Java发送和接收广播的UDP，用于探测局域网中指定类型的设备

注意这是发的广播信息，同一网段中其它机器都会收到这个信息（只有特殊的监听这类消息的机器会做出回应）：

SendUDP.java

1import java.io.BufferedReader;
2import java.io.IOException;
3import java.io.InputStreamReader;
4import java.net.DatagramPacket;
5import java.net.DatagramSocket;
6import java.net.InetAddress;
7import java.net.UnknownHostException;
8
9public class SendUDP {
10    public static void main(String[] args) throws Exception {
11        // Use this port to send broadcast packet
12        @SuppressWarnings("resource")
13        final DatagramSocket detectSocket = new DatagramSocket(8888);
14
15        // Send packet thread
16        new Thread(new Runnable() {
17            @Override
18            public void run() {
19                System.out.println("Send thread started.");
20                while (true) {
21                    try {
22                        byte[] buf = new byte[1024];
23                        int packetPort = 9999;
24
25                        // Broadcast address
26                        InetAddress hostAddress = InetAddress.getByName("192.168.184.255");
27                        BufferedReader stdin = new BufferedReader(
28                                new InputStreamReader(System.in));
29                        String outMessage = stdin.readLine();
30
31                        if (outMessage.equals("bye"))
32                            break;
33                        buf = outMessage.getBytes();
34                        System.out.println("Send " + outMessage + " to " + hostAddress);
35                        // Send packet to hostAddress:9999, server that listen
36                        // 9999 would reply this packet
37                        DatagramPacket out = new DatagramPacket(buf,
38                                buf.length, hostAddress, packetPort);
39                        detectSocket.send(out);
40                    } catch (UnknownHostException e) {
41                        e.printStackTrace();
42                    } catch (IOException e) {
43                        e.printStackTrace();
44                    }
45                }
46            }
47        }).start();
48        
49        // Receive packet thread.
50        new Thread(new Runnable() {
51            @Override
52            public void run() {
53                System.out.println("Receive thread started.");
54                while(true) {
55                    byte[] buf = new byte[1024];
56                    DatagramPacket packet = new DatagramPacket(buf, buf.length);
57                    try {
58                        detectSocket.receive(packet);
59                    } catch (IOException e) {
60                        e.printStackTrace();
61                    }
62                    String rcvd = "Received from " + packet.getSocketAddress() + ", Data="
63                            + new String(packet.getData(), 0, packet.getLength());
64                    System.out.println(rcvd);
65                }
66            }
67        }).start();
68    }
69}

ReceiveUDP.java

1import java.net.DatagramPacket;
2import java.net.DatagramSocket;
3
4public class ReceiveUDP {
5    public static void main(String[] args) throws Exception {
6        int listenPort = 9999;
7        byte[] buf = new byte[1024];
8        DatagramPacket packet = new DatagramPacket(buf, buf.length);
9        @SuppressWarnings("resource")
10        DatagramSocket responseSocket = new DatagramSocket(listenPort);
11        System.out.println("Server started, Listen port: " + listenPort);
12        while (true) {
13            responseSocket.receive(packet);
14            String rcvd = "Received "
15                    + new String(packet.getData(), 0, packet.getLength())
16                    + " from address: " + packet.getSocketAddress();
17            System.out.println(rcvd);
18
19            // Send a response packet to sender
20            String backData = "DCBA";
21            byte[] data = backData.getBytes();
22            System.out.println("Send " + backData + " to " + packet.getSocketAddress());
23            DatagramPacket backPacket = new DatagramPacket(data, 0,
24                    data.length, packet.getSocketAddress());
25            responseSocket.send(backPacket);
26        }
27    }
28}

下图是SendUDP端的执行截图，发送内容为Message：

在SendUDP端发送了消息后，UDP端会立即显示收到消息，如下图：

正如第一幅图看到的，我在同一子网下的两台机器上运行着ReceiveUDP，于是两台机器都做出了回应。

如果将这种方式移植到Android手机上，可以用来探测同一WiFi下的其它设备（前提是这些设备上运行着类似ReceiveUDP的），以获取它们的IP地址。此后可以建立TCP连接，做其他的事情。有人说可以用Ping网段的方式来发现其它设备，但对于Android来说，这个方式并不可靠。因为判定消息不可达的时间难以确定。