Java 即時聊天系統設計：Socket 檔案傳輸核心實作參考

架構概覽

採用 Client-Server 架構，Server 端維護一個執行緒池處理多客戶端連線，傳輸協定自訂簡易二進制協定：

[命令碼:1 byte] [檔名長度:2 bytes] [檔名] [檔案大小:8 bytes] [檔案內容]

Server 端核心程式碼片段

// 接收檔案
public void receiveFile(Socket client) throws IOException {
    DataInputStream in = new DataInputStream(client.getInputStream());
    byte cmd = in.readByte();
    if (cmd == CMD_FILE) {
        int nameLen = in.readShort();
        byte[] nameBytes = new byte[nameLen];
        in.readFully(nameBytes);
        String fileName = new String(nameBytes, StandardCharsets.UTF_8);
        long fileSize = in.readLong();

        try (FileOutputStream fos = new FileOutputStream("upload/" + fileName)) {
            byte[] buffer = new byte[8192];
            long remaining = fileSize;
            while (remaining > 0) {
                int read = in.read(buffer, 0, (int) Math.min(buffer.length, remaining));
                if (read == -1) break;
                fos.write(buffer, 0, read);
                remaining -= read;
            }
        }
        // 回覆成功
        new DataOutputStream(client.getOutputStream()).writeByte(RESP_OK);
    }
}

Client 端發送檔案

public void sendFile(String serverIp, int port, File file) throws IOException {
    try (Socket socket = new Socket(serverIp, port);
         DataOutputStream out = new DataOutputStream(socket.getOutputStream());
         FileInputStream fis = new FileInputStream(file)) {

        out.writeByte(CMD_FILE);
        byte[] nameBytes = file.getName().getBytes(StandardCharsets.UTF_8);
        out.writeShort(nameBytes.length);
        out.write(nameBytes);
        out.writeLong(file.length());

        byte[] buffer = new byte[8192];
        int read;
        while ((read = fis.read(buffer)) != -1) {
            out.write(buffer, 0, read);
        }
        out.flush();

        // 讀取回覆
        DataInputStream in = new DataInputStream(socket.getInputStream());
        byte resp = in.readByte();
        if (resp == RESP_OK) System.out.println("上傳成功");
    }
}

關鍵點提醒

• 使用 DataInputStream/DataOutputStream 處理二進制資料，避免編碼問題。
• 大檔案採用分塊讀寫，防止 OOM。
• 生產環境建議加入斷點續傳、校驗和、TLS 加密與流量控制。