Lập trình Client-Server Android với Socket TCP

Bài viết hướng dẫn xây dựng mô hình **Client-Server** trên Android sử dụng **Socket TCP** (java.net.Socket / ServerSocket). Hai ứng dụng riêng biệt: Server lắng nghe, Client kết nối. Phù hợp chat LAN, điều khiển thiết bị, sync dữ liệu offline.

Yêu cầu chung

2 thiết bị Android (hoặc 1 thật + 1 emulator) cùng mạng WiFi LAN.
Android 6.0+ (API 23+).
Quyền: INTERNET, ACCESS_NETWORK_STATE, Server thêm ACCESS_WIFI_STATE.

📦 SERVER APP

build.gradle.kts (Module: server)

plugins { id("com.android.application") id("org.jetbrains.kotlin.android") }
android {
    namespace = "com.example.socketserver"
    compileSdk = 34
    defaultConfig {
        applicationId = "com.example.socketserver"
        minSdk = 23
        targetSdk = 34
        versionCode = 1
        versionName = "1.0"
    }
}
dependencies {
    implementation("androidx.core:core-ktx:1.12.0")
    implementation("androidx.appcompat:appcompat:1.6.1")
    implementation("com.google.android.material:material:1.11.0")
    implementation("androidx.constraintlayout:constraintlayout:2.1.4")
}

Manifest Server

<manifest xmlns:android="https://schemas.android.com/apk/res/android"
    package="com.example.socketserver">
    <uses-permission android:name="android.permission.INTERNET" />
    <uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />
    <uses-permission android:name="android.permission.ACCESS_WIFI_STATE" />
    <application android:allowBackup="true" android:label="@string/app_name" android:theme="@style/Theme.SocketServer">
        <activity android:name=".MainActivity" android:exported="true">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />
                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
    </application>
</manifest>

Layout Server (res/layout/activity_main.xml)

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout
    xmlns:android="https://schemas.android.com/apk/res/android"
    xmlns:app="https://schemas.android.com/apk/res-auto"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:padding="16dp">

    <TextView android:id="@+id/tvServerInfo" android:layout_width="0dp" android:layout_height="wrap_content"
        android:text="Server IP: ...\nPort: 8080" android:textSize="16sp"
        app:layout_constraintTop_toTopOf="parent" app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintEnd_toEndOf="parent" />

    <TextView android:id="@+id/tvStatus" android:layout_width="0dp" android:layout_height="wrap_content"
        android:text="Trạng thái: Chờ kết nối..." android:textSize="16sp" android:textStyle="bold"
        app:layout_constraintTop_toBottomOf="@id/tvServerInfo" android:layout_marginTop="16dp"
        app:layout_constraintStart_toStartOf="parent" app:layout_constraintEnd_toEndOf="parent" />

    <androidx.recyclerview.widget.RecyclerView
        android:id="@+id/rvMessages"
        android:layout_width="0dp" android:layout_height="0dp"
        app:layout_constraintTop_toBottomOf="@id/tvStatus"
        app:layout_constraintBottom_toTopOf="@id/inputLayout"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        android:layout_marginTop="16dp" />

    <LinearLayout android:id="@+id/inputLayout" android:layout_width="0dp" android:layout_height="wrap_content"
        android:orientation="horizontal" android:layout_marginTop="8dp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintStart_toStartOf="parent" app:layout_constraintEnd_toEndOf="parent">
        <EditText android:id="@+id/etMessage" android:layout_width="0dp" android:layout_height="wrap_content"
            android:layout_weight="1" android:hint="Tin nhắn..." android:inputType="text" />
        <Button android:id="@+id/btnSend" android:layout_width="wrap_content" android:layout_height="wrap_content"
            android:layout_marginStart="8dp" android:text="Gửi" />
    </LinearLayout>
</androidx.constraintlayout.widget.ConstraintLayout>

