数据库应用程序（Database Application）

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2023-12-01

在上一章中，我们创建了一个示例RMI应用程序，其中客户端调用显示GUI窗口（JavaFX）的方法。

在本章中，我们将举例说明客户端程序如何检索驻留在服务器上的MySQL数据库中的表的记录。

假设我们在数据库details有一个名为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 | 
+----+--------+--------+------------+---------------------+

假设用户的名称是myuser ，密码是password 。

创建学生班

使用setter和getter方法创建一个Student类，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; 
   } 
}

定义远程接口

定义远程接口。在这里，我们使用名为getStudents ()的方法定义一个名为Hello的远程接口。此方法返回一个列表，其中包含Student类的对象。

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;  
}

开发实现类

创建一个类并实现上面创建的interface.

这里我们实现Remote interface的getStudents()方法。当您调用此方法时，它将检索名为student_data的表的记录。使用setter方法将这些值设置为Student类，将其添加到列表对象并返回该列表。

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://localhost: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;     
   }  
}

服务器程序

RMI服务器程序应实现远程接口或扩展实现类。在这里，我们应该创建一个远程对象并将其绑定到RMI registry 。

以下是此应用程序的服务器程序。在这里，我们将扩展上面创建的类，创建一个远程对象并使用绑定名称hello将其注册到RMI注册表。

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(); 
      } 
   } 
}

客户计划

以下是此应用程序的客户端程序。在这里，我们获取远程对象并调用名为getStudents()的方法。它从列表对象中检索表的记录并显示它们。

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(); 
      } 
   } 
}