Java Socket 程式設計入門：Server/Client/Applet 三端完整範例

經典 TCP Socket 教學範例：Server 監聽 port 5000，發送歡迎訊息；Application Client 與 Applet Client 分別連線接收訊息。展示 ServerSocket、Socket、DataInputStream/DataOutputStream 基礎用法。

1. Server 端

import java.io.*;
import java.net.*;

public class Server {
    public static void main(String[] args) {
        try (ServerSocket server = new ServerSocket(5000)) {
            System.out.println("Server 啟動，等待連線...");
            try (Socket client = server.accept();
                 DataOutputStream out = new DataOutputStream(client.getOutputStream())) {
                System.out.println("Client 連線: " + client.getInetAddress());
                out.writeUTF("Welcome to Server. Bye!");
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

2. Application Client

import java.io.*;
import java.net.*;

public class Client {
    public static void main(String[] args) {
        String host = (args.length > 0) ? args[0] : "localhost";
        try (Socket socket = new Socket(host, 5000);
             DataInputStream in = new DataInputStream(socket.getInputStream())) {
            String msg = in.readUTF();
            System.out.println("Server 回應: " + msg);
        } catch (IOException e) {
            System.err.println("連線失敗: " + e.getMessage());
        }
    }
}

3. Applet Client（歷史參考，現代瀏覽器已棄用 Applet）

import java.applet.Applet;
import java.awt.Graphics;
import java.io.*;
import java.net.*;

public class AppletClient extends Applet {
    private String info = "連線中...";
    public void init() {
        try (Socket socket = new Socket(getCodeBase().getHost(), 5000);
             DataInputStream in = new DataInputStream(socket.getInputStream())) {
            info = in.readUTF();
        } catch (Exception e) {
            info = "錯誤: " + e.getMessage();
        }
        repaint();
    }
    public void paint(Graphics g) { g.drawString(info, 20, 20); }
}

HTML 引用：<applet code="AppletClient.class" width=200 height=200></applet>

執行步驟

1. javac Server.java Client.java AppletClient.java
2. 啟動 Server：java Server
3. 另開終端啟動 Client：java Client localhost

延伸應用

• 傳輸可序列化物件：改用 ObjectOutputStream/ObjectInputStream。
• 多執行緒 Server：accept() 後交給 ExecutorService 處理每個 Client。
• 心跳/斷線重連：應用層協議加入 Ping/Pong 機制。