Thread: Socket program that worked in the past no longer working, cannot open socket.

I wrote this program back in May that makes use of networking. It's a smallish program that is comprised of 2 parts, a client and a server. The server just receives information from a single client. The client is just a form in which you input an address. If you open the client without opening the server first, it doesn't work. It just says "NO CONNECTION TO SERVER" when you try to add a record. Which is what it's supposed to do.

Anyway, for some reason, every time I open the server program, I get a BindException. I have this handled by a try-catch block, so it throws an error message of my own design "PORT ALREADY IN USE. EXITING." then it exits.

I arbitrarily chose port 10,000 to handle the transmission of data, and port 9,000 to handle task IDs (I had to use a separate socket so that the server would know what to do with the information it was given). I chose these because as far as I know they aren't reserved by any regular TCP/IP protocols. Also, this program worked fine 5 months ago. What's the deal? Here's the code for the server portion if you think you can find anything relevant in there.

 
public class P_AddressServer 
{
	//Text area to log activity.
	private JTextArea jtaServerLog = new JTextArea();
 
	//Socket for listening for objects.
	private Socket listeningSocket;
 
	//Integer input stream from client to get task ID.
	private DataInputStream getTaskID;
 
	//Object input and ouput streams from and to client.
	private ObjectInputStream inputFromClient;
	private ObjectOutputStream outputToClient;
 
	//Integer to store flags indicating what the client wants.
	private int userTask;
 
	//ID of current record.
	private int currentRecord = 0;
 
	//Linked list to store the address objects.
	LinkedList<Address> mailingList = new LinkedList<Address>();
 
	P_AddressServer()
	{
		//Make the JFrame.
		makeWindow();
 
		//Sockets for listening for addresses and the task ID numbers.
		ServerSocket serverSocket = null;
		ServerSocket serverTaskSocket = null;
 
		try 
		{
			//Open socket to client.  Listen on port 10000
			serverSocket = new ServerSocket(10000);		
			//Open socket to listen for the task ID.
			serverTaskSocket = new ServerSocket(9000);
 
			//Wait for input.
			while(true)
			{
				//Begin listening.
				listeningSocket = serverSocket.accept();
				Socket taskListeningSocket = serverTaskSocket.accept();
 
				//Input stream for receiving the task's ID.
				getTaskID = new DataInputStream(taskListeningSocket.getInputStream());
 
				//Get the task ID.
				userTask = getTaskID.readInt();
 
				//Process server task based on user's task choice.
				if(userTask == Address.ADD)
					serverSideAdd();
				else
					serverSideSingleItem(userTask);
			}
 
		} 		
		catch (BindException e)
		{
			JOptionPane.showMessageDialog(null, "PORT ALREADY IN USE.  EXITING.");
			System.exit(0);
		}
		catch (IOException e) 
		{
			// TODO Auto-generated catch block
			e.printStackTrace();
		} 
		finally
		{
			try 
			{
				serverSocket.close();
				serverTaskSocket.close();
			} 
			catch (IOException e1) 
			{
				// TODO Auto-generated catch block
				e1.printStackTrace();
			}
		}
	}
 
	//Method to construct the GUI frame.
	private void makeWindow()
	{
		//Create main frame.
		JFrame logWindow = new JFrame("Server activity log.");
 
		//Lock out user editing of server log.
		jtaServerLog.setEditable(false);
 
		//Log date and time of startup.
		jtaServerLog.append("SERVER STARTED AT: ");
		jtaServerLog.append(new Date().toString());
 
		//Add elements to JFrame.
		logWindow.add(new JScrollPane(jtaServerLog));
 
		//Set attributes of frame.
		logWindow.setVisible(true);
		logWindow.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
		logWindow.setSize(600,350);
 
	}
 
	//Method to add addresses to linked list and server log.
	private void serverSideAdd() throws IOException
	{
		//Create input and output streams.
		inputFromClient = new ObjectInputStream(listeningSocket.getInputStream());
 
		try
		{
			//Current record is last item in list.
			currentRecord = mailingList.size();
 
			Address newAddress = (Address)inputFromClient.readObject();
 
			//Identify if this is the first address, and set the flag if it is.
			if(mailingList.isEmpty())
				newAddress.isFirst = true;
 
 
			//Log address information to the server.
			addStuffToLog(newAddress.name);
			addStuffToLog(newAddress.street);
			addStuffToLog(newAddress.city);
			addStuffToLog(newAddress.state);
			addStuffToLog(newAddress.zip);
 
			//Add address from client to the linked list.
			mailingList.add(newAddress);
 
			//Move pointer of the last item.
			mailingList.get(currentRecord).isLast = true;
 
			//Next to last item is no longer the last item.  
			//Disregard if this is the first item (size = 1, currentRecord = 0).
			if(mailingList.size() > 1)
				mailingList.get(currentRecord - 1).isLast = false;
		}
		catch(ClassNotFoundException e)
		{
			JOptionPane.showMessageDialog(null, "ADDRESS CLASS NOT FOUND.  EXITING.");
			System.exit(0);
		}
		finally
		{
			//Close the input stream from the client.  Reopen when needed next time.
			inputFromClient.close();
		}
 
	}	//End of serverSideAdd method.
 
