Netty客户端抛出UnsupportedOperationException问题排查及修复

问题 #

我正在学习Netty教程，编写了服务端与客户端代码。系统中EPOLL不可用，服务端运行正常，但客户端启动时抛出java.lang.UnsupportedOperationException异常，堆栈信息指向io.netty.channel.socket.nio.NioServerSocketChannel.doConnect(NioServerSocketChannel.java:178)。我的代码与教程完全一致，使用Netty版本4.1.82.Final、Java 17（openjdk 17.0.3-oracle）。

服务端代码：

package yt.ingrim.p01;

import io.netty.bootstrap.ServerBootstrap;
import io.netty.channel.*;
import io.netty.channel.epoll.Epoll;
import io.netty.channel.epoll.EpollEventLoopGroup;
import io.netty.channel.epoll.EpollServerSocketChannel;
import io.netty.channel.nio.NioEventLoopGroup;
import io.netty.channel.socket.nio.NioServerSocketChannel;

public class Server {
    public static class MySimpleChannelInboundHandler extends SimpleChannelInboundHandler<Object> {
        @Override
        public void channelActive(ChannelHandlerContext ctx) throws Exception {
            super.channelActive(ctx);
            System.out.println("channel Connected --> " + ctx.channel());
        }

        @Override
        protected void channelRead0(ChannelHandlerContext ctx, Object msg) throws Exception {

        }
    }
    public static final boolean EPOLL = Epoll.isAvailable();
    Class channelClazz = EPOLL? EpollServerSocketChannel.class : NioServerSocketChannel.class;

    public Server() throws Exception {
        EventLoopGroup eventLoopGroup = EPOLL? new EpollEventLoopGroup(): new NioEventLoopGroup();
        try {
            new ServerBootstrap()
                    .group(eventLoopGroup)
                    .channel(channelClazz)
                    .childHandler(new ChannelInitializer<Channel>() {
                        @Override
                        protected void initChannel(Channel ch) throws Exception {
                            ChannelPipeline cpl = ch.pipeline();
                            cpl.addLast("default_channel_handler", new MySimpleChannelInboundHandler());
                        }
                    }).bind(8000).sync().channel().closeFuture().syncUninterruptibly();
        } finally {
            eventLoopGroup.shutdownGracefully().await();
        }
    }
    public static void main(String[] args) throws Exception {
        new Server();
    }
}

客户端代码：

package yt.ingrim.p01;

import io.netty.bootstrap.Bootstrap;
import io.netty.channel.Channel;
import io.netty.channel.ChannelInitializer;
import io.netty.channel.EventLoopGroup;
import io.netty.channel.epoll.Epoll;
import io.netty.channel.epoll.EpollEventLoopGroup;
import io.netty.channel.epoll.EpollServerSocketChannel;
import io.netty.channel.nio.NioEventLoopGroup;
import io.netty.channel.socket.nio.NioServerSocketChannel;

import java.net.InetSocketAddress;

public class Client {

    public static final boolean EPOLL = Epoll.isAvailable();

    Class channelClazz = EPOLL? EpollServerSocketChannel.class : NioServerSocketChannel.class;

    public Client() throws Exception {
        EventLoopGroup eventLoopGroup = EPOLL? new EpollEventLoopGroup(): new NioEventLoopGroup();
        try {
            new Bootstrap()
                    .group(eventLoopGroup)
                    .channel(channelClazz)
                    .remoteAddress(new InetSocketAddress("127.0.0.1", 8000))
                    .handler(new ChannelInitializer<Channel>() {
                        @Override
                        protected void initChannel(Channel ch) throws Exception {

                        }
                    }).connect().sync().channel().closeFuture().syncUninterruptibly();
        } finally {
            eventLoopGroup.shutdownGracefully().sync();
        }
    }
    public static void main(String []args) throws Exception {
        System.out.println(EPOLL);
        new Client();
    }
}

