ChannelHandlerContext

ChannelHandlerContext代表了一个ChannelHandler和ChannelPipeline之间的关系，ChannelHandlerContext创建于ChannelHandler被载入到ChannelPipeline的时候，ChannelHandlerContext主要功能是管理在同一ChannelPipeline中各个ChannelHandler的交互
ChannelHandlerContext，Channel，ChannelPipeline提供的一些方法，下图时三者者之间的关系

ByteToMessageDecoder

一个抽象类需要我们自己重写decode方法

  1. class ByteToIntegerDecoder extends ByteToMessageDecoder{
  2.         @Override
  3.         protected void decode(ChannelHandlerContext ctx, ByteBuf in, List<Object> out) throws Exception {
  4.             while(in.readableBytes() > 4){
  5.                 out.add(in.readInt());
  6.             }
  7.         }
  8.     }

ctx是decode属于的ChannelHandlerContext,in是用来读取的数据，out是我们转换后的对象列表

channelRead

  1. public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
  2.         if (msg instanceof ByteBuf) {
  3.             RecyclableArrayList out = RecyclableArrayList.newInstance();
  4.             try {
  5.                 ByteBuf data = (ByteBuf) msg;
  6.                 first = cumulation == null;
  7.                 if (first) {
  8.                     cumulation = data;
  9.                 } else {
  10.                     cumulation = cumulator.cumulate(ctx.alloc(), cumulation, data);
  11.                 }
  12.                 callDecode(ctx, cumulation, out);
  13.             } catch (DecoderException e) {
  14.                 throw e;
  15.             } catch (Throwable t) {
  16.                 throw new DecoderException(t);
  17.             } finally {
  18.                 if (cumulation != null && !cumulation.isReadable()) {
  19.                     cumulation.release();
  20.                     cumulation = null;
  21.                 }
  22.                 int size = out.size();
  24.                 for (int i = 0; i < size; i ++) {
  25.                     ctx.fireChannelRead(out.get(i));
  26.                 }
  27.                 out.recycle();
  28.             }
  29.         } else {
  30.             ctx.fireChannelRead(msg);
  31.         }
  32.     }

cumulation是代表累加的byte数据，即上一次decode剩下的byte,cumulator是累加器，默认使用MERGE_CUMULATOR就是使用内存复制来进行累加。累加完之后调用callDecode(ctx, cumulation, out),callDecode中循环调用我们要实现的抽象方法decode(ctx,in,out)来解码，知道不能继续解。在finally中对out列表中的每一个对象调用ctx.fireChannelRead(out.get(i))
触发ChannelPipeline后面的ChannelHandler的channelRead事件