Programming Forums > Application Development > Java

Fall Back Son · Apr 19th, 2008, 5:32 PM

I'm trying to write a multi-threaded server application. The server is just going to be the same computer as the client just because its more convenient to write that way. I have a book, "Absolute Java" to go by, but I can't figure out what is wrong. The amount of code is very small so I'll just post all of it.

Client:

(Toggle Plain Text)

package stuff;
import java.net.*;
import java.io.*;

public class Client1 {

	/**
	 * @param args
	 */
	public static void main(String[] args) {
		// TODO Auto-generated method stub
		try{
			int port = 4444;
			String hostname = "localhost";
			Socket connectionSock = new Socket(hostname, port);
			BufferedReader serverInput = 
				new BufferedReader(new InputStreamReader(connectionSock.getInputStream()));
			DataOutputStream serverOutput = 
				new DataOutputStream(connectionSock.getOutputStream());
			serverOutput.writeBytes("Print this message Mr. server");
		}catch (Exception e){}
		
		
	}
}

Server:

(Toggle Plain Text)

package stuff;
import java.net.*;
import java.io.*;

public class Server extends Thread {

	/**
	 * @param args
	 */
	Socket connection;
	
	
	public static void main(String[] args) {
		// TODO Auto-generated method stub
		try{
			ServerSocket serverSock = new ServerSocket(4444);
			while(true)
			{
			Socket connectionSock = serverSock.accept();
			System.out.println("Client connection accepted.");
			Server handler = new Server(connectionSock);
			handler.start();
			}
		}catch(Exception e){}
		
		
	}
	
	public Server(Socket connectionSocket)
	{
		connection = connectionSocket;
	}
	
	public void run()
	{
		try{
		BufferedReader clientInput = 
			new BufferedReader(new InputStreamReader(connection.getInputStream()));
		DataOutputStream clientOutput = new DataOutputStream(connection.getOutputStream());
		String test = clientInput.readLine();
		System.out.println("read in " + test);
		}catch(Exception e){}
		
	}

}

Its supposed to be fairly easy to set up something like what I've attempted above, so perhaps I haven't gotten the concepts down. But I think I do. One thing I don't understand is how can we initially ask the server which port we should try to connect to (the one we initially connect to before the server spins us off to another port)? Or do we just have to know that in advance?

Also, is there something wrong with how I'm trying to make the application multi-threaded? Thats what I thought was wrong at first, but when I run the program, the run method is invoked, it just never reads in the text that the client sends.

Fall Back Son · Apr 19th, 2008, 6:07 PM

Its also throwing an exception every other time I run it, but when I change the port number it no longer throws the exception. But when I run it again, under the port number I changed it to, it once again throws an exception.

Fall Back Son · Apr 19th, 2008, 6:12 PM

If anybody who reads this has an example of an easy to understand multi-threaded Client-Server type application, I'd appreciate if you could post it. I can make an application with one thread but thats not helpful

Fall Back Son · Apr 19th, 2008, 6:44 PM

I changed my code to the following, and now it appears to be working. However, I'm sure there's probably still something wrong with it... *sighs*

(Toggle Plain Text)

package stuff;
import java.net.*;
import java.io.*;

public class Client1 {

	/**
	 * @param args
	 */
	public static void main(String[] args) {
		// TODO Auto-generated method stub
		 Socket echoSocket = null;
	     PrintWriter out = null;
	     BufferedReader in = null;
		try{
			int port = 1235;
			String hostname = "localhost";
			Socket connectionSock = new Socket(hostname, port);
			out = new PrintWriter(connectionSock.getOutputStream(), true);
	        in = new BufferedReader(new InputStreamReader(
	                                        connectionSock.getInputStream()));
			out.println("Print this message Mr. server");
			out.println("Print this other message Mr. server");
		}catch (Exception e){}
		
		
	}

}

Freaky Chris · Apr 22nd, 2008, 3:41 AM

Im new to Java myself but i that changing the way you have threaded your server may be an idea.

For example you are spawning a new tread of the entire program. Perhaps you would be better of having the main class run an infinite loop accepting connection from clients. Once it has recieved a client from socket.accept() the you can pass that to a new Threaded class that deals with all of the input and outputs to that socket.

