Java UDP 聊天室實作：序列化物件傳輸與多執行緒架構

架構設計

採用 UDP 協定實作簡易聊天室，核心元件：

• Message：可序列化訊息物件，含內容、發送者 ID、接收者 ID
• Sender：發送執行緒，將 Message 序列化為位元組陣列並封裝成 DatagramPacket 發送
• Receiver：接收執行緒，阻塞接收封包並反序列化還原 Message

Message 類別

public class Message implements Serializable {
    private String msg, id, destId;
    public Message(String msg, String id, String destId) {
        this.msg = msg; this.id = id; this.destId = destId;
    }
    // getter 方法...
}

發送端實作

public class Sender extends Thread {
    private DatagramSocket socket;
    private DatagramPacket packet;

    public Sender(Message msg) throws IOException {
        this.msg = msg;
        ByteArrayOutputStream bos = new ByteArrayOutputStream();
        ObjectOutputStream oos = new ObjectOutputStream(bos);
        oos.writeObject(msg);
        oos.flush();
        byte[] data = bos.toByteArray();
        packet = new DatagramPacket(data, data.length,
            InetAddress.getByName(msg.getDestId()), 7002);
        socket = new DatagramSocket(7000);
    }

    public void run() {
        try { socket.send(packet); } catch (IOException e) { e.printStackTrace(); }
        finally { socket.close(); }
    }
}

接收端實作

public class Receiver extends Thread {
    private DatagramSocket socket;
    private DatagramPacket packet;
    private byte[] buf = new byte[500];

    public Receiver() throws SocketException {
        socket = new DatagramSocket(7002);
        packet = new DatagramPacket(buf, buf.length);
    }

    public void run() {
        try {
            while (true) {
                socket.receive(packet);
                ObjectInputStream ois = new ObjectInputStream(
                    new ByteArrayInputStream(buf));
                Message msg = (Message) ois.readObject();
                System.out.println(msg.getId() + ": " + msg.getMessage());
            }
        } catch (Exception e) { e.printStackTrace(); }
    }
}

關鍵技術點

• 使用 ByteArrayOutputStream + ObjectOutputStream 實現物件序列化
• 發送/接收埠號分離（7000/7002）避免衝突
• 接收緩衝區大小需大於預期最大封包
• UDP 無連線特性：不保證送達順序與可靠性

此為教學級範例，生產環境需加入訊息確認、重傳、心跳偵測、加密等機制。