Client/Server & NIO

Hi.
This is my first post - welcome everybody!:)
My goal is to create a client sending messages and a server which echoes it back.

Here is Server.java code:

Code:

---

public class Server
{
    ByteBuffer buffer = ByteBuffer.allocate(1024);
   
    public Server(int port) throws IOException
    {
        startServer(port);
    }
   
    public void startServer(int port) throws IOException
    {
        Selector selector = Selector.open();
        ServerSocketChannel ssc = ServerSocketChannel.open();
        ssc.configureBlocking(false);
        ServerSocket ss = ssc.socket();
        InetSocketAddress address = new InetSocketAddress("localhost", port);
        ss.bind(address);
        ssc.register(selector, SelectionKey.OP_ACCEPT);
        for(;;)
        {           
            selector.select();
            Set selectedKeys = selector.selectedKeys();
            Iterator it = selectedKeys.iterator();
            while(it.hasNext())
            {
                System.out.println("1. Something happens...");
                SelectionKey key = (SelectionKey)it.next();
                if(key.isAcceptable())
                {
                    ServerSocketChannel ssc2 = (ServerSocketChannel)key.channel();
                    SocketChannel sc = ssc2.accept();
                    sc.configureBlocking(false);
                    SelectionKey newKey = sc.register(selector, SelectionKey.OP_READ);
                    it.remove();
                    System.out.println("2. Someone has connected...");
                    continue;
                }               
                if(key.isReadable())
                {
                    System.out.println("3. Ready for reading...");                   
                    SocketChannel sc = (SocketChannel)key.channel();
                    while(true)
                    {                       
                        buffer.clear();                                       
                        int r = sc.read(buffer);
                        if(r <= 0)
                            break;
                        buffer.flip();
                        sc.write(buffer);
                    }
                    it.remove();
                }               
            }           
        }
    }
   
    public static void main(String[] args)
    {
        try
        {
            new Server(9999);
        } catch (IOException e)
        {
            System.out.println("Server exception: " + e.getMessage());
        }
    }
}

---

And Client.java code:

Code:

---

public class Client
{
    public Client(int port) throws IOException
    {
        SocketChannel socketChannel = SocketChannel.open();
        socketChannel.configureBlocking(false);
        socketChannel.connect(new InetSocketAddress("localhost", port));
       
        try
        {
            Thread.sleep(3000);
        } catch (InterruptedException e)
        {
            e.printStackTrace();
        }
       
        socketChannel.close();           
    }

    public static void main(String[] args)
    {
        try
        {
            new Client(9999);
        } catch (IOException e)
        {
            System.out.println("Client exception: " + e.getMessage());
        }
    }
}

---

My problems are following:

1. When client closes after 3 seconds, server permanently generates such messages:
(...)
1. Something happens...
3. Ready for reading...
(...)
How can I disconnect and display (on the server's side) only one message:
"Client has disconnected" and avoid repeating those above?

2. Could I have an example of sending message from client to server and vice versa? I've been trying to do this, but some exceptions are generated...

3. How can I send whole objects through channels (serialization)?

Thanks in advance for help!;)

Welcome to Java Forums. :)

Please use code tags next time when you posting here in the forum. Unformated codes are really hard to read. If you don't know how do it, please check on my forum signature. You can find a link to the relevant page.

Quote:

---

Originally Posted by Pav

1. When client closes after 3 seconds, server permanently generates such messages:
(...)
1. Something happens...
3. Ready for reading...
(...)
How can I disconnect and display (on the server's side) only one message:
"Client has disconnected" and avoid repeating those above?

---

Think about this, once a client disconnect how can you find out that from the server side? Do you have a logic in your implementation? Seems to me that your server doesn't know that client is disconnected.

Quote:

---

Originally Posted by Eranga

Think about this, once a client disconnect how can you find out that from the server side? Do you have a logic in your implementation? Seems to me that your server doesn't know that client is disconnected.

---

I thought that there is implemented some mechanism which informs server about disconnection (socketChannel.close();)... How can I let the server know about this event then? Shall I send a special message? As I wrote above, I don't know how to write information properly into the channel...

Actually those things are depends on with the implementations.

For an example you can design something like this. Keep track all the connected clients, say in a map. Within a predefined times if one of the client not send a request to the server, means that the client is disconnected from the system. In map you've to record the last request time for each client.

If you want to you can do some implementation on the client side as well. For an example if 1000 clients are connect to the server, to check that all are connect, server has to send 1000 request to clients. Obviously that's not a good practice. So what you can do is, that connection validation part can be handle from the client side as well, up to some extend.

I've managed with disconnection problem with this simple way:

Code:

---

//Server.java snippet
if(key.isReadable())
{
    System.out.println("3. Ready for reading...");                   
    SocketChannel sc = (SocketChannel)key.channel();
    while(true)
    {                       
        buffer.clear();                                       
        int r = sc.read(buffer);
        if(r <= 0)
                {
                        sc.close(); //here is amendment
                        break;
                }
        buffer.flip();
        sc.write(buffer);
    }
    it.remove();
}     
//...

---

Moreover, I realised that my client isn't able to connect with server:

Code:

---

SocketChannel socketChannel = SocketChannel.open();
socketChannel.configureBlocking(false);
socketChannel.connect(new InetSocketAddress("localhost", port)); //port is set to 9999

while(!socketChannel.isConnected())
{
        System.out.println("Not connected");
}

---

Client falls into infinite loop... Why?

I solved this problem by deleting such a line from a client application:
socketChannel.configureBlocking(false);

Quote:

---

Originally Posted by Pav

I solved this problem by deleting such a line from a client application:
socketChannel.configureBlocking(false);

---

Glad to here that. So please mark the thread solved.

And also I hope that you really aware whats really going on there. I mean the reason to delete it.

Not really, but it's the only way to run my system I've found:)

So why don't you read the API about that. So you can find what's really happen there.

SelectableChannel (Java 2 Platform SE v1.4.2)