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J2ME 透過 Servlet 存取資料庫完整實作指南

編輯：J2ME

架構概覽：J2ME → HTTP → Servlet → JDBC → Database

J2ME 裝置無法直接連接資料庫 (無 JDBC 驅動、防火牆限制、資安考量)。標準架構採用 中介層 Servlet：J2ME 發送 HTTP 請求 → Servlet 接收、呼叫 JDBC → 回傳資料 (文字/JSON/XML)。本文以 Tomcat 5.5 + JDK 1.6 + ODBC 資料源為例，完整示範從伺服器端部署到用戶端呼叫的全流程。

伺服器端環境建置

1. Tomcat 5.5 安裝與環境變數

JAVA_HOME=C:\jdk1.6
CATALINA_HOME=C:\Tomcat5.5
PATH=%JAVA_HOME%\bin;%CATALINA_HOME%\bin;...
CLASSPATH=%JAVA_HOME%\lib\tools.jar;%CATALINA_HOME%\lib\servlet-api.jar

2. 設定 ODBC 資料源

控制台 → 系統管理工具 → 資料來源 (ODBC) → 系統 DSN → 新增
選擇 SQL Server 或 Access 驅動
資料源名稱: mydata
指向資料庫 6D1，資料表 users(id char(4), name varchar(20))
登入帳號: sa，密碼: 空白

3. 部署 Servlet (returnMsg.java)

目錄結構：

Tomcat5.5/webapps/6D1/
├── WEB-INF/
│   ├── web.xml
│   └── classes/
│       └── returnMsg.class

returnMsg.java 完整原始碼

import java.sql.*;
import javax.sql.*;
import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;

public class returnMsg extends HttpServlet {
    public void doGet(HttpServletRequest req, HttpServletResponse res)
            throws ServletException, IOException {
        
        res.setContentType("text/html; charset=gb2312");
        ServletOutputStream out = res.getOutputStream();
        DataOutputStream dos = new DataOutputStream(out);
        
        dos.writeUTF("這是測試結果");
        dos.writeUTF("下面是數據信息");
        
        Connection conn = null;
        Statement stmt = null;
        ResultSet rs = null;
        
        try {
            Class.forName("sun.jdbc.odbc.JdbcOdbcDriver");
            conn = DriverManager.getConnection("jdbc:odbc:mydata", "sa", "");
            stmt = conn.createStatement();
            rs = stmt.executeQuery("SELECT * FROM users");
            
            while (rs.next()) {
                dos.writeUTF("\n用戶ID: " + rs.getString("id"));
                dos.writeUTF("\n用戶名: " + rs.getString("name"));
            }
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            try { if (rs != null) rs.close(); } catch (Exception ignore) {}
            try { if (stmt != null) stmt.close(); } catch (Exception ignore) {}
            try { if (conn != null) conn.close(); } catch (Exception ignore) {}
        }
    }
}

web.xml 部署描述檔

<?xml version="1.0" encoding="ISO-8859-1"?>
<web-app xmlns="https://java.sun.com/xml/ns/j2ee"
         xmlns:xsi="https://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="https://java.sun.com/xml/ns/j2ee
         https://java.sun.com/xml/ns/j2ee/web-app_2_4.xsd"
         version="2.4">
    <servlet>
        <servlet-name>returnMsg</servlet-name>
        <servlet-class>returnMsg</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>returnMsg</servlet-name>
        <url-pattern>/returnMsg</url-pattern>
    </servlet-mapping>
</web-app>

編譯：javac -classpath %CATALINA_HOME%\lib\servlet-api.jar returnMsg.java，將 .class 放入 classes 目錄。啟動 Tomcat 驗證：https://localhost:8080/6D1/returnMsg 應顯示資料庫內容。

用戶端 MIDlet 實作

專案建立 (Wireless Toolkit 2.5.2)

專案名: 6D1
MIDlet 類別: NetMain
原始碼目錄: apps/6D1/src/

NetMain.java (主 MIDlet 與 UI)

import javax.microedition.midlet.*;
import javax.microedition.lcdui.*;

public class NetMain extends MIDlet implements CommandListener {
    private Display display;
    private SendMsg sender;
    private Command cmdConnect = new Command("連接", Command.OK, 1);
    
    public NetMain() {
        display = Display.getDisplay(this);
    }
    
    public void startApp() {
        Form form = new Form("聯網測試");
        form.append("點選下方按鈕連接伺服器");
        form.addCommand(cmdConnect);
        form.setCommandListener(this);
        display.setCurrent(form);
    }
    
    public void pauseApp() {}
    public void destroyApp(boolean un) {}
    
    public void commandAction(Command c, Displayable d) {
        if (c == cmdConnect) {
            sender = new SendMsg(this);
            display.setCurrent(sender);
        }
    }
}

SendMsg.java (網路通訊邏輯)

import javax.microedition.lcdui.*;
import javax.microedition.io.*;
import java.io.*;

public class SendMsg extends Canvas implements Runnable {
    private NetMain parent;
    private String response = "連線中...";
    private Thread thread;
    
    public SendMsg(NetMain parent) {
        this.parent = parent;
        thread = new Thread(this);
        thread.start();
    }
    
    protected void paint(Graphics g) {
        g.setColor(0xFFFFFF);
        g.fillRect(0, 0, getWidth(), getHeight());
        g.setColor(0x000000);
        g.drawString(response, 5, 5, Graphics.TOP | Graphics.LEFT);
    }
    
    public void run() {
        HttpConnection conn = null;
        DataInputStream dis = null;
        try {
            conn = (HttpConnection) Connector.open(
                "https://localhost:8080/6D1/returnMsg");
            conn.setRequestMethod(HttpConnection.GET);
            
            dis = new DataInputStream(conn.openDataInputStream());
            StringBuilder sb = new StringBuilder();
            String line;
            while ((line = dis.readUTF()) != null) {
                sb.append(line).append("\n");
            }
            response = sb.toString();
        } catch (Exception e) {
            response = "錯誤: " + e.getMessage();
            e.printStackTrace();
        } finally {
            try { if (dis != null) dis.close(); } catch (Exception ignore) {}
            try { if (conn != null) conn.close(); } catch (Exception ignore) {}
        }
        repaint();
    }
    
    // 按任意鍵返回主選單
    protected void keyPressed(int keyCode) {
        parent.display.setCurrent(parent);
    }
}

關鍵技術點總結

層面	說明
通訊協定	HTTP GET，支援所有 MIDP 裝置
資料格式	DataOutputStream/DataInputStream (UTF 字串)，簡單輕量
資料庫連線	Servlet 端使用 JDBC-ODBC Bridge (舊版範例)，生產環境建議改用 DataSource (JNDI) + 連線池
錯誤處理	用戶端捕捉 IOException，顯示友善訊息；伺服器端記錄堆疊追蹤
安全性	範例未加密，實務上必須使用 HTTPS + 驗證機制