What is NIO?
New IO, which started in Java 1.4, provides a new non-blocking Java IO operation API.
Non-Blocking IO
Instead of the old version of Blocking IO, it is mostly used for network-related API s.

Why use NIO?
After using NIO, the performance of WEB network program can be further improved.

Simulate Tomcat 7, block IO to process Http requests:

public class BIOHttpServer {
public static void main(String[] args) throws IOException {
ServerSocket socket = new ServerSocket(8080);
System. out. println (Thread. current Thread (). getName ()+ "Start:" +8080);
while(true){
Socket accept = socket.accept();
InputStream inputStream = accept.getInputStream();
byte[] b = new byte[1024];
inputStream.read(b);
System.out.println(new String(b));

            // The http response header must write as follows:
            String response = "HTTP/1.1 200 ok\r\nContent-Length: 11\r\n\r\nHello World\r\n";
            accept.getOutputStream().write(response.getBytes());
            accept.getOutputStream().flush();
            accept.close();         
    }
}

}

NIO's core principles of high performance:
Initiate connection
Operating system receiving connection
TCP Module+Multiplexing Mechanism
A Java thread is selectively processed through the Selector tool
Delivery thread pool with data transfer
Ultimately, threads make the most of it

Using NIO:

Simulate Tomcat 8.5, NIO processes Http requests:
public class NIOHttpServer {
public static void main(String[] args) throws IOException {

    // 1. Server Socket Channel Binding Port
    ServerSocketChannel socket = ServerSocketChannel.open();
    socket.configureBlocking(false); // no-Blocking
    socket.bind(new InetSocketAddress(8080));

    System.out.println("NIO Server startup, port:"+8080);

    // 2. Getting new connections 
    // selector gets different tcp connection dynamics under different operating systems
    Selector selector = Selector.open();

    // Selector, queries TCP connections according to conditions
    socket.register(selector, SelectionKey.OP_ACCEPT);

    while(true){
        selector.select(1000); //If there is no new connection, wait

        // 3. Processing query results
        Set<SelectionKey> keys = selector.selectedKeys();
        Iterator<SelectionKey> iterator = keys.iterator();

        while(iterator.hasNext()){
            SelectionKey result = iterator.next();

            //Processing according to different types
            if(result.isAcceptable()){ //3.1 Get the new connection object
                // nio implies that accept is not blocked and returns null without connection
                SocketChannel accept = socket.accept();
                if(accept!=null){
                    // Register Connection Objects for Attention
                    accept.configureBlocking(false);// no-Blocking
                    accept.register(selector, SelectionKey.OP_READ);
                }
            }
            if(result.isReadable()){ //3.2 Connections with Data Requests
                SocketChannel channel = (SocketChannel) result.channel();
                // In the process of processing, cancel the registration of selector corresponding connection to avoid duplication
                result.cancel();

                // NIO Reading and Writing: Byte Buffer
                ByteBuffer byteBuffer = ByteBuffer.allocate(1024);
                channel.read(byteBuffer);
                byteBuffer.flip(); //Mode transformation
                byte[] b = byteBuffer.array();
                String request = new String(b);

                //Processing requests... 
                System.out.println(request);

                //Data Response: NIO Write Data
                String response = "HTTP/1.1 200 ok\r\nContent-Length: 11\r\n\r\nHello World\r\n";
                channel.write(ByteBuffer.wrap(response.getBytes()));

                // Processing completed, re-registered, continue to receive and process new connections
                // channel.register(selector, SelectionKey.OP_READ);
            }
            // Delete processed results (events)
            iterator.remove();
        }   

        // Check that the process is in place to clear the previous call effect
        selector.selectNow();
    }
}

}

(This article was originally created by the technology teacher of the source code era. Please indicate the source for reprinting!)