io.netty5.example.dns.udp.DnsClient is DNS client to lookup A record of domain "www.example.com" by UDP.

Setup io.netty5.channel.Channel instance

Please read the following code.

EventLoopGroup group = new MultithreadEventLoopGroup(NioHandler.newFactory())
Bootstrap b = new Bootstrap();
b.group(group)
 .channel(NioDatagramChannel.class)
 .handler(new ChannelInitializer<DatagramChannel>() {
     @Override
     protected void initChannel(DatagramChannel ch) {
         ChannelPipeline p = ch.pipeline();
         p.addLast(new DatagramDnsQueryEncoder())
         .addLast(new DatagramDnsResponseDecoder())
         .addLast(new SimpleChannelInboundHandler<DatagramDnsResponse>() {
            @Override
            protected void messageReceived(ChannelHandlerContext ctx, DatagramDnsResponse msg) {
                try {
                    handleQueryResp(msg);
                } finally {
                    ctx.close();
                }
            }
        });
     }
 });

anything related to communication is executed in one of the threads of EventLoopGroup
NioDatagramChannel.class states that the underlying network library is Java NIO DatagramChannel
handler() accepts a subclass of io.netty5.channel.ChannelInitializer which implements the function initChannel(DatagramChannel). It is to setup list of ChannelHandler in ChannelPipeline. I called this class setup channel class

Channel instances will be built and obtained in the following.

final Channel ch = b.bind(0).asStage().get();

When bind(0) is called, io.netty5.bootstrap.Bootstrap#init is triggered.

@Override
Future<Channel> init(Channel channel) {
    ChannelPipeline p = channel.pipeline();

    setChannelOptions(channel, newOptionsArray(), logger);
    setAttributes(channel, newAttributesArray());

    p.addLast(config.handler());

    return channel.executor().newSucceededFuture(channel);
}

Value of config.handler() is setup channel class. And then io.netty5.channel.ChannelInitializer#handlerAdded is triggered.

@Override
public void handlerAdded(ChannelHandlerContext ctx) throws Exception {
    try {
        initChannel((C) ctx.channel());
    } catch (Throwable cause) {
        // Explicitly call exceptionCaught(...) as we removed the handler before calling initChannel(...).
        // We do so to prevent multiple calls to initChannel(...).
        channelExceptionCaught(ctx, cause);
    } finally {
        if (!ctx.isRemoved()) {
            ctx.pipeline().remove(this);
        }
    }
}

Setup channel class will be removed after ChannelPipeline is setup. Then the following will be in ChannelPipeline

DefaultChannelPipeline$HeadHandler
DatagramDnsQueryEncoder
DatagramDnsResponseDecoder
subclass of SimpleChannelInboundHandler
DefaultChannelPipeline$TailHandler

netty5: UDP DNS client example (1)

Setup io.netty5.channel.Channel instance