[JAVA] I tried to implement a server using Netty

at first

Last time, I tried to create a TCP / IP + NIO / BIO transfer path using libraries such as webSocket and SocketChannel, but I found that the code became quite complicated. This time I will try to realize a server again using Netty.

What is Netty

Simply put, Netty is a framework for developing applications that perform asynchronous communication. Compared to NIO processing realized by SocketChannel, it is no longer necessary to directly operate low-layer APIs (select (), read (), etc.), and it is hidden on the Netty side.


NIO implementation on ServerSocket Channel

First of all, I will repost the previous Implementing NIO with ServerSocket Channel.

➀ Create a ServerSocketChannel and listen for client-side access

public class ChannelServer {

    public static void main(String[] args) throws IOException {
        // I/Prepare a thread pool to process O requests
        ThreadPoolExecutor executor = new ThreadPoolExecutor(3, 10, 1000, TimeUnit.MILLISECONDS, 
                new ArrayBlockingQueue<Runnable>(100));

        ServerSocketChannel serverSocketChannel = ServerSocketChannel.open();
        serverSocketChannel.bind(new InetSocketAddress(1234));

        while (true) {
            SocketChannel socketChannel = serverSocketChannel.accept();

            if (socketChannel != null) {
                //Commit the request to the thread pool
                executor.submit(new ChannelServerThread(socketChannel));

② Thread that processes the request

public class ChannelServerThread implements Runnable {

    private SocketChannel socketChannel;
    private String remoteName;

    public ChannelServerThread(SocketChannel socketChannel) throws IOException {
        this.socketChannel = socketChannel;
        this.remoteName = socketChannel.getRemoteAddress().toString();
        System.out.println("client:" + remoteName + " access successfully!");

    // I/Handle O request
    public void run() {
        ByteBuffer buffer = ByteBuffer.allocate(1024);
        ByteBuffer sizeBuffer = ByteBuffer.allocate(4);
        StringBuilder sb = new StringBuilder();
        byte b[];
        //Read data and length from socketChannel and output to standard output
        while(true) {
            try {
                int read = socketChannel.read(sizeBuffer);
                if (read != -1) {
                    int size = sizeBuffer.getInt();
                    int readCount = 0;
                    b = new byte[1024];
                    while (readCount < size) {
                        read = socketChannel.read(buffer);
                        if (read != -1) {
                            readCount += read;
                            int index = 0 ;
                            while(buffer.hasRemaining()) {
                                b[index++] = buffer.get();
                                if (index >= b.length) {
                                    index = 0;
                                    sb.append(new String(b,"UTF-8"));
                            if (index > 0) {
                                sb.append(new String(b,"UTF-8"));
                    System.out.println(remoteName +  ":" + sb.toString());
            } catch (Exception e) {
                System.out.println(remoteName + "access colsed");
                try {
                } catch (IOException ex) {

NIO implementation with Netty

➀ Process that waits for access on the client side

public class EchoServer {
    private final int port;

    public EchoServer(int port) {
        this.port = port;

    public void start() throws Exception {
        EventLoopGroup group = new NioEventLoopGroup();
        try {
            ServerBootstrap sb = new ServerBootstrap();
            //Thread pool binding
                     //Port number binding
                    .childHandler(new ChannelInitializer<SocketChannel>() {

                        protected void initChannel(SocketChannel ch) throws Exception {
                            System.out.println("connected...; Client:" + ch.remoteAddress());
                            ch.pipeline().addLast(new EchoServerHandler());
            //Asynchronous binding of server
            ChannelFuture cf = sb.bind().sync();
            System.out.println(EchoServer.class + " started and listen on " + cf.channel().localAddress());
            //Close server channel
        } finally {
            //Unbind thread pool

    public static void main(String[] args) throws Exception {
        new EchoServer(66666).start();

② The one who processes the request

public class EchoServerHandler extends ChannelInboundHandlerAdapter {
    public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
        System.out.println("server channelRead...; received:" + msg);

    public void channelReadComplete(ChannelHandlerContext ctx) throws Exception {
        System.out.println("server channelReadComplete..");
        //WriteAndFlush in empty buffer,Close the connection


    public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception {
        System.out.println("server occur exception:" + cause.getMessage());

change point

** The amount of code has been greatly reduced. ** ** This is to hide APIs such as read () and Buffer operations from the Netty side and provide a convenient high-layer API. For example, channelRead () method of EchoServerHandler class: When data from the client side is accepted, it is converted to String internally, so this method is called.


This time I tried to implement the server using Netty. I will introduce the cooperation with the client side next time.

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