异常堆栈：

Exception in thread "main" java.lang.UnsupportedOperationException
at io.netty.channel.socket.nio.NioServerSocketChannel.doConnect(NioServerSocketChannel.java:178)
at io.netty.channel.nio.AbstractNioChannel$AbstractNioUnsafe.connect(AbstractNioChannel.java:248)
at io.netty.channel.DefaultChannelPipeline$HeadContext.connect(DefaultChannelPipeline.java:1342)
at io.netty.channel.AbstractChannelHandlerContext.invokeConnect(AbstractChannelHandlerContext.java:548)
at io.netty.channel.AbstractChannelHandlerContext.connect(AbstractChannelHandlerContext.java:533)
at io.netty.channel.AbstractChannelHandlerContext.connect(AbstractChannelHandlerContext.java:517)
at io.netty.channel.DefaultChannelPipeline.connect(DefaultChannelPipeline.java:978)
at io.netty.channel.AbstractChannel.connect(AbstractChannel.java:265)
at io.netty.bootstrap.Bootstrap$3.run(Bootstrap.java:250)
at io.netty.util.concurrent.AbstractEventExecutor.runTask(AbstractEventExecutor.java:174)
at io.netty.util.concurrent.AbstractEventExecutor.safeExecute(AbstractEventExecutor.java:167)
at io.netty.util.concurrent.SingleThreadEventExecutor.runAllTasks(SingleThreadEventExecutor.java:470)
at io.netty.channel.nio.NioEventLoop.run(NioEventLoop.java:569)
at io.netty.util.concurrent.SingleThreadEventExecutor$4.run(SingleThreadEventExecutor.java:997)
at io.netty.util.internal.ThreadExecutorMap$2.run(ThreadExecutorMap.java:74)
at io.netty.util.concurrent.FastThreadLocalRunnable.run(FastThreadLocalRunnable.java:30)
at java.base/java.lang.Thread.run(Thread.java:829)

问题原因 #

客户端代码错误使用了服务端专属的Channel类型：NioServerSocketChannel和EpollServerSocketChannel。这两类Channel的设计目的是监听TCP端口、接受客户端连接，不支持主动发起连接的connect操作，调用该方法时必然抛出UnsupportedOperationException。

修复方案 #

将客户端的Channel类型替换为对应的客户端Channel：

• 当EPOLL可用时，使用EpollSocketChannel
• 当EPOLL不可用时，使用NioSocketChannel

修改后的客户端代码：

package yt.ingrim.p01;

import io.netty.bootstrap.Bootstrap;
import io.netty.channel.Channel;
import io.netty.channel.ChannelInitializer;
import io.netty.channel.EventLoopGroup;
import io.netty.channel.epoll.Epoll;
import io.netty.channel.epoll.EpollEventLoopGroup;
import io.netty.channel.epoll.EpollSocketChannel;
import io.netty.channel.nio.NioEventLoopGroup;
import io.netty.channel.socket.nio.NioSocketChannel;

import java.net.InetSocketAddress;

public class Client {

    public static final boolean EPOLL = Epoll.isAvailable();

    // 修正Channel类型为客户端版本
    Class channelClazz = EPOLL ? EpollSocketChannel.class : NioSocketChannel.class;

    public Client() throws Exception {
        EventLoopGroup eventLoopGroup = EPOLL ? new EpollEventLoopGroup() : new NioEventLoopGroup();
        try {
            new Bootstrap()
                    .group(eventLoopGroup)
                    .channel(channelClazz)
                    .remoteAddress(new InetSocketAddress("127.0.0.1", 8000))
                    .handler(new ChannelInitializer<Channel>() {
                        @Override
                        protected void initChannel(Channel ch) throws Exception {

                        }
                    }).connect().sync().channel().closeFuture().syncUninterruptibly();
        } finally {
            eventLoopGroup.shutdownGracefully().sync();
        }
    }
    public static void main(String []args) throws Exception {
        System.out.println(EPOLL);
        new Client();
    }
}

内容的提问来源于stack exchange，提问作者user51