Non-Blocking Multicast Sockets in JDK 1.4?

Question

Hi,
I've been trying to create non-blocking multicast sockets in JDK1.4, which essentially seems (at this stage) to boil down to the simpler problem of creating a DatagramChannel that uses MulticastSockets, or at least DatagramSockets that can join a Multicast group. Not having found any obvious way to do it, I created this extraordinary hack:
package java.net; // Wicked, wicked!
import java.io.*;
public class MyDatagramSocket {
public static void join(java.net.DatagramSocket socket, InetAddress addr)
throws IOExceptio DatagramSocket ds = new DatagramSocket(port);
ds.setReuseAddress(true);
MyDatagramSocket.join(ds, InetAddress.getByName("224.0.0.104"));
DatagramPacket dp = new DatagramPacket(array, 5000);
ds.receive(dp);          /* READS FINE */
n
socket.impl.join(addr); // Uses knowledge of DatagramSocket culled from examining source to access DatagramSocketImpl
Now I compile this, and drop the class file into my rt.jar files (in the JDK and the JRE), so that I can use MyDatagramSocket.join (DatagramSocket, InetAddress), which looks like it should work from code like this:
try {
int port = 58501;
DatagramChannel dc = DatagramChannel.open();
dc.socket().setReuseAddress(true);
dc.socket().bind(new InetSocketAddress(port));
MyDatagramSocket.join(dc.socket(), InetAddress.getByName("224.0.0.104"));
byte [] array = new byte[5000];
ByteBuffer bb = ByteBuffer.wrap(array);
dc.receive(bb);
System.out.println("Read from dc");
} catch (Exception x) {
x.printStackTrace();
But it doesn't work - it just doesn't read. A simpler example is this:
DatagramSocket ds = new DatagramSocket(port);
ds.setReuseAddress(true);
MyDatagramSocket.join(ds, InetAddress.getByName("224.0.0.104"));
DatagramPacket dp = new DatagramPacket(array, 5000);
ds.receive(dp);          /* READS FINE */
So I know that my hack is working, but this fails:
DatagramChannel dc = DatagramChannel.open();
dc.socket().bind(new InetSocketAddress(port));
dc.socket().setReuseAddress(true);
MyDatagramSocket.join(dc.socket(), InetAddress.getByName("224.0.0.104"));
DatagramPacket dp = new DatagramPacket(array, 5000);
dc.socket().receive(dp);     /* NEVER READS */
I've reduced the problem to one of the difference between a java.net.DatagramSocket - the standard DatagramSocket, and a sun.nio.ch.DatagramSocketAdaptor, which is what DatagramChannels seem to use.
My questions are:
a) Is there a proper way to do this, without my adding my own classes to java.net?
b) If NO is the answer to a), any ideas what I'm doing wrong in my code?
Many thanks for any assistance,
Craig

Answer 1

I've encountered the same problem in my code. The datagramChannel never receives incoming data. Doesn't matter the rate at which you send it or anything else. I don't see any way around this problem at the moment. If i find something i'll post it. Interesting enough, my friend who programs with C++ got non-blocking I/O with datagrams to work in windows. So this might just be java.