Java中的UDP广播
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UDP Broadcast in Java
提问by user355528
Morning.
早晨。
I'm pretty new in Java and socket connections but I'm trying to send out a UDP packet/broadcast on 255.255.255.255 on port 8001 to a device. I can get the data to send just fine, however when it comes time to receive the data the connection times out. I have a packet sniffer and I can see the packet send and then the device respond.
我在 Java 和套接字连接方面很新，但我试图在端口 8001 上的 255.255.255.255 上向设备发送 UDP 数据包/广播。我可以让数据发送得很好，但是当需要接收数据时，连接超时。我有一个数据包嗅探器，我可以看到数据包发送，然后设备响应。
I'm pretty sure it is a rookie mistake that I'm missing in my code but I've been stuck on it for awhile and any help would be appreciated.
我很确定这是我在代码中遗漏的新手错误，但我已经坚持了一段时间，任何帮助将不胜感激。
m_Socket = new DatagramSocket(m_SERVERPORT);
InetAddress address = InetAddress.getByName(m_SERVERIP);
m_DataPack = new DatagramPacket(m_SERVERCMD.getBytes(), m_SERVERCMD.getBytes().length,
address, m_SERVERPORT);
m_Socket.setBroadcast(true);
m_Socket.connect(address, m_SERVERPORT);
m_Socket.send(m_DataPack);
m_DataPack = new DatagramPacket(data, data.length,
address, m_SERVERPORT);
m_Socket.receive(m_DataPack); // This is where it times out
data = m_DataPack.getData();
String received = data.toString();
System.out.println("Received: " + received);
m_Socket.close();
Thanks and Gig'Em.
谢谢和 Gig'Em。
EDIT:
编辑：
I'm not sure if this helps but when I watch the m_Socket object I can see the following right before it sends:
我不确定这是否有帮助，但是当我观看 m_Socket 对象时，我可以在它发送之前看到以下内容：
bound = true;
close = false;
connectedAddress = Inet4Address (id = 32) (-1,-1,-1,-1);
connectedPort = 8001;
connectState = 1;
created = true;
impl = PlainDatagramSocketImpl;
oldImpl = false;
and the m_DataPack object is the following:
m_DataPack 对象如下：
address = Inet4Address (id = 32) (-1,-1,-1,-1);
bufLength = 6 (size of packet I'm sending is 6 char long);
offset = 0;
port = 8001;
回答by John Kugelman
回答by user207421
This doesn't make sense. You are broadcasting, which is 1-to-many, and you are also connecting, which is 1-to-1. Which is it?
这没有意义。你在广播，这是一对多，你也在连接，这是一对一。是哪个？
Lose the connect. And lose the 255.255.255.255. This has been heavily deprecated for about 20 years. Use a subnet-local broadcast address, e.g. 192.168.1.255.
失去连接。并失去 255.255.255.255。这已经被严重弃用了大约 20 年。使用子网本地广播地址，例如 192.168.1.255。
回答by Zorayr
You can also take a look at MulticastSocket described at Broadcasting to Multiple Recipients. Hope this helps.
您还可以查看广播到多个接收者中描述的 MulticastSocket 。希望这可以帮助。
回答by user12043
You are sending and receiving the packet on same device. You could separate send and receive ports (e.g send on 8001, receive on 8002). And run send and receive codes as separate threads. If both device must find each other (or one device find itself).
您正在同一台设备上发送和接收数据包。您可以将发送和接收端口分开（例如在 8001 上发送，在 8002 上接收）。并将发送和接收代码作为单独的线程运行。如果两个设备都必须找到对方（或一个设备找到自己）。
import java.io.IOException;
import java.net.*;
Receiving:
接收：
private DatagramSocket getReceiveSocket() throws UnknownHostException, SocketException {
if (receiveSocket == null) {
receiveSocket = new DatagramSocket(8002, InetAddress.getByName("0.0.0.0")); // 0.0.0.0 for listen to all ips
receiveSocket.setBroadcast(true);
}
return receiveSocket;
}
public void receive() throws IOException {
// Discovery request command
byte[] buffer = "__DISCOVERY_REQUEST__".getBytes();
DatagramPacket packet = new DatagramPacket(buffer, buffer.length);
getReceiveSocket().receive(packet);
System.out.println("Discovery package received! -> " + packet.getAddress() + ":" + packet.getPort());
// Get received data
String data = new String(packet.getData()).trim();
if (data.equals("__DISCOVERY_REQUEST__")) { // validate command
// Send response
byte[] response = new byte["__DISCOVERY_RESPONSE".length()];
DatagramPacket responsePacket = new DatagramPacket(response, response.length, packet.getAddress(), packet.getPort());
getReceiveSocket().send(responsePacket);
System.out.println("Response sent to: " + packet.getAddress() + ":" + packet.getPort());
} else {
System.err.println("Error, not valid package!" + packet.getAddress() + ":" + packet.getPort());
}
}
Sending:
发送：
private DatagramSocket getSendSocket() throws UnknownHostException, SocketException {
if (sendSocket == null) {
sendSocket = new DatagramSocket(8001, InetAddress.getLocalHost());
sendSocket.setBroadcast(true);
}
return sendSocket;
}
public void send() throws IOException {
// Discovery request command
byte[] data = "__DISCOVERY_REQUEST__".getBytes();
DatagramPacket packet = new DatagramPacket(data, data.length, InetAddress.getByName("255.255.255.255"), 8002);
getSendSocket().send(packet);
System.out.println("Discovery package sent!" + packet.getAddress() + ":" + packet.getPort());
// Discovery response command
byte[] response = new byte["__DISCOVERY_RESPONSE__".length()];
DatagramPacket responsePacket = new DatagramPacket(response, response.length);
getSendSocket().receive(responsePacket);
System.out.println("Discovery response received!" + responsePacket.getAddress() + ":" + responsePacket.getPort());
String responseData = new String(responsePacket.getData()).trim();
if (responseData.equals("__DISCOVERY_RESPONSE__")) { // Check valid command
System.out.println("Found buddy!" + responsePacket.getAddress() + ":" + responsePacket.getPort());
}
}
Of course should put this code in a loop in a thread. Based on this example: https://demey.io/network-discovery-using-udp-broadcast/
当然应该把这段代码放在一个线程的循环中。基于此示例：https: //demey.io/network-discovery-using-udp-broadcast/
UPDATE
更新
The link above is dead. But same method described here also: https://www.baeldung.com/java-broadcast-multicast
上面的链接已经死了。但这里也描述了相同的方法：https: //www.baeldung.com/java-broadcast-multicast
