Java RMI數據庫應用程序
在上一章中，我們創建了一個RMI圖形用戶界面應用程序的示例，客戶端調用服務器端顯示GUI窗口(JavaFX)的方法。
在本章中，我們將以一個例子來看看客戶端程序如何檢索位於服務器上的MySQL數據庫中的表的記錄。
假設在數據庫：testdb中有一個名稱爲student的表，其表結構和數據如下所示 -
+----+--------+--------+------------+---------------------+
| ID | NAME | BRANCH | PERCENTAGE | EMAIL |
+----+--------+--------+------------+---------------------+
| 1 | Maxsu | IT | 85 | [email protected] |
| 2 | Curry | EEE | 95 | [email protected] |
| 3 | Alex | ECE | 90 | [email protected] |
+----+--------+--------+------------+---------------------+創建數據庫和表如下語句 -
CREATE DATABASE IF NOT EXISTS testdb DEFAULT CHARSET utf8 COLLATE utf8_general_ci;
use testdb;
-- ----------------------------
-- Table structure for `student`
-- ----------------------------
DROP TABLE IF EXISTS `student`;
CREATE TABLE `student` (
`id` int(10) NOT NULL AUTO_INCREMENT,
`name` varchar(32) NOT NULL DEFAULT '',
`branch` varchar(16) NOT NULL DEFAULT '',
`percentage` float unsigned DEFAULT '0',
`email` varchar(48) NOT NULL DEFAULT '',
PRIMARY KEY (`id`)
) ENGINE=MyISAM AUTO_INCREMENT=1 DEFAULT CHARSET=utf8;
-- ----------------------------
-- Records of student
-- ----------------------------
INSERT INTO `student` VALUES ('1', 'Maxsu', 'IT', '85', '[email protected]');
INSERT INTO `student` VALUES ('2', 'Curry', 'EEE', '96', '[email protected]');
INSERT INTO `student` VALUES ('3', 'Alex', 'ECE', '92', '[email protected]');假設所使用的MySQL服務器的用戶名是:root，其密碼是:123456。
創建 Student 類
創建一個Student類,並創建setter和getter方法，創建一個Java源代碼文件：Student.java，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;
}
}定義遠程接口
在這裏，我們定義一個名爲Hello的遠程接口，Hello接口中有一個getStudents()方法。此方法返回一個包含Student類對象的列表。
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;
}開發實現類
創建一個類並實現上面創建的Hello遠程接口。
這裏我們實現遠程接口的getStudents()方法。當調用此方法時，它將檢索數據庫：students表中的記錄。使用Student類的setter方法設置對象的值並返回該對象列表。創建一個JAVA源文件：ImplExample.java 如下所示 -
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/testdb";
// Database credentials
String USER = "root";
String PASS = "123456";
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;
}
}服務器程序
RMI服務器程序應實現遠程接口或擴展實現類。 在這裏，我們將創建一個遠程對象並將其綁定到RMI註冊表。
以下是本應用程序的服務器程序。 在這裏，我們將擴展上述創建的類，創建一個遠程對象並使用綁定名稱爲：hello ，將其註冊到RMI註冊表。創建一個JAVA源文件：Server.java 如下所示 -
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();
}
}
}客戶端程序
以下是本應用程序的客戶端程序。 在這裏，我們獲取遠程對象並調用getStudents()方法。它從列表對象中檢索表 - student的行記錄並顯示它們。創建一個JAVA源文件：Client.java 如下所示 -
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();
}
}
}運行示例的步驟
以下是運行RMI示例的步驟。
注意：如果手動編譯，需要把上面代碼存放到目錄：F:\worksp\java_rmi\java-dbapp中，並創建一個目錄：F:\worksp\java_rmi\java-dbapp\libs，把下載的
mysql-connector-java-5.1.40-bin.jar庫(下載地址：http://downloads.mysql.com/archives/c-j/ )放入到 F:\worksp\java_rmi\java-dbapp\libs 目錄中。
第一步 - 打開存儲所有程序的文件夾，並編譯所有Java文件，如下所示。
F:\worksp\java_rmi\java-dbapp> javac -Djava.ext.dirs=F:\worksp\java_rmi\java-dbapp\libs *.java第二步 - 使用以下命令啓動rmi註冊表。
F:\worksp\java_rmi\java-dbapp> start rmiregistry執行上面命令後，將在單獨的窗口中啓動一個rmi註冊表。
第三步 - 運行服務器類代碼，如下所示。
F:\worksp\java_rmi\java-dbapp>java -Djava.ext.dirs=F:\worksp\java_rmi\java-dbapp\libs Server
Server ready第三步 - 運行客戶端類文件，如下所示。
F:\worksp\java_rmi\java-dbapp>java -Djava.ext.dirs=F:\worksp\java_rmi\java-dbapp\libs Client執行上面命令後，輸出結果如下 -
F:\worksp\java_rmi\java-dbapp>java -Djava.ext.dirs=F:\worksp\java_rmi\java-dbapp\libs Client
ID: 1
name: Maxsu
branch: IT
percent: 85
email: [email protected]
ID: 2
name: Curry
branch: EEE
percent: 96
email: [email protected]
ID: 3
name: Alex
branch: ECE
percent: 92
email: [email protected]
F:\worksp\java_rmi\java-dbapp>