编写一个Netty框架MODBUS-RTU的java代码

好的，以下是一个简单的Netty框架下的MODBUS-RTU协议的Java代码示例：

import io.netty.bootstrap.Bootstrap;
import io.netty.buffer.ByteBuf;
import io.netty.channel.ChannelFuture;
import io.netty.channel.ChannelHandlerContext;
import io.netty.channel.ChannelInitializer;
import io.netty.channel.ChannelOption;
import io.netty.channel.SimpleChannelInboundHandler;
import io.netty.channel.socket.nio.NioSocketChannel;
import io.netty.handler.codec.DelimiterBasedFrameDecoder;
import io.netty.handler.codec.MessageToByteEncoder;
import io.netty.handler.codec.string.StringDecoder;
import io.netty.handler.codec.string.StringEncoder;
import io.netty.util.CharsetUtil;

public class ModbusRtuClient {
    private static final String HOST = "127.0.0.1";
    private static final int PORT = 502;

    public static void main(String[] args) {
        Bootstrap bootstrap = new Bootstrap();
        bootstrap.group(new NioEventLoopGroup())
                .channel(NioSocketChannel.class)
                .option(ChannelOption.TCP_NODELAY, true)
                .handler(new ChannelInitializer<NioSocketChannel>() {
                    @Override
                    protected void initChannel(NioSocketChannel ch) {
                        ch.pipeline()
                                .addLast(new DelimiterBasedFrameDecoder(1024, Unpooled.copiedBuffer(new byte[]{0x0d, 0x0a})))
                                .addLast(new StringDecoder(CharsetUtil.UTF_8))
                                .addLast(new ModbusRtuEncoder())
                                .addLast(new ModbusRtuClientHandler());
                    }
                });
        try {
            ChannelFuture future = bootstrap.connect(HOST, PORT).sync();
            future.channel().closeFuture().sync();
        } catch (InterruptedException e) {
            e.printStackTrace();
        } finally {
            bootstrap.group().shutdownGracefully();
        }
    }

    private static class ModbusRtuEncoder extends MessageToByteEncoder<String> {
        @Override
        protected void encode(ChannelHandlerContext ctx, String msg, ByteBuf out) {
            byte[] data = msg.getBytes(CharsetUtil.UTF_8);
            byte[] message = new byte[data.length + 2];
            System.arraycopy(data, 0, message, 0, data.length);
            int crc = calculateCRC(message, message.length - 2);
            message[message.length - 2] = (byte) (crc & 0xff);
            message[message.length - 1] = (byte) ((crc >> 8) & 0xff);
            out.writeBytes(message);
        }

        private int calculateCRC(byte[] data, int length) {
            int crc = 0xffff;
            for (int i = 0; i < length; i++) {
                crc ^= data[i] & 0xff;
                for (int j = 0; j < 8; j++) {
                    if ((crc & 0x0001) != 0) {
                        crc >>= 1;
                        crc ^= 0xA001;
                    } else {
                        crc >>= 1;
                    }
                }
            }
            return crc;
        }
    }

    private static class ModbusRtuClientHandler extends SimpleChannelInboundHandler<String> {
        @Override
        protected void channelRead0(ChannelHandlerContext ctx, String msg) {
            System.out.println("Received response: " + msg);
        }

        @Override
        public void channelActive(ChannelHandlerContext ctx) {
            String request = "01 03 00 00 00 02 C4 0B";
            System.out.println("Send request: " + request);
            ctx.writeAndFlush(request + "\r\n");
        }

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

该代码实现了一个简单的MODBUS-RTU客户端，使用Netty框架进行网络通信，涉及到了编码器、解码器、帧分隔符、CRC校验等操作。请根据实际需求自行修改和完善。