使用netty框架写一个mqtt的java代码客服端

好的，下面是使用Netty框架写一个MQTT的Java客户端的代码示例：

import io.netty.bootstrap.Bootstrap;
import io.netty.buffer.ByteBuf;
import io.netty.buffer.Unpooled;
import io.netty.channel.ChannelFuture;
import io.netty.channel.ChannelFutureListener;
import io.netty.channel.ChannelHandlerContext;
import io.netty.channel.ChannelInboundHandlerAdapter;
import io.netty.channel.ChannelInitializer;
import io.netty.channel.ChannelPipeline;
import io.netty.channel.EventLoopGroup;
import io.netty.channel.nio.NioEventLoopGroup;
import io.netty.channel.socket.SocketChannel;
import io.netty.channel.socket.nio.NioSocketChannel;
import io.netty.handler.codec.mqtt.MqttConnectMessage;
import io.netty.handler.codec.mqtt.MqttConnectReturnCode;
import io.netty.handler.codec.mqtt.MqttFixedHeader;
import io.netty.handler.codec.mqtt.MqttMessage;
import io.netty.handler.codec.mqtt.MqttMessageFactory;
import io.netty.handler.codec.mqtt.MqttMessageType;
import io.netty.handler.codec.mqtt.MqttQoS;
import io.netty.handler.codec.mqtt.MqttVersion;
import io.netty.util.CharsetUtil;

public class MqttClient {
    private static final String HOST = "127.0.0.1";
    private static final int PORT = 1883;

    public static void main(String[] args) {
        EventLoopGroup group = new NioEventLoopGroup();

        try {
            Bootstrap bootstrap = new Bootstrap();
            bootstrap.group(group)
                    .channel(NioSocketChannel.class)
                    .handler(new ChannelInitializer<SocketChannel>() {
                        @Override
                        protected void initChannel(SocketChannel ch) throws Exception {
                            ChannelPipeline pipeline = ch.pipeline();
                            pipeline.addLast(new MqttEncoder());
                            pipeline.addLast(new MqttDecoder());
                            pipeline.addLast(new MqttClientHandler());
                        }
                    });

            ChannelFuture future = bootstrap.connect(HOST, PORT).sync();
            future.channel().closeFuture().sync();
        } catch (InterruptedException e) {
            e.printStackTrace();
        } finally {
            group.shutdownGracefully();
        }
    }

    private static class MqttClientHandler extends ChannelInboundHandlerAdapter {
        @Override
        public void channelActive(ChannelHandlerContext ctx) throws Exception {
            MqttFixedHeader fixedHeader = new MqttFixedHeader(
                    MqttMessageType.CONNECT,
                    false,
                    MqttQoS.AT_MOST_ONCE,
                    false,
                    0
            );
            MqttConnectMessage connectMessage = new MqttConnectMessage(
                    fixedHeader,
                    MqttVersion.MQTT_3_1_1.protocolName(),
                    MqttVersion.MQTT_3_1_1.protocolLevel(),
                    true,
                    0,
                    "client",
                    "username",
                    "password".getBytes(CharsetUtil.UTF_8)
            );
            ctx.writeAndFlush(connectMessage).addListener((ChannelFutureListener) future -> {
                if (future.isSuccess()) {
                    System.out.println("Connect success");
                } else {
                    System.out.println("Connect failed");
                }
            });
        }

        @Override
        public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
            if (msg instanceof MqttMessage) {
                MqttMessage message = (MqttMessage) msg;
                switch (message.fixedHeader().messageType()) {
                    case CONNACK:
                        MqttConnectReturnCode returnCode = ((MqttMessageFactory.MqttConnectAckVariableHeader) message.variableHeader()).connectReturnCode();
                        if (returnCode == MqttConnectReturnCode.CONNECTION_ACCEPTED) {
                            System.out.println("Connect accepted");
                        } else {
                            System.out.println("Connect refused: " + returnCode);
                        }
                        break;
                    case PUBLISH:
                        ByteBuf payload = message.payload();
                        String messageContent = payload.toString(CharsetUtil.UTF_8);
                        System.out.println("Received message: " + messageContent);
                        break;
                    default:
                        System.out.println("Unknown message type: " + message.fixedHeader().messageType());
                        break;
                }
            }
        }

        @Override
        public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception {
            cause.printStackTrace();
            ctx.close();
        }
    }
}

这个示例代码使用了Netty框架内置的MQTT编解码器，也就是`MqttEncoder`和`MqttDecoder`。在`MqttClientHandler`中，我们重写了`channelActive`方法，当连接建立成功后，会发送一个连接请求，并在`channelRead`方法中处理服务端返回的消息，包括`CONNACK`和`PUBLISH`消息。在`exceptionCaught`方法中，我们处理了异常情况，关闭连接。需要注意的是，这个示例代码中的用户名和密码是明文的，实际使用中应该使用加密方式进行传输。