如何在Java中监听UDP端口？

Question

我在Java项目中需要监听3个不同的UDP端口。我按照以下方式实现了我的系统：

我有一个名为UDPMessageListener的类，它实现了Runnable接口。我想要创建3个线程，以此对象作为运行实例。

我有一个名为“UDPPacketProcessor”的接口，其中只有一个onPacketReceived方法。

每个UDPMessageListener都有一个UDPPacketProcessor实例，并将UDP数据包定向到已注册为其UDPPacketProcessor的对象。

我还有一个名为DatabaseProc的类，它需要为来自3个不同UDP端口的消息提供服务。DatabaseProc实现了UDPPacketProcessor以向这3个UDPMessageListener类注册自身。

理论上，根据我的Java知识，不应该有任何问题，3个Runnable线程监听3个端口，并在收到数据包时调用我的主单例对象的方法。然而，当我运行程序时，只有一个端口被监听。我发现只有HBMessageListener正常工作，其他两个不起作用。当我绕过HBMessageListener（注释掉我运行它的那一行）后，我现在可以看到只有AlarmMessageListener起作用。我错在哪里了？

UDPMessageListener.java

public class UDPMessageListener implements Runnable {

int port;
byte[] receiveData;
DatagramSocket udpListeningSocket;
UDPPacketProcessor processor;
public UDPMessageListener(UDPPacketProcessor listener,int localPort){
    port = localPort;
    this.processor = listener;
    receiveData = new byte[SRPDefinitions.BYTE_BUFFER_MAX_LENGTH];
    try {
        udpListeningSocket = new DatagramSocket(port);
    } catch (SocketException e) {
        System.out.println("Socket bind error in port: " + port);
        e.printStackTrace();
    }
}

@Override
public void run() {
    while(true){
        DatagramPacket receivePacket = new DatagramPacket(receiveData, receiveData.length);
        try {
            udpListeningSocket.receive(receivePacket);
            System.out.println("Received UDP Packet from Port:" + port);
            processor.onPacketReceived(receivePacket, port);

        } catch (IOException e) {
            System.out.println("UDP Listener end up with an exception:");
            e.printStackTrace();
        }

    }
}
}

UDPPacketProcessor.java

public interface UDPPacketProcessor {
public void onPacketReceived(DatagramPacket receivedPacket,int localPort);
}

DatabaseProc.java

public class DatabaseProc implements UDPPacketProcessor{

private static DatabaseProc instance = null; // for singleton.

byte[] receiveData;
byte[] sendData;
ByteBuffer systemMessageByteBuffer;

UDPMessageListener HBMessageListener;
UDPMessageListener AlarmMessageListener;
UDPMessageListener TrackMessageListener;

private DatabaseProc(){
    receiveData = new byte[SRPDefinitions.BYTE_BUFFER_MAX_LENGTH];
    sendData = new byte[SRPDefinitions.BYTE_BUFFER_MAX_LENGTH];
    HBMessageListener = new UDPMessageListener(this,SRPDefinitions.HB_PORT);
    AlarmMessageListener = new UDPMessageListener(this,SRPDefinitions.ALARM_PORT);
    TrackMessageListener = new UDPMessageListener(this,SRPDefinitions.TRACK_PORT);
}

public void run(){
    runListeners();
}

private void runListeners(){
    HBMessageListener.run();
    AlarmMessageListener.run();
    TrackMessageListener.run();
}

public static DatabaseProc getInstance(){
    if(instance == null){
        instance = new DatabaseProc();
    }
    return instance;
}

@Override
public void onPacketReceived(DatagramPacket receivedPacket, int localPort) {
    String strIPAddress =receivedPacket.getAddress().toString();

    ByteBuffer buffer = ByteBuffer.allocate(receivedPacket.getLength());
    System.out.println("Received Packet Length: " + receivedPacket.getLength() + "/" + receivedPacket.getData().length);
    buffer.order(ByteOrder.LITTLE_ENDIAN);

    buffer.put(receivedPacket.getData(),0,receivedPacket.getLength());
    buffer.position(0);

    if(localPort == SRPDefinitions.HB_PORT){
        System.out.println("HB Message Received from " + strIPAddress + "!");
        SRPHeartBeatMessage message = new SRPHeartBeatMessage(buffer);
        //message.print();
    }
    if(localPort == SRPDefinitions.ALARM_PORT){
        System.out.println("ALARM Message Received from " + strIPAddress + "!");
        SRPAlarmMessage message = new SRPAlarmMessage(buffer);
        message.print();
    }
    if(localPort == SRPDefinitions.TRACK_PORT){
        System.out.println("TRACK Message Received from " + strIPAddress + "!");
    }   
}

- fercis

---

你是如何验证只有一个端口是开放的？请发布 netstat 的输出（正确列出监听 UDP 套接字的选项取决于你的操作系统）。 - chrylis -cautiouslyoptimistic-

感谢您的即时回复。我的3个端口是42010、42020和42030。HB端口是42030（我只能得到响应的端口，也是第一个运行的端口）。我从终端查询了每个3个端口的netstat输出，所有端口的进程ID都相同： - fercis

`fercis@fercis2014:~$ sudo netstat -lpn |grep :42030 udp6 0 0 :::42030 :::* 3458/java fercis@fercis2014:~$ sudo netstat -lpn |grep :42010 udp6 4224 0 :::42010 :::* 3458/java fercis@fercis2014:~$ sudo netstat -lpn |grep :42020 udp6 0 0 :::42020 :::* 3458/java` - fercis

Answer 1

问题出在 DatabaseProc 类中。将 runListeners 改为以下内容。

private void runListeners(){
    new Thread(HBMessageListener).start();
    new Thread(AlarmMessageListener).start();
    new Thread(TrackMessageListener).start();
}

编辑

当直接调用UDPMessageListener.run()时，解释就是它不会创建新的线程并异步执行。在您的代码中，由于HBMessageListener.run()方法处于无限循环中，因此执行永远不会跳出该方法。您需要并行化监听器以便所有监听器都可以监听。

- shazin