URGENT:Handle multiple connection in a multithreaded server.

Question

I hava a multithreaded server where it can handle multiple connection.
The program works in this way. First, it will wait for connection. Once it accepts a connection, it will pass the socket to a thread class which will in turn process the connection. Then, the server will go back to the infinite loop to wait for new connection.
Since I am not very familiar on using thread, I would like to post the following questions:
1) Let say that my server can handle multiple connections. This is to cater for the different requests from the client. For instance, the client may make one connection for chatting and another connection for sending file(something like we have in Yahoo Messanger). So, at the server side, how can I differentiate between the connection for CHAT and the connection for SEND FILE so that I can handle the requests separately. It means I can save the file sent from client and display the chat message in the JTextArea?
2) How can I stop the server, clean up the thread and start the server again for waiting new connection? Currently, I can just start my server once.
For reference,following are parts of my program:
public void listenSocket(){
try{
server = new ServerSocket(5000);
} catch(IOException e) {
System.out.println("Failed to listen on port 5000");
System.exit(0);
try{
while(true){
server1frame sframe=null;
ChatHandler handler;
handler = new ChatHandler(sframe,server.accept(),jTextArea1,jTextField1,jFileChooser1);
handler.start();
} catch(IOException e) {
System.out.println("Accept failed:5000");
System.exit(0);
//to close the connection as the final steps before program exit.
} finally {
try{
server.close();
} catch (IOException ioe) {
ioe.printStackTrace();
public void run() {
out.println("Connection Established");
out.flush();
try {
String line;
     while(!(line =in.readLine()).equalsIgnoreCase("QUIT")) {
if(line.startsWith("CLIENT"))
jTextArea1.append("\n"+line);
else if(line.startsWith("Connecting")) {
jTextArea1.append("\n"+line);
else if (line.startsWith("Finish")) {
JOptionPane.showMessageDialog(sframe,"File has finished downloading",
"File Download Complete",JOptionPane.INFORMATION_MESSAGE);
else {
BufferedWriter fout = new BufferedWriter(
new FileWriter("e:/num1.txt",true));
fout.write(line);
fout.newLine();
fout.flush(); }
     } catch(IOException ioe) {
     ioe.printStackTrace();
     } finally {
     try {
          in.close();
          socket.close();
     } catch(IOException ioe){
System.out.println("Fail to close connection.");

Answer 1

1) Let say that my server can handle multiple
connections. This is to cater for the different
requests from the client. For instance, the client may
make one connection for chatting and another
connection for sending file(something like we have in
Yahoo Messanger). So, at the server side, how can I
differentiate between the connection for CHAT and the
connection for SEND FILE so that I can handle the
requests separately. It means I can save the file sent
from client and display the chat message in the
JTextArea?Either your server listens on two different ports for two different protocolls, (then you would a server thread for each port) or you accept connections for both protocolls on both ports, and have the client state what protocoll it is using :
eg:
accept Connection - start new Thread - read first line - depending on protocoll pass protocoll object (CHAT, FTP) to run method
There is an example for a simple protocoll at : http://java.sun.com/docs/books/tutorial/networking/sockets/clientServer.html
2) How can I stop the server, clean up the thread and
start the server again for waiting new connection?
Currently, I can just start my server once.You normally start servers only once. It should then be up and listening for it's entire lifetime. Restarting a server should only be needed if it hangs or crashes.