	//Method to send single address from the list to the client.
	private void serverSideSingleItem(int userTask) throws IOException
	{		
		//Open data stream to send record information to client.
		outputToClient = new ObjectOutputStream(listeningSocket.getOutputStream());
 
		switch(userTask)
		{	
		//Current record is the first record.
		case Address.FIRST:		currentRecord = 0;
								break;
		//Current record is one more than the previous.
		case Address.NEXT:		currentRecord++;
								break;
		//Current record is one less than the previous.
		case Address.PREVIOUS:	currentRecord--;
								break;
		//Current record is one less than the size of the whole list
		case Address.LAST:		currentRecord = mailingList.size() - 1;
								break;
		//Just quit if somehow an integer other than one of our 4 constants got in.
		default:				System.exit(0);
								break;
		}
		/*
		Check current record.  If it was record 0 when user entered this method,
		and user hit the Previous button, set it back to 0 by incrementing.
		If it was the last record and user hit Next, set it to the ID of the last
		by decrementing.  
		Set the respective flags to indicate if they are the first or last.
		In this way, hitting previous on the first record will show the first record
		and hitting next on the last record will show the last record.
		*/
		if(currentRecord == -1)
			currentRecord++;
		if(currentRecord == mailingList.size())
			currentRecord--;
 
		//Send current record.
		outputToClient.writeObject(mailingList.get(currentRecord));	
	}
 
	//Method to add strings to the server log.
	private void addStuffToLog(String addThis)
	{
		jtaServerLog.append("\n" +addThis);
	}
 
	//Program entry point.  Simply creates new server object.
	public static void main(String[] args)
	{
		new P_AddressServer();
	}
}

Originally Posted by aussiemcgr

What ports do XAMPP use?
Try opening your command prompt and entering: "netstat -a".
It will list your ports. Have a look if the port in question is actually being used at the time.
If it isn't being used and you are getting that exception, maybe some other condition is occurring to make that exception get thrown (perhaps a different exception "should" be thrown instead).

XAMPP I think is only using 80 and 443. But I changed around to some other arbitrary port numbers and still got the same thing. Netstat doesn't show port 10,000 as being in use, so you're probably right... the port isn't actually in use. Some other condition is causing that BindException, and my error handling code just gave the misleading notion that the port was in use. Probably because when I coded it, I thought that a port being in use was the only thing that could cause a Bind Exception.

The stack trace says "Address already in use." Could it be that the IP address (127.0.0.1) is getting hogged by XAMPP and that's what's causing the Bind Exception? But I get this same thing even when XAMPP isn't running. And the IP address shouldn't matter as long as you have an open TCP port, right? What else could cause the Bind Exception?

Hmmm...

Earlier today tried the May export and the (newer) October export on a third computer and they both worked. Just now tried both on this computer, and they both worked. Then closed them and tried again, and neither work.


...wait... I think I missed this before. Netstat shows both ports 9000 and 10000 (the ports I arbitrarily chose) as being in use use in a "LISTENING" state. I'm a little fuzzy on how this stuff works. Will a port eventually reopen and become available again if it goes long enough without activity? Maybe it worked a few minutes ago because the port freed itself up after some time. Maybe this is happening because I didn't put anything in my code to close the sockets when the program closes. Or did I? I have two statements to close the sockets, but they're in a finally clause. I don't use finally that often--do they execute whether the "try" condition or the "catch" condition is executed? Would my finally clause be closing the socket whether it was opened successfully or not?

Tell me if my thought processes are screwed up here.... I'm thinking...

1) Program tries to open sockets (serverSocket = new ServerSocket(10000) and serverTaskSocket = new ServerSocket(9000))
2) On success, program enters the infinite while loop and listens for object data on port 10000 and integers on port 9000.
3) When user closes program, while loop terminates and the code in the finally clause executes.
4) On failure, the code in the catch clause executes, which includes closing out the program via System.exit(0).... Does this termination stop the finally from executing?

Yeah so it looks like ports 9000 and 10000 are still in use. Changed it over to 9001 and 10001 and the program works. So I guess I have a resource leak? How do I get those ports to free up when the program closes?

Originally Posted by aussiemcgr

Ok, so clearly you are opening the port with your program and it is staying open. That is a good information for us to work with.
The finally statement will almost always execute, even if an exception is thrown. In fact, I think the finally statement even executes if the program is exited during the try statement through conventional means. It is very reliable. HOWEVER, System.exit(int) is not a great idea to use if you can avoid it. It might stop the finally block from executing. If the exit() method throws a SecurityException, the finally statement will execute, but otherwise it usually won't. Ideally, a program should exit "naturally" every time (meaning it runs out of procedures to execute, instead of force exiting with System.exit()).

Hmm. Is there a better way? The System.exit(0) statement is inside that catch block. If it hits that, there are no other statements to execute, so it wouldn't do anything. But if I got rid of the exit statement, the window would stay open, and I don't want that.

Do you actually exit the while loop, or are you relying on it to just close on itself when the user exits? Because there is a time delay between when a java program "exits" and when the java VM finishes executing. If you exit a program in an infinite loop, the vm may still continue to run for a while (I've noticed like 2 or 3 minutes at most, which is a long time in computer terms).

No, there's no exit condition for the loop. I just rely on the user closing to terminate it. I can't think of a condition that would suffice. And it seems inappropriate to have an exit condition. The program is essentially emulating a server, and servers stay on until you shut them off. Also, stuffing the "listen for data" part inside an infinite loop is how I was taught to do it.

Isn't there an event handler of some sort that can detect when a window is closed? I can't remember it off the top of my head, but I have used it recently. Maybe I could attach that to my window, and the action would be to close the sockets when the window is closed.

Ok. Attaching a simple WindowListener got this all working correctly. I'm not crazy about the clutter caused by the 6 other methods the interface requires me to implement, but it is what it is. Thanks for helping me suss this out guys.