Android Socket 通訊實作：手機當 Server 實現多裝置點對點傳輸

Android 裝置間透過 Socket 通訊，一台當 Server (ServerSocket)，多台當 Client (Socket)，實現局域網內即時訊息/檔案傳輸。本文提供完整雙端代碼架構與關鍵點。

架構概覽

+----------------+      Wi-Fi Hotspot / LAN       +----------------+
|  Server 端      | <----------------------------> |  Client 端 x N  |
|  ServerSocket  |        TCP Socket                |  Socket         |
|  accept() 迴圈  |                                |  connect()      |
+----------------+                                +----------------+

Server 端核心代碼 (Kotlin)

class SocketServer(private val port: Int = 8888) {
    private var serverSocket: ServerSocket? = null
    private val clients = mutableListOf()
    private val executor = Executors.newCachedThreadPool()

    fun start() {
        executor.execute {
            try {
                serverSocket = ServerSocket(port)
                while (!Thread.currentThread().isInterrupted) {
                    val client = serverSocket!!.accept()  // 阻塞等待連線
                    clients.add(client)
                    executor.execute(ClientHandler(client))
                }
            } catch (e: IOException) { e.printStackTrace() }
        }
    }

    fun broadcast(msg: String) {
        clients.forEach { it.outputStream.write(msg.toByteArray()) }
    }

    inner class ClientHandler(private val socket: Socket) : Runnable {
        override fun run() {
            try {
                val input = socket.inputStream.bufferedReader()
                input.forEachLine { line ->
                    // 處理收到訊息，可再 broadcast 給其他客戶端
                    handleMessage(line)
                }
            } catch (e: IOException) { } finally { closeQuietly(socket) }
        }
    }

    fun stop() {
        serverSocket?.close()
        executor.shutdownNow()
    }
}

// AndroidManifest.xml 需權限
// 
// 
//  (開熱點需)

Client 端核心代碼

class SocketClient(private val serverIp: String, private val port: Int = 8888) {
    private var socket: Socket? = null
    private val handler = Handler(Looper.getMainLooper())

    fun connect(onConnected: () -> Unit, onMessage: (String) -> Unit, onError: (Exception) -> Unit) {
        Thread {
            try {
                socket = Socket(serverIp, port)
                handler.post(onConnected)
                val reader = socket!!.inputStream.bufferedReader()
                reader.forEachLine { line ->
                    handler.post { onMessage(line) }
                }
            } catch (e: Exception) {
                handler.post { onError(e) }
            }
        }.start()
    }

    fun send(msg: String) {
        Thread { socket?.outputStream?.write("$msg\n".toByteArray()) }.start()
    }

    fun disconnect() { socket?.close() }
}

關鍵實作細節

現成開源庫推薦

• KryoNet：高效序列化 + UDP/TCP，適合遊戲/即時同步。
• Netty (Android 版)：事件驅動、零拷貝、支援 WebSocket/HTTP/自定義協議。
• Okio + OkHttp WebSocket：現代 HTTP/WS 基礎，穩定易用。
• Nearby Connections API：Google 官方離線 P2P (藍牙/BLE/Wi-Fi Direct)，無需手動開熱點。

權限配置 (Android 10+)

<uses-permission android:name="android.permission.ACCESS_FINE_LOCATION" /> 
<!-- 掃描/連接 Wi-Fi Direct 需定位權限 -->
<uses-permission android:name="android.permission.NEARBY_WIFI_DEVICES" /> 
<!-- Android 13+ 專用 -->

提問者回饋：「Android的socket不就是java的socket麼，比如說應用什麼快傳就是開wifi熱點然後用socket啊」。掌握 Java Socket 基礎，結合 Android 生命週期管理與權限配置，即可實作穩定的裝置間通訊。