This may help to tidy up the server and make it clearer what is going on. Also it means that multiple clients can connect at the same time if you require this.

Cheers,
Chris

Fall Back Son · May 11th, 2008, 7:24 PM

Quote:

Originally Posted by Freaky Chris

Perhaps you would be better of having the main class run an infinite loop accepting connection from clients. Once it has recieved a client from socket.accept() the you can pass that to a new Threaded class that deals with all of the input and outputs to that socket.

It is already doing that lol. Recheck the code. Main has a while loop that is always true so that it can connect to multiple clients. A new thread is spun off for each client and the run method is invoked.

Jimbo · May 12th, 2008, 8:32 PM

Quote:

Originally Posted by Fall Back Son

One thing I don't understand is how can we initially ask the server which port we should try to connect to (the one we initially connect to before the server spins us off to another port)? Or do we just have to know that in advance?

You do have to know it in advance. Some programs are smart though, and know that certain protocols use certain ports. That's why in your browser, you put http://somesite.com/somepage.html instead of being required to specify a port (e.g. http://somesite.com:80/somepage.html). Any port under 1024 is considered a "well known" port and is reserved for a specific protocol (e.g. 80 = HTTP, 443= HTTPS, 25 = SMTP, 666 = Doom (the game)).

There are programs that will check what ports are open on a machine (see: port scanning), and from that you can guess what is being hosted on those ports. If you see port 80 open, odds are that it's hosting a web server, but that's not necessarily the case, rather it's simply by convention.

Apr 19th, 2008, 5:32 PM	#1
Fall Back Son Hobbyist Programmer Join Date: Oct 2006 Posts: 188 Rep Power: 2	Confused about simple multi-threaded server I'm trying to write a multi-threaded server application. The server is just going to be the same computer as the client just because its more convenient to write that way. I have a book, "Absolute Java" to go by, but I can't figure out what is wrong. The amount of code is very small so I'll just post all of it. Client: (Toggle Plain Text) package stuff; import java.net.; import java.io.; public class Client1 { /** * @param args / public static void main(String[] args) { // TODO Auto-generated method stub try{ int port = 4444; String hostname = "localhost"; Socket connectionSock = new Socket(hostname, port); BufferedReader serverInput = new BufferedReader(new InputStreamReader(connectionSock.getInputStream())); DataOutputStream serverOutput = new DataOutputStream(connectionSock.getOutputStream()); serverOutput.writeBytes("Print this message Mr. server"); }catch (Exception e){} } } package stuff; import java.net.; import java.io.; public class Client1 { /* * @param args / public static void main(String[] args) { // TODO Auto-generated method stub try{ int port = 4444; String hostname = "localhost"; Socket connectionSock = new Socket(hostname, port); BufferedReader serverInput = new BufferedReader(new InputStreamReader(connectionSock.getInputStream())); DataOutputStream serverOutput = new DataOutputStream(connectionSock.getOutputStream()); serverOutput.writeBytes("Print this message Mr. server"); }catch (Exception e){} } } Server: (Toggle Plain Text) package stuff; import java.net.; import java.io.; public class Server extends Thread { /* * @param args / Socket connection; public static void main(String[] args) { // TODO Auto-generated method stub try{ ServerSocket serverSock = new ServerSocket(4444); while(true) { Socket connectionSock = serverSock.accept(); System.out.println("Client connection accepted."); Server handler = new Server(connectionSock); handler.start(); } }catch(Exception e){} } public Server(Socket connectionSocket) { connection = connectionSocket; } public void run() { try{ BufferedReader clientInput = new BufferedReader(new InputStreamReader(connection.getInputStream())); DataOutputStream clientOutput = new DataOutputStream(connection.getOutputStream()); String test = clientInput.readLine(); System.out.println("read in " + test); }catch(Exception e){} } } package stuff; import java.net.; import java.io.; public class Server extends Thread { /* * @param args */ Socket connection; public static void main(String[] args) { // TODO Auto-generated method stub try{ ServerSocket serverSock = new ServerSocket(4444); while(true) { Socket connectionSock = serverSock.accept(); System.out.println("Client connection accepted."); Server handler = new Server(connectionSock); handler.start(); } }catch(Exception e){} } public Server(Socket connectionSocket) { connection = connectionSocket; } public void run() { try{ BufferedReader clientInput = new BufferedReader(new InputStreamReader(connection.getInputStream())); DataOutputStream clientOutput = new DataOutputStream(connection.getOutputStream()); String test = clientInput.readLine(); System.out.println("read in " + test); }catch(Exception e){} } } Its supposed to be fairly easy to set up something like what I've attempted above, so perhaps I haven't gotten the concepts down. But I think I do. One thing I don't understand is how can we initially ask the server which port we should try to connect to (the one we initially connect to before the server spins us off to another port)? Or do we just have to know that in advance? Also, is there something wrong with how I'm trying to make the application multi-threaded? Thats what I thought was wrong at first, but when I run the program, the run method is invoked, it just never reads in the text that the client sends.

