。 

4. Skeleton實現 

Java代碼 
1.import java.io.ObjectOutputStream;   
2.import java.io.ObjectInputStream;   
3.import java.net.Socket;   
4.import java.net.ServerSocket;   
5.  
6.public class Person_Skeleton extends Thread {   
7.    PersonServer myServer;   
8.  
9.    public Person_Skeleton(PersonServer server); {   
10.        // get reference of object server   
11.        this.myServer = server;   
12.    }   
13.  
14.    public void run(); {   
15.        try {   
16.            // new socket at port 9000   
17.            ServerSocket serverSocket = new ServerSocket(9000);;   
18.            // accept stub's request   
19.            Socket socket = serverSocket.accept();;   
20.  
21.            while (socket != null); {   
22.                // get stub's request   
23.                ObjectInputStream inStream =   
24.                    new ObjectInputStream(socket.getInputStream(););;   
25.                String method = (String);inStream.readObject();;   
26.  
27.                // check method name   
28.                if (method.equals("age");); {   
29.                    // execute object server's business method   
30.                    int age = myServer.getAge();;   
31.                    ObjectOutputStream outStream =   
32.                        new ObjectOutputStream(socket.getOutputStream(););;   
33.  
34.                    // return result to stub   
35.                    outStream.writeInt(age);;   
36.                    outStream.flush();;   
37.                }   
38.  
39.                if(method.equals("name");); {   
40.                    // execute object server's business method   
41.                    String name = myServer.getName();;   
42.                    ObjectOutputStream outStream =   
43.                        new ObjectOutputStream(socket.getOutputStream(););;   
44.  
45.                    // return result to stub   
46.                    outStream.writeObject(name);;   
47.                    outStream.flush();;   
48.                }   
49.            }   
50.        } catch(Throwable t); {   
51.            t.printStackTrace();;   
52.            System.exit(0);;   
53.        }   
54.    }   
55.  
56.    public static void main(String args []); {   
57.        // new object server   
58.        PersonServer person = new PersonServer("Richard", 34);;   
59.  
60.        Person_Skeleton skel = new Person_Skeleton(person);;   
61.        skel.start();;   
62.    }   
63.}  
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類 extends from Thread，它長駐在后臺運行，隨時接收client發過來的request。并根據發送過來的key去調用相應的business method。 

5. 最后一個，Client的實現 

Java代碼 
1.public class PersonClient {   
2.    public static void main(String [] args); {   
3.        try {   
4.            Person person = new Person_Stub();;   
5.            int age = person.getAge();;   
6.            String name = person.getName();;   
7.            System.out.println(name + " is " + age + " years old");;   
8.        } catch(Throwable t); {   
9.            t.printStackTrace();;   
10.        }   
11.    }   
12.}  
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的本質是，它要知道Person接口的定義，并實例一個Person_Stub，通過Stub來調用business method，至于Stub怎么去和Server溝通，Client就不用管了。 

注意它的寫法： 
Person person = new Person_Stub(); 
而不是 
Person_Stub person = new Person_Stub(); 

為什么？因為要面向接口編程嘛，呵呵。 

感謝您有耐心看到這里，關于RMI，我想說的就這么多了。但是好象還沒寫到EJB，本人就累了個半死，算了，我還是先去睡覺，明天再往下續吧。。。 

本人沒有用過Weblogic，這里就結合WebSphere來講講各個類的調用關系吧。 

假定我們要創建一個讀取User信息的SessionBean，需要我們寫的有3個文件： 
1. UserServiceHome.java 
Home接口 

2. UserService.java 
Remote接口 

3. UserServiceBean.java 
Bean實現 

WSAD最終會生成10個class。其它7個是什么呢？我們一個一個數過來： 

4. _UserServiceHome_Stub.java 
這個當然就是Home接口在Client端(動態加載)的Stub類了，它implements UserServiceHome。 

5. _EJSRemoteStatelessUserServiceHome_a940aa04_Tie.java 
Home接口在Server端的Skeleton類，"a940aa04"應該是隨機生成的，所有其他的相關class名里都會有這個標志串，Tie是Corba對Skeleton的叫法。 

6. EJSRemoteStatelessUserServiceHome_a940aa04.java