要實現在Android平臺上發UDP廣播，可能需要先了解一下什麼是廣播地址

廣播地址

廣播地址（Broadcast Address）是專門用於同時向網路中所有工作站進行傳送的一個地址。在使用TCP/IP協議的網路中，主機標識端host ID 為全1的IP地址為廣播地址，廣播的分組傳送給host ID段所涉及的所有計算機。例如，對於10.1.1.0（255.255.255.0）網段，其廣播地址為10.1.1.255（255即為2進位制的11111111），當發出一個目的地址為10.1.1.255的分組（封包）時，它將被分發給該網段上的所有計算機。

[圖片來自百度百科]

廣播地址有兩類：

• 受限廣播
  它不被路由傳送，但會被送到相同物理網路段上的所有主機，IP地址的網路欄位和主機欄位全為1就是地址255.255.255.255。
• 直接廣播
  網路廣播會被路由傳送，並會發送到專門網路上的每臺主機，IP地址的網路欄位定義這個網路，主機欄位通常為全為1，如 192.168.10.255

關於廣播地址的其他知識，大家可以自行搜尋學習。

當我們知道有廣播地址這個東西之後，就能很方便地在Android平臺上實現傳送廣播和接收廣播了。

一臺Android作為Server端傳送廣播，那麼此時的廣播地址怎麼確定，因為，作為Server端的手機可能是連線到一個路由器上，也有可能是自己作為AP裝置發熱點，讓Client端去連線。對於以上的兩種情況，廣播地址是有所不同的：

• 第一種情況（server端連線到路由器）：見下面程式碼片段。
• 第二種情況（server端作為AP裝置傳送熱點）：
  在這種情況下，IP也是可以確定，有人在分析Android原始碼的時候，發現如果Android裝置開啟了wifi熱點，那麼，該裝置的本地IP是固定的，是192.168.43.1，那麼我們就可以知道此時對應的廣播地址就是192.168.43.255。

通過以下程式碼可以獲取到本地IP（java）

    public static String getLocalIPAddress() {
        Enumeration<NetworkInterface> enumeration = null;
        try
 
 {
            enumeration = NetworkInterface.getNetworkInterfaces();
        } catch (SocketException e) {
            Logger.w(e);
        }
        if (enumeration != null) {
            // 遍歷所用的網路介面
            while (enumeration.hasMoreElements()) {
                NetworkInterface nif = enumeration.nextElement();// 得到每一個網路介面繫結的地址
                Enumeration<InetAddress> inetAddresses = nif.getInetAddresses();
                // 遍歷每一個介面繫結的所有ip
                if (inetAddresses != null)
                    while (inetAddresses.hasMoreElements()) {
                        InetAddress ip = inetAddresses.nextElement();
                        if (!ip.isLoopbackAddress() && isIPv4Address(ip.getHostAddress())) {
                            return ip.getHostAddress();
                        }
                    }
            }
        }
        return "";
    }
     /**
     * Ipv4 address check.
     */
    private static final Pattern IPV4_PATTERN = Pattern.compile("^(" +
            "([0-9]|[1-9][0-9]|1[0-9]{2}|2[0-4][0-9]|25[0-5])\\.){3}" +
            "([0-9]|[1-9][0-9]|1[0-9]{2}|2[0-4][0-9]|25[0-5])$");

    /**
     * Check if valid IPV4 address.
     *
     * @param input the address string to check for validity.
     * @return True if the input parameter is a valid IPv4 address.
     */
    public static boolean isIPv4Address(String input) {
        return IPV4_PATTERN.matcher(input).matches();
    }

通過以下程式碼可以直接獲取廣播地址，如果是開啟wifi熱點直接返回“192.168.43.255”（kotlin）：

    companion object {
        fun getBroadcastAddress(context: Context): InetAddress {
            if (isWifiApEnabled(context)) { //判斷wifi熱點是否開啟
                return InetAddress.getByName("192.168.43.255")  //直接返回
            }
            val wifi: WifiManager = context.getSystemService(Context.WIFI_SERVICE) as WifiManager
            val dhcp: DhcpInfo = wifi.dhcpInfo ?: return InetAddress.getByName("255.255.255.255")
            val broadcast = (dhcp.ipAddress and dhcp.netmask) or dhcp.netmask.inv()
            val quads = ByteArray(4)
            for (k in 0..3) {
                quads[k] = ((broadcast shr k * 8) and 0xFF).toByte()
            }
            return InetAddress.getByAddress(quads)
        }

