DatagramSocket只允許數據報發送給指定的目標地址，而MulticastSocket可以將數據報以廣播的方式發送至多個客戶端。其主要思想是設置一組特殊網絡地址作為多點廣播地址，每個多點廣播地址都被看做一個組，當客戶端需要發送，接收廣播消息時，加入到該組即可。
IP協議為多點廣播提供了這些特殊的IP地址，這些IP地址的范圍是224.0.0.0至239.255.255.255。當MulticastSocket把一個DatagramPacket發送到多點廣播IP地址時，該數據將被自動廣播到加入該地址的所有MulticastSocket，同時也可以設置該MulticastSocket接收自身發送的數據。
如果僅僅是用于發送數據報的MulticastSocket對象，使用默認地址，隨機端口即可。但如果創建接收用的MulticastSocket對象，則該MulticastSocket對象必須指定端口，否則發送方無法確定發送數據報的目標端口。
下面通過一個簡單的例子實現多點廣播圖片：
多點廣播的工具類：
public class ComUtil{ public static final String BROADCAST_IP = "224.2.2.2"; public static final int BOADCAST_PORT = 30000; private static final int DATA_LEN = 100 * 1024; //定義本程序的MulticastSocket實例 private MulticastSocket socket = null; //定義廣播的IP地址 private InetAddress broadcastAddress = null; //定義接收網絡數據的字符數組 byte[] inBuff = new byte[DATA_LEN]; //以指定字節數組創建準備接受的DatagramPacket對象 private DatagramPacket inPacket = new DatagramPacket(inBuff , inBuff.length); //定義一個用于發送的DatagramPacket對象 private DatagramPacket outPacket = null; private Handler handler; //構造器，初始化資源 public ComUtil(Handler handler) throws Exception { this.handler = handler; //因為該MultcastSocket對象需要接受數據，所以有指定端口 socket = new MulticastSocket(BOADCAST_PORT); broadcastAddress = InetAddress.getByName(BROADCAST_IP); //將該socket加入指定的多點廣播地址 socket.joinGroup(broadcastAddress); //設置本MultcastSocket發送的數據報將被送到本身 socket.setLoopbackMode(false); //初始化發送用的DatagramSocket，它包含一個長度為0的字節數組 outPacket = new DatagramPacket(new byte[0] , 0 , broadcastAddress , BOADCAST_PORT); new ReadBroad().start(); } //廣播消息的工具方法 public void broadCast(byte[] msg) { try { //將msg字符串轉換為字節數組 byte[] buff = msg; //設置發送用的DatagramPacket里的字節數組 outPacket.setData(buff); //發送數據 socket.send(outPacket); } catch (IOException e) { e.printStackTrace(); } } //持續讀取MulticastSocket的線程 class ReadBroad extends Thread { public void run() { while (true) { try { //讀取Socket中的數據 socket.receive(inPacket); Message msg = new Message(); msg.what = 0x123; msg.obj = inBuff; handler.sendMessage(msg); } catch (IOException e) { e.printStackTrace(); } } } }}MainActivity類：
public class MainActivity extends Activity{ private Button button; private ImageView img; private ComUtil comUitl; Handler handler = new Handler() { @Override public void handleMessage(Message msg) { if (msg.what == 0x123) { byte[] result = (byte[]) msg.obj; img.setImageBitmap(BitmapFactory.decodeByteArray(result , 0 , result.length)); } } }; @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.main_activity); try { comUitl = new ComUtil(handler); } catch (Exception e) { e.printStackTrace(); } button = (Button) findViewById(R.id.send_img_all); img = (ImageView) findViewById(R.id.receiver_img); button.setOnClickListener(new View.OnClickListener() { @Override public void onClick(View view) { sendData(); } }); } private void sendData() { Bitmap bitmap = BitmapFactory.decodeResource(getResources() , R.drawable.wenqing2); ByteArrayOutputStream byteArray = new ByteArrayOutputStream(); bitmap.compress(Bitmap.CompressFormat.PNG , 100 , byteArray); bitmap.recycle(); final byte[] msg = byteArray.toByteArray(); new Thread() { @Override public void run() { comUitl.broadCast(msg); } }.start(); try { byteArray.close(); } catch (IOException e) { e.printStackTrace(); } }}以上就是本文的全部內容，希望對大家的學習有所幫助，也希望大家多多支持VEVB武林網。
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