import java.net.*;
import java.nio.*;
import java.nio.channels.*;
import java.nio.charset.*;

public class Server {

    /** sempre da assisini */
    public static void main(String[] args) throws Throwable {
        ServerSocket server = new ServerSocket(9999);
        Socket client = server.accept();
        ReadableByteChannel channel =
Channels.newChannel(client.getInputStream());
        ByteBuffer len = ByteBuffer.allocate(4); //4 byte = 1 int
        channel.read(len);
        len.flip();
               ByteBuffer message =
ByteBuffer.allocateDirect(len.asIntBuffer().
        while(message.hasRemaining()) {
            channel.read(message);
        }
        message.flip();
        Charset ch = Charset.forName("ISO-8859-1");
        channel.close();
        server.close();
    }
}

package client;

import java.io.BufferedOutputStream;
import java.io.ByteArrayOutputStream;
import java.io.DataOutputStream;
import java.io.FileInputStream;
import java.io.OutputStreamWriter;
import java.net.*;
import java.nio.*;
import java.nio.channels.*;
import java.nio.charset.*;

public class Client {
    public static void main(String[] args) throws Throwable {
        Socket socket = new Socket("localhost", 9999);
        DataOutputStream os = new
DataOutputStream(socket.getOutputStream());
        String messaggio = "ciao";
        os.writeBytes(messaggio);
        os.flush();
        os.close();
        socket.close();
    }
}

Exception:
Exception in thread "main" java.nio.BufferUnderflowException
    at java.nio.Buffer.nextGetIndex(Unknown Source)
    at java.nio.ByteBufferAsIntBufferB.get(Unknown Source)
    at server.Server.main(Server.java:26)

I look the examples at the java guide but there isn't an example that
explain like a client io can send a msg to server NIO.

Because this is an example for understanding the comunication. The my
real problem is that i've a framework implemented with NIO and i must
implementing a client J2ME that send msg to it.