Message Model & Adapter (Kotlin)

data class ChatMessage(
    val text: String,
    val isSent: Boolean, // true = Server gửi, false = Client gửi
    val timestamp: Long = System.currentTimeMillis()
)

class MessageAdapter : ListAdapter(DiffCallback()) {
    inner class VH(view: View) : RecyclerView.ViewHolder(view) {
        val tvText: TextView = view.findViewById(R.id.tvMsgText)
        val tvTime: TextView = view.findViewById(R.id.tvMsgTime)
        val container: LinearLayout = view.findViewById(R.id.msgContainer)
    }
    override fun onCreateViewHolder(p: ViewGroup, vt: Int): VH {
        val view = LayoutInflater.from(p.context).inflate(R.layout.item_message, p, false)
        return VH(view)
    }
    override fun onBindViewHolder(h: VH, pos: Int) {
        val msg = getItem(pos)
        h.tvText.text = msg.text
        h.tvTime.text = android.text.format.DateFormat.format("HH:mm", msg.timestamp)
        // Căn trái/phải, màu nền khác nhau
        val params = h.container.layoutParams as LinearLayout.LayoutParams
        params.gravity = if (msg.isSent) Gravity.END else Gravity.START
        h.container.layoutParams = params
        h.container.setBackgroundResource(if (msg.isSent) R.drawable.bg_msg_sent else R.drawable.bg_msg_received)
    }
    class DiffCallback : DiffUtil.ItemCallback() {
        override fun areItemsTheSame(o: ChatMessage, n: ChatMessage) = o.timestamp == n.timestamp
        override fun areContentsTheSame(o: ChatMessage, n: ChatMessage) = o == n
    }
}

Message Item Layout (res/layout/item_message.xml)

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="https://schemas.android.com/apk/res/android"
    android:id="@+id/msgContainer"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:orientation="vertical"
    android:padding="12dp"
    android:maxWidth="280dp">
    <TextView android:id="@+id/tvMsgText" android:layout_width="wrap_content" android:layout_height="wrap_content"
        android:textSize="15sp" android:textColor="@android:color/black" />
    <TextView android:id="@+id/tvMsgTime" android:layout_width="wrap_content" android:layout_height="wrap_content"
        android:textSize="10sp" android:textColor="#999" android:layout_marginTop="2dp" android:layout_gravity="end" />
</LinearLayout>

Drawables Bubble (res/drawable/bg_msg_sent.xml)

<shape xmlns:android="https://schemas.android.com/apk/res/android" android:shape="rectangle">
    <solid android:color="#2196F3" />
    <corners android:radius="16dp" android:bottomRightRadius="2dp" />
</shape>

<!-- bg_msg_received.xml -->
<shape xmlns:android="https://schemas.android.com/apk/res/android" android:shape="rectangle">
    <solid android:color="#E0E0E0" />
    <corners android:radius="16dp" android:bottomLeftRadius="2dp" />
</shape>

MainActivity Server (Kotlin + Coroutines)

class MainActivity : AppCompatActivity() {
    private lateinit var binding: ActivityMainBinding
    private val adapter = MessageAdapter()
    private var serverSocket: ServerSocket? = null
    private var clientSocket: Socket? = null
    private var out: PrintWriter? = null
    private val scope = CoroutineScope(Dispatchers.IO)
    private val uiScope = CoroutineScope(Dispatchers.Main)

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        binding = ActivityMainBinding.inflate(layoutInflater)
        setContentView(binding.root)

        binding.rvMessages.apply {
            layoutManager = LinearLayoutManager(this@MainActivity)
            adapter = this@MainActivity.adapter
        }

        val localIp = getLocalIpAddress() ?: "Không xác định"
        binding.tvServerInfo.text = "Server IP: $localIp\nPort: 8080"

        binding.btnSend.setOnClickListener {
            val text = binding.etMessage.text.toString().trim()
            if (text.isNotEmpty()) sendMessage(text)
        }

        startServer()
    }

    private fun getLocalIpAddress(): String? {
        try {
            val wifiManager = getSystemService(WIFI_SERVICE) as WifiManager
            val ipInt = wifiManager.connectionInfo.ipAddress
            if (ipInt == 0) return null
            return InetAddress.getByAddress(ipInt.toByteArray(ByteOrder.LITTLE_ENDIAN)).hostAddress
        } catch (e: Exception) { return null }
    }

    private fun startServer() {
        scope.launch {
            try {
                serverSocket = ServerSocket(8080)
                uiScope.launch { binding.tvStatus.text = "Đang lắng nghe port 8080..." }

                while (!Thread.currentThread().isInterrupted) {
                    clientSocket = serverSocket!!.accept()
                    uiScope.launch { binding.tvStatus.text = "Client kết nối: ${clientSocket!!.inetAddress}" }

                    out = PrintWriter(clientSocket!!.getOutputStream(), true)
                    val in = BufferedReader(InputStreamReader(clientSocket!!.getInputStream()))

