Java RMI - 資料庫應用程式

在上一章中，我們建立了一個示例 RMI 應用程式，其中客戶端呼叫一個顯示 GUI 視窗（JavaFX）的方法。

在本章中，我們將透過一個示例來了解客戶端程式如何檢索駐留在伺服器上的 MySQL 資料庫中表的記錄。

假設我們在資料庫details中有一個名為student_data的表，如下所示。

+----+--------+--------+------------+---------------------+ 
| ID | NAME   | BRANCH | PERCENTAGE | EMAIL               | 
+----+--------+--------+------------+---------------------+ 
|  1 | Ram    | IT     |         85 | ram123@gmail.com    | 
|  2 | Rahim  | EEE    |         95 | rahim123@gmail.com  | 
|  3 | Robert | ECE    |         90 | robert123@gmail.com | 
+----+--------+--------+------------+---------------------+

假設使用者名稱為myuser，密碼為password。

建立學生類

建立一個Student類，其中包含setter和getter方法，如下所示。

public class Student implements java.io.Serializable {   
   private int id, percent;   
   private String name, branch, email;    
  
   public int getId() { 
      return id; 
   } 
   public String getName() { 
      return name; 
   } 
   public String getBranch() { 
      return branch; 
   } 
   public int getPercent() { 
      return percent; 
   } 
   public String getEmail() { 
      return email; 
   } 
   public void setID(int id) { 
      this.id = id; 
   } 
   public void setName(String name) { 
      this.name = name; 
   } 
   public void setBranch(String branch) { 
      this.branch = branch; 
   } 
   public void setPercent(int percent) { 
      this.percent = percent; 
   } 
   public void setEmail(String email) { 
      this.email = email; 
   } 
}

定義遠端介面

定義遠端介面。在這裡，我們定義了一個名為Hello的遠端介面，其中包含一個名為getStudents()的方法。此方法返回一個包含Student類物件的列表。

import java.rmi.Remote; 
import java.rmi.RemoteException; 
import java.util.*;

// Creating Remote interface for our application 
public interface Hello extends Remote {  
   public List<Student> getStudents() throws Exception;  
}

開發實現類

建立一個類並實現上面建立的介面。

在這裡，我們實現了遠端介面的getStudents()方法。當您呼叫此方法時，它將檢索名為student_data表的記錄。使用其setter方法將這些值設定到Student類，將其新增到列表物件中並返回該列表。

import java.sql.*; 
import java.util.*;  

// Implementing the remote interface 
public class ImplExample implements Hello {  
   
   // Implementing the interface method 
   public List<Student> getStudents() throws Exception {  
      List<Student> list = new ArrayList<Student>();   
    
      // JDBC driver name and database URL 
      String JDBC_DRIVER = "com.mysql.jdbc.Driver";   
      String DB_URL = "jdbc:mysql://:3306/details";  
      
      // Database credentials 
      String USER = "myuser"; 
      String PASS = "password";  
      
      Connection conn = null; 
      Statement stmt = null;  
      
      //Register JDBC driver 
      Class.forName("com.mysql.jdbc.Driver");   
      
      //Open a connection
      System.out.println("Connecting to a selected database..."); 
      conn = DriverManager.getConnection(DB_URL, USER, PASS); 
      System.out.println("Connected database successfully...");  
      
      //Execute a query 
      System.out.println("Creating statement..."); 
      
      stmt = conn.createStatement();  
      String sql = "SELECT * FROM student_data"; 
      ResultSet rs = stmt.executeQuery(sql);  
      
      //Extract data from result set 
      while(rs.next()) { 
         // Retrieve by column name 
         int id  = rs.getInt("id"); 
         
         String name = rs.getString("name"); 
         String branch = rs.getString("branch"); 
         
         int percent = rs.getInt("percentage"); 
         String email = rs.getString("email");  
         
         // Setting the values 
         Student student = new Student(); 
         student.setID(id); 
         student.setName(name); 
         student.setBranch(branch); 
         student.setPercent(percent); 
         student.setEmail(email); 
         list.add(student); 
      } 
      rs.close(); 
      return list;     
   }  
}

伺服器程式

RMI 伺服器程式應實現遠端介面或擴充套件實現類。在這裡，我們應該建立一個遠端物件並將其繫結到RMI 登錄檔。

以下是此應用程式的伺服器程式。在這裡，我們將擴充套件上面建立的類，建立一個遠端物件並將其註冊到 RMI 登錄檔中，繫結名稱為hello。

import java.rmi.registry.Registry; 
import java.rmi.registry.LocateRegistry; 
import java.rmi.RemoteException; 
import java.rmi.server.UnicastRemoteObject; 

public class Server extends ImplExample { 
   public Server() {} 
   public static void main(String args[]) { 
      try { 
         // Instantiating the implementation class 
         ImplExample obj = new ImplExample(); 
    
         // Exporting the object of implementation class (
            here we are exporting the remote object to the stub) 
         Hello stub = (Hello) UnicastRemoteObject.exportObject(obj, 0);  
         
         // Binding the remote object (stub) in the registry 
         Registry registry = LocateRegistry.getRegistry(); 
         
         registry.bind("Hello", stub);  
         System.err.println("Server ready"); 
      } catch (Exception e) { 
         System.err.println("Server exception: " + e.toString()); 
         e.printStackTrace(); 
      } 
   } 
}

客戶端程式

以下是此應用程式的客戶端程式。在這裡，我們正在獲取遠端物件並呼叫名為getStudents()的方法。它從列表物件中檢索表的記錄並顯示它們。

import java.rmi.registry.LocateRegistry; 
import java.rmi.registry.Registry; 
import java.util.*;  

public class Client {  
   private Client() {}  
   public static void main(String[] args)throws Exception {  
      try { 
         // Getting the registry 
         Registry registry = LocateRegistry.getRegistry(null); 
    
         // Looking up the registry for the remote object 
         Hello stub = (Hello) registry.lookup("Hello"); 
    
         // Calling the remote method using the obtained object 
         List<Student> list = (List)stub.getStudents(); 
         for (Student s:list)v { 
            
            // System.out.println("bc "+s.getBranch()); 
            System.out.println("ID: " + s.getId()); 
            System.out.println("name: " + s.getName()); 
            System.out.println("branch: " + s.getBranch()); 
            System.out.println("percent: " + s.getPercent()); 
            System.out.println("email: " + s.getEmail()); 
         }  
      // System.out.println(list); 
      } catch (Exception e) { 
         System.err.println("Client exception: " + e.toString()); 
         e.printStackTrace(); 
      } 
   } 
}