        /**
         * check whether the wifiAp is Enable
         */
        private fun isWifiApEnabled(context: Context): Boolean {
            try {
                val manager = context.getSystemService(Context.WIFI_SERVICE) as WifiManager
                val method = manager.javaClass.getMethod("isWifiApEnabled")
                return method.invoke(manager) as Boolean
            } catch (e: NoSuchMethodException) {
                e.printStackTrace()
            } catch (e: IllegalAccessException) {
                e.printStackTrace()
            } catch (e: InvocationTargetException) {
                e.printStackTrace()
            }
            return false
        }
    }

如果，廣播地址能確定了，下面就可以進行實現傳送廣播（Server）和接收廣播（Client）了

我們先定義一個通用的UDP廣播類：包括（獲取廣播地址，開啟和關閉廣播、傳送廣播包和接收廣播包）

UDPBroadcaster.kt:（kotlin）

class UDPBroadcaster(var mContext: Context) {
    private val TAG:String = UDPBroadcaster::class.java.simpleName
    private var mDestPort = 0
    private var mSocket: DatagramSocket? = null
    private val ROOT_PATH:String = Environment.getExternalStorageDirectory().path
    /**
     * 開啟
     */
    fun open(localPort: Int, destPort: Int): Boolean {
        mDestPort = destPort
        try {
            mSocket = DatagramSocket(localPort)
            mSocket?.broadcast = true
            mSocket?.reuseAddress = true
            return true
        } catch (e: SocketException) {
            e.printStackTrace()
        }
        return false
    }

    /**
     * 關閉
     */
    fun close(): Boolean {
        if (mSocket != null && mSocket?.isClosed?.not() as Boolean) {
            mSocket?.close()
        }
        return true
    }

    /**
     * 傳送廣播包
     */
    fun sendPacket(buffer: ByteArray): Boolean {
        try {
            val addr = getBroadcastAddress(mContext)
            Log.d("$TAG addr",addr.toString())
            val packet = DatagramPacket(buffer, buffer.size)
            packet.address = addr
            packet.port = mDestPort
            mSocket?.send(packet)
            return true
        } catch (e1: UnknownHostException) {
            e1.printStackTrace()
        } catch (e: IOException) {
            e.printStackTrace()
        }
        return false
    }

    /**
     * 接收廣播
     */
    fun recvPacket(buffer: ByteArray): Boolean {
        val packet = DatagramPacket(buffer, buffer.size)
        try {
            mSocket?.receive(packet)
            return true
        } catch (e: IOException) {
            e.printStackTrace()
        }
        return false
    }

    companion object {
        /**
         * 獲取廣播地址
         */
        fun getBroadcastAddress(context: Context): InetAddress {
            if (isWifiApEnabled(context)) { //判斷wifi熱點是否開啟
                return InetAddress.getByName("192.168.43.255")  //直接返回
            }
            val wifi: WifiManager = context.getSystemService(Context.WIFI_SERVICE) as WifiManager
            val dhcp: DhcpInfo = wifi.dhcpInfo ?: return InetAddress.getByName("255.255.255.255")
            val broadcast = (dhcp.ipAddress and dhcp.netmask) or dhcp.netmask.inv()
            val quads = ByteArray(4)
            for (k in 0..3) {
                quads[k] = ((broadcast shr k * 8) and 0xFF).toByte()
            }
            return InetAddress.getByAddress(quads)
        }

        /**
         * check whether the wifiAp is Enable
         */
        private fun isWifiApEnabled(context: Context): Boolean {
            try {
                val manager = context.getSystemService(Context.WIFI_SERVICE) as WifiManager
                val method = manager.javaClass.getMethod("isWifiApEnabled")
                return method.invoke(manager) as Boolean
            } catch (e: NoSuchMethodException) {
                e.printStackTrace()
            } catch (e: IllegalAccessException) {
                e.printStackTrace()
            } catch (e: InvocationTargetException) {
                e.printStackTrace()
            }
            return false
        }
    }
}

宣告許可權

<uses-permission android:name="android.permission.ACCESS_WIFI_STATE"/>
<uses-permission android:name="android.permission.INTERNET"/>

