突破netty单机最大连接数

实现单机的百万连接，瓶颈有以下几点：
1、如何模拟百万连接
2、突破局部文件句柄的限制
3、突破全局文件句柄的限制
在linux系统里面，单个进程打开的句柄数是非常有限的，一条TCP连接就对应一个文件句柄，而对于我们应用程序来说，一个服务端默认建立的连接数是有限制的。

如下图所示，通常一个客户端去除一些被占用的端口之后，可用的端口大于只有6w个左右，要想模拟百万连接要起比较多的客户端，而且比较麻烦，所以这种方案不合适。

在服务端启动800~8100，而客户端依旧使用1025-65535范围内可用的端口号，让同一个端口号，可以连接Server的不同端口。这样的话，6W的端口可以连接Server的100个端口，累加起来就能实现近600W左右的连接，TCP是以一个四元组概念，以原IP、原端口号、目的IP、目的端口号来确定的，当原IP 和原端口号相同，但目的端口号不同，最终系统会把他当成两条TCP 连接来处理，所以TCP连接可以如此设计。

测试环境

netty客户端和netty服务端,都是springboot项目。
运行环境：linux
netty版本：4.1.6.Final

netty服务端代码

netty maven
 <properties>
 <netty-all.version>4.1.6.Final</netty-all.version>
 </properties>
 <dependency>
 <groupId>io.netty</groupId>
 <artifactId>netty-all</artifactId>
 <version>${netty-all.version}</version>
 </dependency>

@SpringBootApplication
public class NettyserverApplication {
    private static final int BEGIN_PORT = 8000;
    private static final int N_PORT = 100;
    public static void main(String[] args) {
        SpringApplication.run(NettyserverApplication.class, args);
        new Server().start(BEGIN_PORT, N_PORT);
    }
}
/----------------------------------------------------------------------------------
import io.netty.bootstrap.ServerBootstrap;
import io.netty.channel.ChannelFutureListener;
import io.netty.channel.ChannelOption;
import io.netty.channel.EventLoopGroup;
import io.netty.channel.nio.NioEventLoopGroup;
import io.netty.channel.socket.nio.NioServerSocketChannel;
public final class Server {
    public void start(int beginPort, int nPort) {
        System.out.println("server starting....");

        EventLoopGroup bossGroup = new NioEventLoopGroup();
        EventLoopGroup workerGroup = new NioEventLoopGroup();

        ServerBootstrap bootstrap = new ServerBootstrap();
        bootstrap.group(bossGroup, workerGroup);
        bootstrap.channel(NioServerSocketChannel.class);
        bootstrap.childOption(ChannelOption.SO_REUSEADDR, true);

        bootstrap.childHandler(new ConnectionCountHandler());

        /**
         *  绑定100个端口号
         */
        for (int i = 0; i < nPort; i++) {
            int port = beginPort + i;
            bootstrap.bind(port).addListener((ChannelFutureListener) future -> {
                System.out.println("bind success in port: " + port);
            });
        }
        System.out.println("server started!");
    }
}
/-------------------------------------------------------------------------------------------------
import io.netty.channel.Channel;
import io.netty.channel.ChannelHandler.Sharable;
import io.netty.channel.ChannelHandlerContext;
import io.netty.channel.ChannelInboundHandlerAdapter;

import java.util.concurrent.Executors;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicInteger;

@Sharable
public class ConnectionCountHandler extends ChannelInboundHandlerAdapter {
  //jdk1.5 并发包中的用于计数的类
    private AtomicInteger nConnection = new AtomicInteger();

    public ConnectionCountHandler() {
           /**
         *  每两秒统计一下连接数
         */
Executors.newSingleThreadScheduledExecutor().scheduleAtFixedRate(() -> {
            System.out.println("connections: " + nConnection.get());
        }, 0, 2, TimeUnit.SECONDS);

    }
   /**
     *  每次过来一个新连接就对连接数加一
     * @param ctx
     */
    @Override
    public void channelActive(ChannelHandlerContext ctx) {
        nConnection.incrementAndGet();
    }
   /**
     *  端口的时候减一
     * @param ctx
     */
    @Override
    public void channelInactive(ChannelHandlerContext ctx) {
        nConnection.decrementAndGet();
    }

    @Override
    public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception {
        super.exceptionCaught(ctx, cause);
        Channel channel = ctx.channel();
        if(channel.isActive()){
            ctx.close();
        }
        //……
    }
}

netty客户端代码

netty maven
 <properties>
 <netty-all.version>4.1.6.Final</netty-all.version>
 </properties>
 <dependency>
 <groupId>io.netty</groupId>
 <artifactId>netty-all</artifactId>
 <version>${netty-all.version}</version>
 </dependency>

@SpringBootApplication
public class NettyclientApplication {
    private static final int BEGIN_PORT = 8000;
    private static final int N_PORT = 100;
    public static void main(String[] args) {
        SpringApplication.run(NettyclientApplication.class, args);
        new Client().start(BEGIN_PORT, N_PORT);
    }
}
//----------------------------------------------------------------------------------------
package com.nettyclient.test;

import io.netty.bootstrap.Bootstrap;
import io.netty.channel.*;
import io.netty.channel.nio.NioEventLoopGroup;
import io.netty.channel.socket.SocketChannel;
import io.netty.channel.socket.nio.NioSocketChannel;

public class Client {

    private static final String SERVER_HOST = "127.0.0.1";

    public void start(final int beginPort, int nPort) {
        System.out.println("client starting....");
        EventLoopGroup eventLoopGroup = new NioEventLoopGroup();
        final Bootstrap bootstrap = new Bootstrap();
        bootstrap.group(eventLoopGroup);
        bootstrap.channel(NioSocketChannel.class);
        bootstrap.option(ChannelOption.SO_REUSEADDR, true);
        bootstrap.handler(new ChannelInitializer<SocketChannel>() {
            @Override
            protected void initChannel(SocketChannel ch) {
            }
        });
        int index = 0;
        int port;
        while (!Thread.interrupted()) {
            port = beginPort + index;
            try {
                ChannelFuture channelFuture = bootstrap.connect(SERVER_HOST, port);
                channelFuture.addListener((ChannelFutureListener) future -> {
                    if (!future.isSuccess()) {
                        System.out.println("连接失败, 退出!");
                        System.exit(0);
                    }
                });
                channelFuture.get();
            } catch (Exception e) {
            }
            if (++index == nPort) {
                index = 0;
            }
        }
    }
}