Question

I’m writing a piece of UDP networking program (client – server), and I’ve run into some trouble.

I want to use streams to I/O data, so I googled “udp inputstream” and found UDPInputStream and UDPOutputStream. When I try to use these, however, the program gets stuck when trying to initialize the UDPOutputStream.

This is the line in my code that freezes:

outStream = new UDPOutputStream(InetAddress.getByName("127.0.0.1"), port);
System.out.println("UDP output stream initialized."); // <-- doesn't get called

I checked out the source of the UDPOutputStream, the code gets stuck on this line:

dsock = new DatagramSocket();

Why does the execution hang up on this line? On the server side, I still use my “old”, non-stream version of a simple UDP code, and it works. The socket is initialized the same way and it doesn’t hang up. I tried to put a port number to the initialization, but it doesn’t solve the problem.

Answer 1

Host machines have more than one network interface (for example, 127.0.0.1 for the loopback interface and some other address for the network card; there may be more than one network card).

If you bind to the loopback address 127.0.0.1 then you’ll only be able to receive packets sent locally. If want to receive packets sent over the network from a remote machine you must bind to the local IP address (e.g. 192.168.1.100).

Try following:

InetAddress addr = InetAddress.InetAddress.getLocalHost();
outStream = new UDPOutputStream(addr, port);