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Salad Lam
Salad Lam

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netty5: UDP DNS client example (1)

#netty

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(); } } }); } });
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  1. anything related to communication is executed in one of the threads of EventLoopGroup
  2. NioDatagramChannel.class states that the underlying network library is Java NIO DatagramChannel
  3. 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();
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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); }
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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); } } }
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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

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