DatagramSocket.receive and DatagramSocket.close are both synchronized on the same monitor, so any thread attempting to close a socket blocks until a packet is received on the socket (if another thread is currently waiting for a packet). This is not right.

This code demonstrates the problem:

import java.net.*;
import java.io.*;
public class SocketClose
implements Runnable
{
public static void main(String args[])
{
try {
DatagramSocket sock = new DatagramSocket();
System.out.println("Opened socket on port "+sock.getLocalPort());
Thread t = new Thread(new SocketClose(sock));
t.start();
Thread.sleep(1000);
System.out.println("Attempting to close...");
sock.close();
System.out.println("Closed!");
}
catch (Exception e)
{
System.err.println("Closer thread exception:");
e.printStackTrace();
}
}
DatagramSocket sock;
public SocketClose(DatagramSocket sock)
{
this.sock = sock;
}
public void run()
{
try {sock.receive(new DatagramPacket(new byte[1000],1000));}
catch (IOException e)
{
System.err.println("Receiver thread exception:");
e.printStackTrace();
}
}
}