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); {
		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。

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。。。