ejb 原理

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()

 

Java代码 

public interface Person { 

    public int getAge(); throws Throwable; 

    public String getName(); throws Throwable; 

} 

 

 

2. Person的实现PersonServer类

Java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：

Java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

Java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

Java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。