A good, simple tutorial for making sockets work over the Internet.

[John McClellan]

Hello. Yesterday and today I've learned a few basic things about making sockets in Java an using them to create client/server-type applications. The problem here is that I'm not seeing much in the way of really simple, beginner's tutorials about how to apply that across more than a single computer (specifically across the Internet). All I've seen so far at least looks like it would only work on a single computer (they use localhost as the server, don't get into URLs, etc.). Does anybody have a good link to such a tutorial please? Thanks!

[Paul Clapham]

The reason you aren't seeing those specialized tutorials is that there isn't any difference between a socket connecting your application to a server on localhost and a socket connecting your browser to www.coderanch.com. You don't have to write your code any differently either.

Do you have a specific problem behind this question, or are you just asking out of curiosity?

[John McClellan]

Well, I haven't actually tried doing this across multiple computers, but it's just hard for me to see how it would know which thing to connect to if I didn't pass it some sort of information that describes where the server is and what it is. Below I've included the actual code I was using for the client side. What would I have to change if I were to try to access the server across the Internet, instead of on the same computer?

package bdn;

// The java.net package contains the basics needed for network operations. */
import java.net.*;
// The java.io package contains the basics needed for IO operations. */
import java.io.*;

/**
 * The SocketClient class is a simple example of a TCP/IP Socket Client.
 *
 */
public class SocketClient {
	public static void main(String[] args) {
		/** Define a host server */
		String host = "localhost";
		/** Define a port */
		int port = 19999;

		StringBuffer instr = new StringBuffer();
		String TimeStamp;
		System.out.println("SocketClient initialized");

		try {
			/** Obtain an address object of the server */
			InetAddress address = InetAddress.getByName(host);
			/** Establish a socket connection */
			Socket connection = new Socket(address, port);
			/** Instantiate a BufferedOutputStream object */
			BufferedOutputStream bos = new BufferedOutputStream(connection.getOutputStream());

			/** Instantiate an OutputStreamWriter object with the optional character encoding. */
			OutputStreamWriter osw = new OutputStreamWriter(bos, "US-ASCII");

			TimeStamp = new java.util.Date().toString();
			String process = "Calling the Socket Server on " + host + " port " + port + " at "
					+ TimeStamp + (char)13;

			/** Write across the socket connection and flush the buffer */
			osw.write(process);
			osw.flush();

			/**
			 * Instantiate a BufferedInputStream object for reading incoming socket streams.
			 */
			BufferedInputStream bis = new BufferedInputStream(connection.getInputStream());
			/**
			 * Instantiate an InputStreamReader with the optional character encoding.
			 */
			InputStreamReader isr = new InputStreamReader(bis, "US-ASCII");

			/** Read the socket's InputStream and append to a StringBuffer */
			int c;
			while ((c = isr.read()) != 13)
				instr.append((char)c);

			/** Close the socket connection. */
			connection.close();
			System.out.println(instr);
			}
		catch (IOException f) {
			System.out.println("IOException: " + f);
		}
		catch (Exception g) {
			System.out.println("Exception: " + g);
		}
	}
}

[John McClellan]

I mean, it probably has something to do with changing the name "localhost" to something different, but I'm not sure of the specifics. Also would it still be using that port number the same way?

[Tim Moores]

Spot on: you'd change "localhost" to the name or IP address of the machine you're trying to connect to, and you're in business. Ports are the same everywhere.

Note that you may encounter problems using Reader and Writer classes with sockets: http://www.fenestrated.net/~macman/mirrors/Apple%20Technotes%20%28As%20of%202002%29/tn/tn1157.html. But the code seems to be aware of how to handle newlines, so you're probably good.

[John McClellan]

Now what I'm having trouble with is the following: I'm using a wireless router, and when I call InetAddress.getByName(<some sort of internal, local ip>) on my own computer, that works just fine. But then when I try to call InetAddress.getByName(<the router's ip>), either on my own computer or on another one on the same router, that doesn't work. What's the problem?

[Paul Clapham]

In what way doesn't it work?

[John McClellan]

IOException: java.net.ConnectException: Connection refused: connect

[Rob Spoor]

Check your firewall settings.

[John McClellan]

I've tried turning it off and running the program, but I still got the same error.

[John de Michele]

John:

Did you try running it against localhost? What was the output?

John.

[John McClellan]

SingleSocketServer responded at Wed Nov 02 10:57:55 EDT 2011

The nature of this output has to do with what the program was made to do.

[Keerthivasan Ramanathan]

you are getting this exception because the socket server is not running at the specified port of the specified host. When you get 'Connection refused', it is either your socket server doesn't run or firewall/security rights issue.

[Keerthivasan Ramanathan]

I forgot to add one more. this exception will occur, when the maximum incoming requests queue of the socket server is full.