Java 中的 UDP 广播

Question

早晨。

我对 Java 和套接字连接很陌生，但我正在尝试在端口 8001 上的 255.255.255.255 上向设备发送 UDP 数据包/广播。我可以很好地发送数据，但是当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。

我很确定这是我在代码中遗漏的菜鸟错误，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();

谢谢，还有 Gig'Em。

编辑：

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;

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;

Answer 1

这没有道理。你在广播，这是1对多，你也在连接，这是1对1。是哪一个？

失去连接。并丢失 255.255.255.255。大约 20 年来，该方法已被严重弃用。使用子网本地广播地址，例如 192.168.1.255。

Answer 2

您还可以查看 Broadcasting to Multiple Recipients。希望这有帮助。

Answer 3

如果您想接收数据报，您需要 bind() 到本地端点（地址+端口）。

Answer 4

您正在同一设备上发送和接收数据包。您可以分离发送和接收端口（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。如果两个设备必须找到对方（或一个设备找到自己）。

import java.io.IOException;
import java.net.*;

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());
    }
}

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());
    }
}

当然应该将这段代码放在线程中的循环中。
基于此示例： https://demey.io/network-discovery-using- udp-broadcast/

更新

上面的链接已失效。但这里也描述了相同的方法： https://www.baeldung.com/java-broadcast-multicast