Java NIO still blocking GUI

Edit: GUI now pops up(thanks to matt ), but when I press the start button, the program completely freezes and I have to end it in jGrasp.

I have an issue with java NIO where my GUI doesn't pop-up when I run the server code.

Here is the code:

import java.io.*;
import java.nio.*;
import java.nio.channels.*;
import javax.swing.*;
import java.awt.*;
import java.util.*;
import java.awt.event.*;
public class Server extends JFrame implements ActionListener{
JButton start = null;
public Server(){
JPanel panel = new JPanel(new FlowLayout());
start = new JButton("Start");
add(panel);
panel.add(start);
start.addActionListener(this);
}
public void start(){
try{
Selector selector = Selector.open();
ServerSocketChannel serverChannel = ServerSocketChannel.open();
serverChannel.configureBlocking(false);
InetSocketAddress hostAddress = new InetSocketAddress("localhost", 0);
serverChannel.bind(hostAddress);
serverChannel.register(selector, SelectionKey.OP_ACCEPT);
while (true) {
int readyCount = selector.select();
if (readyCount == 0) {
continue;
}
// process selected keys...
Set<SelectionKey> readyKeys = selector.selectedKeys();
Iterator<SelectionKey> iterator = readyKeys.iterator();
while (iterator.hasNext()) {
SelectionKey key = iterator.next();
// Remove key from set so we don't process it twice
iterator.remove();
// operate on the channel...
// client requires a connection
if (key.isAcceptable()) {
ServerSocketChannel server = (ServerSocketChannel)  key.channel();    
// get client socket channel
SocketChannel client = server.accept();
// Non Blocking I/O
client.configureBlocking(false);
// record it for read/write operations (Here we have used it for read)
client.register(selector, SelectionKey.OP_READ);
continue;
}
}
}
}
catch(IOException ioe){}
}
public void actionPerformed(ActionEvent e) {    
if(e.getSource()==start){
start();
}
}
public static void main(String []args){
Server gui = new Server();
gui.setTitle("Server");
gui.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
gui.pack();
gui.setLocationRelativeTo(null);
gui.setResizable(false);
gui.setVisible(true);
}
} 

What am I doing wrong here?
I followed this tutorial and after simple debugging(Iterator<SelectionKey> iterator = readyKeys.iterator(); was missing the <SelectionKey> part), I compiledit, ran, and...nothing.
This is the entire code that I wrote and I don't understand what I'm doing wrong.

You need to separate your tasks. One is to start a gui, and the other one is to start a server.

Make one task the server.

public void runServer() throws Exception{
Selector selector = Selector.open();
ServerSocketChannel serverChannel = ServerSocketChannel.open();
serverChannel.configureBlocking(false);
InetSocketAddress hostAddress = new InetSocketAddress("localhost", 0);
serverChannel.bind(hostAddress);
serverChannel.register(selector, SelectionKey.OP_ACCEPT);
while (true) {
int readyCount = selector.select();
if (readyCount == 0) {
continue;
}
Set<SelectionKey> readyKeys = selector.selectedKeys();
Iterator<SelectionKey> iterator = readyKeys.iterator();
while (iterator.hasNext()) {
SelectionKey key = iterator.next();
iterator.remove();
if (key.isAcceptable()) {
ServerSocketChannel server = (ServerSocketChannel)  key.channel();    
// get client socket channel
SocketChannel client = server.accept();
// Non Blocking I/O
client.configureBlocking(false);
// record it for read/write operations (Here we have used it for read)
client.register(selector, SelectionKey.OP_READ);
continue;
}
}
}
}
}

Then make the other task the gui.

public void startGui(){
JPanel panel = new JPanel(new FlowLayout());
JButton start = new JButton("Start");
add(panel);
panel.add(start);
setTitle("Server");
setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
pack();
setLocationRelativeTo(null);
setResizable(false);
setVisible(true);
}

Now your main method can be reduced to.

public static void main(String[] args) throws Exception{
Server server = new Server();
EventQueue.invokeAndWait( server::startGui );
server.runServer();
}

This way, the gui is started on the EDT, and the server loop which never finishes takes up the main thread. Another small change. Don't extend JFrame, just create a JFrame in the startGui method. That way all gui work is done on the EDT.

Not that I have also removed your Exception handling and just made the methods throw Exception. That way you'll see a StackTrace.

In regards to your newer question, why is the gui freezing. It's because your blocking on the EDT. Your start() method never exits. The most crude method to fix this is to.

public void actionPerformed(ActionEvent e) {    
if(e.getSource()==start){
new Thread( ()->start();).start();
}

}

What that will do, is start a new thread to run your server. Notice the most obvious problem, if you click start more than once, it start more than one thread!

In this case, using non-blocking I/O is probably not necessary. Instead, the main thread could continue to perform I/O (using traditional blocking calls), while scheduling updates to the UI with SwingWorker. These updates will occur in the Swing Event Dispatch Thread, so you don't even need to worry about creating your own threads.

Non-blocking IO allows you to check whether channels are ready to perform certain operations without blocking. If no I/O operations are ready, the thread can perform other tasks and check back later.

But in your program, you've written code that effectively waits (in a "busy-loop") until the operations are ready. Because the constructor never returns, the call to setVisible() is never even invoked. You never give the thread a chance to do any other work, including painting the UI.

Further, with Java Swing, there is a special thread that performs all updates to the UI. When you create components or update what they display, that work has to be done with that thread. Swing provides the SwingWorker class to schedule short tasks that manipulate the UI. You can learn more about concurrency in Swing by completing the <i>Concurrency in Swing</i> tutorial.