RMI 原理2

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); {
        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：

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

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

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。