Java Rmi 简明教程
Java RMI - Database Application
在上一章中，我们创建了一个示例 RMI 应用程序，其中客户端调用一个显示 GUI 窗口（JavaFX）的方法。
在本章中，我们将举例了解客户端程序如何检索驻留在服务器上的 MySQL 数据库中的一个表中的记录。
假设我们在数据库 details 中有一个表，名为 student_data 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 |
+----+--------+--------+------------+---------------------+假设用户名为 myuser ，密码为 password 。
Creating a Student Class
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;
}
}Defining the Remote Interface
定义远程接口。此处，我们定义名为 Hello 的远程接口，其中有一个名为 getStudents () 的方法。此方法返回包含 Student 类对象的列表。
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;
}Developing the Implementation Class
创建一个类并实现上面创建的 interface. 。
此处，我们正在实现 Remote interface 的 getStudents() 方法。当您调用此方法时，它会检索名为 student_data 的表的记录。使用其 setter 方法将这些值设置到 Student 类，将其添加到一个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;
}
}Server Program
RMI 服务器程序应实现远程接口或扩展实现类。此处，我们应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。此处，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 注册表中。
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();
}
}
}Client Program
以下是此应用程序的客户端程序。此处，我们正在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();
}
}
}