Apr 19th, 2008, 6:07 PM	#2
Fall Back Son Hobbyist Programmer Join Date: Oct 2006 Posts: 188 Rep Power: 2	Re: Confused about simple multi-threaded server Its also throwing an exception every other time I run it, but when I change the port number it no longer throws the exception. But when I run it again, under the port number I changed it to, it once again throws an exception.

Apr 19th, 2008, 6:12 PM	#3
Fall Back Son Hobbyist Programmer Join Date: Oct 2006 Posts: 188 Rep Power: 2	Re: Confused about simple multi-threaded server If anybody who reads this has an example of an easy to understand multi-threaded Client-Server type application, I'd appreciate if you could post it. I can make an application with one thread but thats not helpful

Apr 19th, 2008, 6:44 PM	#4
Fall Back Son Hobbyist Programmer Join Date: Oct 2006 Posts: 188 Rep Power: 2	Re: Confused about simple multi-threaded server I changed my code to the following, and now it appears to be working. However, I'm sure there's probably still something wrong with it... sighs (Toggle Plain Text) package stuff; import java.net.; import java.io.; public class Client1 { /** * @param args / public static void main(String[] args) { // TODO Auto-generated method stub Socket echoSocket = null; PrintWriter out = null; BufferedReader in = null; try{ int port = 1235; String hostname = "localhost"; Socket connectionSock = new Socket(hostname, port); out = new PrintWriter(connectionSock.getOutputStream(), true); in = new BufferedReader(new InputStreamReader( connectionSock.getInputStream())); out.println("Print this message Mr. server"); out.println("Print this other message Mr. server"); }catch (Exception e){} } } package stuff; import java.net.; import java.io.; public class Client1 { /* * @param args */ public static void main(String[] args) { // TODO Auto-generated method stub Socket echoSocket = null; PrintWriter out = null; BufferedReader in = null; try{ int port = 1235; String hostname = "localhost"; Socket connectionSock = new Socket(hostname, port); out = new PrintWriter(connectionSock.getOutputStream(), true); in = new BufferedReader(new InputStreamReader( connectionSock.getInputStream())); out.println("Print this message Mr. server"); out.println("Print this other message Mr. server"); }catch (Exception e){} } }

Apr 22nd, 2008, 3:41 AM	#5
Freaky Chris Hobbyist Programmer Join Date: Dec 2007 Location: England Posts: 169 Rep Power: 1	Re: Confused about simple multi-threaded server Im new to Java myself but i that changing the way you have threaded your server may be an idea. For example you are spawning a new tread of the entire program. Perhaps you would be better of having the main class run an infinite loop accepting connection from clients. Once it has recieved a client from socket.accept() the you can pass that to a new Threaded class that deals with all of the input and outputs to that socket. This may help to tidy up the server and make it clearer what is going on. Also it means that multiple clients can connect at the same time if you require this. Cheers, Chris __________________ Who said i couldn't program `sarcasm = raw_input('Type in a sarcastic remark: ') print sarcasm`

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Programming Forums > Application Development > Java

Confused about simple multi-threaded server