                    // Loop đọc tin nhắn từ client
                    while (true) {
                        val line = in.readLine() ?: break
                        uiScope.launch {
                            adapter.submitList(adapter.currentList + ChatMessage(line, false))
                            binding.rvMessages.scrollToPosition(adapter.itemCount - 1)
                        }
                    }
                    // Client ngắt kết nối
                    uiScope.launch { binding.tvStatus.text = "Client ngắt kết nối. Đang chờ mới..." }
                    closeClientResources()
                }
            } catch (e: IOException) { e.printStackTrace() }
        }
    }

    private fun sendMessage(text: String) {
        out?.println(text)
        uiScope.launch {
            adapter.submitList(adapter.currentList + ChatMessage(text, true))
            binding.rvMessages.scrollToPosition(adapter.itemCount - 1)
            binding.etMessage.setText("")
        }
    }

    private fun closeClientResources() {
        try { out?.close(); clientSocket?.close() } catch (e: IOException) {}
        out = null; clientSocket = null
    }

    override fun onDestroy() {
        super.onDestroy()
        scope.cancel()
        try { serverSocket?.close() } catch (e: IOException) {}
    }
}

📱 CLIENT APP

build.gradle.kts (Module: client) - Tương tự, đổi applicationId

namespace = "com.example.socketclient"
defaultConfig { applicationId = "com.example.socketclient" ... }

Manifest Client

<manifest ... package="com.example.socketclient">
    <uses-permission android:name="android.permission.INTERNET" />
    <uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />
    <application ...>
        <activity android:name=".MainActivity" android:exported="true">...</activity>
    </application>
</manifest>

Layout Client (res/layout/activity_main.xml)

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout ... android:padding="16dp">
    <EditText android:id="@+id/etServerIp" android:layout_width="0dp" android:layout_height="wrap_content"
        android:hint="IP Server (vd 192.168.1.15)" android:inputType="text"
        app:layout_constraintTop_toTopOf="parent" app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintEnd_toEndOf="parent" />
    <EditText android:id="@+id/etServerPort" android:layout_width="0dp" android:layout_height="wrap_content"
        android:hint="Port (8080)" android:inputType="number"
        app:layout_constraintTop_toBottomOf="@id/etServerIp" app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintEnd_toEndOf="parent" android:layout_marginTop="8dp" />
    <Button android:id="@+id/btnConnect" android:layout_width="0dp" android:layout_height="wrap_content"
        android:text="KẾT NỐI"
        app:layout_constraintTop_toBottomOf="@id/etServerPort" app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintEnd_toEndOf="parent" android:layout_marginTop="16dp" />

    <androidx.recyclerview.widget.RecyclerView
        android:id="@+id/rvMessages" android:layout_width="0dp" android:layout_height="0dp"
        app:layout_constraintTop_toBottomOf="@id/btnConnect"
        app:layout_constraintBottom_toTopOf="@id/inputLayout"
        app:layout_constraintStart_toStartOf="parent" app:layout_constraintEnd_toEndOf="parent"
        android:layout_marginTop="16dp" />

    <LinearLayout android:id="@+id/inputLayout" android:layout_width="0dp" android:layout_height="wrap_content"
        android:orientation="horizontal" android:layout_marginTop="8dp" android:visibility="gone"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintStart_toStartOf="parent" app:layout_constraintEnd_toEndOf="parent">
        <EditText android:id="@+id/etMessage" android:layout_width="0dp" android:layout_height="wrap_content"
            android:layout_weight="1" android:hint="Tin nhắn..." />
        <Button android:id="@+id/btnSend" android:layout_width="wrap_content" android:layout_height="wrap_content"
            android:layout_marginStart="8dp" android:text="Gửi" />
    </LinearLayout>
</androidx.constraintlayout.widget.ConstraintLayout>

MainActivity Client (Kotlin Coroutines)

class MainActivity : AppCompatActivity() {
    private lateinit var binding: ActivityMainBinding
    private val adapter = MessageAdapter() // Cùng class MessageAdapter
    private var socket: Socket? = null
    private var out: PrintWriter? = null
    private val scope = CoroutineScope(Dispatchers.IO)
    private val uiScope = CoroutineScope(Dispatchers.Main)

