stub 和 skeleton 的講解,自己實現一個stub和skeleton程式
RMI的本質就是實現在不同JVM之間的呼叫,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通訊來實現引數和返回值的傳遞。
有關RMI的例子程式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,不免使人有霧裡看花的感覺。下面的例子是我在《Enterprise JavaBeans》裡看到的,雖然很粗糙,但很直觀,利於很快了解它的工作原理。
Ⅰ:RMI工作原理(自定義實現一個Stub 和 Skeleton)
1. 定義一個Person的介面,其中有兩個business method, getAge() 和getName()
Person程式碼:
- public interface Person {
- public int getAge() throws Throwable;
- public String getName() throws Throwable;
- }
2. Person的實現PersonServer類
PersonServer程式碼:
- public class PersonServer implements Person {
- private int age;
- private 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機器上呼叫getAge()和getName()這兩個business method,那麼就得編寫相應的Stub(Client端)和Skeleton(Server端)程式。這是Stub的實現:
Person_Stub程式碼:
//存根(stub)Person_Stub的實現:
- import java.io.ObjectOutputStream;
- import java.io.ObjectInputStream;
- import java.net.Socket;
- public class Person_Stub implements Person {
- private 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連線,並向Skeleton發請求,然後通過Skeleton呼叫PersonServer的方法,最後接收返回的結果。
4. 骨架(Skeleton)的實現
Person_Skeleton程式碼:
- import java.io.ObjectOutputStream;
- import java.io.ObjectInputStream;
- import java.net.Socket;
- import java.net.ServerSocket;
- public class Person_Skeleton extends Thread {
- private 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,它長駐在後臺執行,隨時接收client發過來的request。並根據傳送過來的key去呼叫相應的business method。
5. 最後一個,Client的實現
PersonClient 程式碼:
- 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(PersonClient)的本質是,它要知道Person介面的定義,並例項一個Person_Stub,通過Stub來呼叫business method,至於Stub怎麼去和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();為什麼?因為要面向介面程式設計嘛,呵呵。
//RMI實質上就是生成2個類stub,skeleton來進行引數和返回值的傳遞,採用值傳遞方式
//類似於以前寫的聊天室程式,被傳遞的物件應實現java.io.Serializable介面