Server：

傳送廣播包（kotlin）：

class MainActivity : AppCompatActivity(), View.OnClickListener {

    val TAG: String = MainActivity::class.java.simpleName
    val SEND_PORT: Int = 8008
    val DEST_PORT: Int = 8009
    var isClosed: Boolean = false
    var sendBuffer: String = "This is UDP Server"
    lateinit var mSendBtn: Button
    lateinit var mCloseBtn: Button
    lateinit var mScrollView: ScrollView
    lateinit var mLogTx: TextView
    lateinit var mUDPBroadCast: UDPBroadcaster
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        mUDPBroadCast = UDPBroadcaster(this)
        initView()
        initEvent()
    }

    private fun initView() {
        mSendBtn = findViewById(R.id.btn_send) as Button
        mCloseBtn = findViewById(R.id.btn_close) as Button
        mScrollView = findViewById(R.id.scrollview) as ScrollView
        mLogTx = findViewById(R.id.log) as TextView
    }

    private fun initEvent() {
        mSendBtn.setOnClickListener(this)
        mCloseBtn.setOnClickListener(this)
    }

    override fun onClick(v: View?) {
        when (v?.id) {
            R.id.btn_send -> sendUDPBroadcast()
            R.id.btn_close -> closeUDPBroadcast()
        }
    }

    private fun closeUDPBroadcast() {
        isClosed = true
    }


    private fun sendUDPBroadcast() {
        isClosed = false
        mUDPBroadCast.open(SEND_PORT, DEST_PORT) //開啟廣播
        val buffer: ByteArray = sendBuffer.toByteArray()
        Thread(Runnable {
            while (!isClosed) {
                try {
                    Thread.sleep(500) //500ms 延時
                } catch (e: Exception) {
                    e.printStackTrace()
                }
                mUDPBroadCast.sendPacket(buffer) //傳送廣播包
                addLog("$TAG data: ${String(buffer)}")
            }
            mUDPBroadCast.close() //關閉廣播
        }).start()
    }

    private fun addLog(log: String) {
        var mLog: String = log
        if (mLog.endsWith("\n").not()) {
            mLog += "\n"
        }
        mScrollView.post(Runnable {
            mLogTx.append(mLog)
            mScrollView.fullScroll(ScrollView.FOCUS_DOWN)
        })
    }
}

Client：

接收廣播（kotlin）：

class MainActivity : AppCompatActivity(), View.OnClickListener {

    val TAG:String = MainActivity::class.java.simpleName
    val LOCAL_PORT:Int = 8009
    val DEST_PORT:Int = 8008
    lateinit var mRecvBtn: Button
    lateinit var mCloseBtn: Button
    lateinit var mScrollView: ScrollView
    lateinit var mLogTx: TextView
    var isClosed:Boolean = false
    lateinit var mUDPBroadCaster:UDPBroadcaster
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        mUDPBroadCaster = UDPBroadcaster(this)
        initView()
        initEvent()
    }

    private fun initEvent() {
        mRecvBtn.setOnClickListener(this)
        mCloseBtn.setOnClickListener(this)
    }

    private fun initView() {
        mRecvBtn = findViewById(R.id.btn_receive) as Button
        mCloseBtn = findViewById(R.id.btn_close) as Button
        mScrollView = findViewById(R.id.scrollView) as ScrollView
        mLogTx = findViewById(R.id.log) as TextView

    }
    override fun onClick(v: View?) {
        when(v?.id){
            R.id.btn_receive->recvUDPBroadcast()
            R.id.btn_close->cancelRecv()
        }
    }

    private fun cancelRecv() {
        isClosed = true
    }

    private fun recvUDPBroadcast() {
        isClosed = false
        mUDPBroadCaster.open(LOCAL_PORT,DEST_PORT)
        var buffer:ByteArray = kotlin.ByteArray(1024)
        val packet = DatagramPacket(buffer, buffer.size)
        Thread(Runnable {
            while (!isClosed){
                try{
                    Thread.sleep(500) //500ms延時
                }catch (e:Exception){e.printStackTrace()}
                mUDPBroadCaster.recvPacket(packet) //接收廣播
                val data:String = String(packet.data)
                addLog("$TAG data: $data")
                addLog("$TAG addr: ${packet.address}")
                addLog("$TAG port: ${packet.port}")
            }
            mUDPBroadCaster.close() //退出接收廣播
        }).start()
    }
    private fun addLog(log: String) {
        var mLog: String = log
        if (mLog.endsWith("\n").not()) {
            mLog += "\n"
        }
        mScrollView.post(Runnable {
            mLogTx.append(mLog)
            mScrollView.fullScroll(ScrollView.FOCUS_DOWN)
        })
    }
}

程式碼很少，也很容易看懂。傳送和接收廣播的截圖如下：

• Server

  

• Client

  

原始碼下載地址