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        binding = ActivityMainBinding.inflate(layoutInflater)
        setContentView(binding.root)

        binding.rvMessages.apply {
            layoutManager = LinearLayoutManager(this@MainActivity)
            adapter = this@MainActivity.adapter
        }

        binding.btnConnect.setOnClickListener { connect() }
        binding.btnSend.setOnClickListener { sendMessage() }
    }

    private fun connect() {
        val ip = binding.etServerIp.text.toString().trim()
        val portStr = binding.etServerPort.text.toString().trim()
        if (ip.isEmpty() || portStr.isEmpty()) return
        val port = portStr.toIntOrNull() ?: return

        scope.launch {
            try {
                socket = Socket(ip, port)
                out = PrintWriter(socket!!.getOutputStream(), true)
                val in = BufferedReader(InputStreamReader(socket!!.getInputStream()))

                uiScope.launch {
                    binding.tvMessages.text = "Đã kết nối $ip:$port"
                    binding.btnConnect.isEnabled = false
                    binding.etServerIp.isEnabled = false
                    binding.etServerPort.isEnabled = false
                    binding.inputLayout.visibility = View.VISIBLE
                    binding.btnSend.isEnabled = true
                }

                // Loop nhận từ server
                while (true) {
                    val line = in.readLine() ?: break
                    uiScope.launch {
                        adapter.submitList(adapter.currentList + ChatMessage(line, false))
                        binding.rvMessages.scrollToPosition(adapter.itemCount - 1)
                    }
                }
            } catch (e: IOException) {
                uiScope.launch { binding.tvMessages.text = "Lỗi kết nối: ${e.message}" }
            } finally {
                onDisconnected()
            }
        }
    }

    private fun sendMessage() {
        val text = binding.etMessage.text.toString().trim()
        if (text.isNotEmpty()) {
            out?.println(text)
            uiScope.launch {
                adapter.submitList(adapter.currentList + ChatMessage(text, true))
                binding.rvMessages.scrollToPosition(adapter.itemCount - 1)
                binding.etMessage.setText("")
            }
        }
    }

    private fun onDisconnected() {
        uiScope.launch {
            binding.btnConnect.isEnabled = true
            binding.etServerIp.isEnabled = true
            binding.etServerPort.isEnabled = true
            binding.inputLayout.visibility = View.GONE
            binding.btnSend.isEnabled = false
            adapter.submitList(adapter.currentList + ChatMessage("-- Mất kết nối --", false))
        }
        try { socket?.close() } catch (e: IOException) {}
        socket = null; out = null
    }

    override fun onDestroy() {
        super.onDestroy()
        scope.cancel()
        try { socket?.close() } catch (e: IOException) {}
    }
}

🚀 Chạy và Test

Build & cài **Server App** lên thiết bị A. Mở app, ghi nhớ IP hiển thị (ví dụ: 192.168.1.42).
Build & cài **Client App** lên thiết bị B. Nhập IP của A, Port 8080, nhấn **KẾT NỐI**.
Hai bên chat realtime. Thử tắt WiFi một bên để test reconnect logic.

⚠️ Các lỗi thường gặp & Fix

Lỗi	Nguyên nhân	Khắc phục
Connection refused	Server chưa start, sai IP/Port, Firewall	Kiểm tra Server chạy, IP đúng, cùng subnet, tắt tường lửa test
NetworkOnMainThreadException	Làm network trên Main Thread	Dùng Coroutine Dispatchers.IO / Thread / RxJava
SocketTimeout / Read timeout	Mạng không ổn định	Set SO_TIMEOUT, implement heartbeat
Emulator không kết nối được thiết bị thật	Emulator dùng NAT riêng (10.0.2.x)	Dùng `10.0.2.2` để truy cập localhost máy host, hoặc cấu hình port forwarding
IPv6 vs IPv4	ServerSocket mặc định IPv6 dual-stack	Client connect bằng IPv4 address, hoặc dùng InetAddress.getByName

🔧 Nâng cao cho Production

Protocol: Thay plain text bằng JSON/Protobuf (kotlinx.serialization).
Security: SSLSocketFactory + Certificate pinning.
Discovery: NSD (Network Service Discovery) hoặc mDNS/Bonjour để tự tìm Server IP.
Reconnection: Exponential backoff retry logic.
Architecture: Repository + ViewModel + Flow, tách Network Module.

Kết luận

Socket TCP trên Android cho phép giao tiếp realtime, hai chiều, độ trễ thấp trên mạng cục bộ. Triển khai đúng đắn cần: Coroutine/Thread cho I/O, quản lý vòng đời Socket, xử lý reconnection, và protocol rõ ràng. Code Kotlin + Coroutines + RecyclerView ở trên là nền tảng sẵn sàng mở rộng.