1 ByteToMessageDecoder.java类的基本结构如下：
其中ByteToMessageDecoder类是累加器的基础类，其核心方法如下：
// ByteToMessageDecoder.java
@Override
public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
if (msg instanceof ByteBuf) {
// 首先会从RECYCLE的stack中取一个对象 return RECYCLER.get();
CodecOutputList out = CodecOutputList.newInstance();
try {
ByteBuf data = (ByteBuf) msg;
first = cumulation == null;
// 判断是否第一次读，第一次读直接赋值
if (first) {
cumulation = data;
} else {
// A点： 非第一次进行字节流的累加
cumulation = cumulator.cumulate(ctx.alloc(), cumulation, data);
}
// B点：对累加的字节流进行解析，并解析到out中(B点会提供一个decode(ctx, in, out);给子类进行实现 --模板方法模式)
callDecode(ctx, cumulation, out);
} catch (DecoderException e) {
throw e;
} catch (Throwable t) {
throw new DecoderException(t);
} finally {
if (cumulation != null && !cumulation.isReadable()) {
numReads = 0;
cumulation.release();
cumulation = null;
} else if (++ numReads >= discardAfterReads) {
// We did enough reads already try to discard some bytes so we not risk to see a OOME.
// See https://github.com/netty/netty/issues/4275
numReads = 0;
discardSomeReadBytes();
}
int size = out.size();
decodeWasNull = !out.insertSinceRecycled();
// C点：将字节流byteBuf向下传播
fireChannelRead(ctx, out, size);
out.recycle();
}
} else {
ctx.fireChannelRead(msg);
}
}
首先分析上面A点：非第一次进行字节流的累加 ，跟踪源码如下：
• private Cumulator cumulator = MERGE_CUMULATOR;
• 继续跟踪源码如下：
// ByteToMessageDecoder.java
public static final Cumulator MERGE_CUMULATOR = new Cumulator() {
@Override
public ByteBuf cumulate(ByteBufAllocator alloc, ByteBuf cumulation, ByteBuf in) {
ByteBuf buffer;
// 判断当前是否还有空间可写
if (cumulation.writerIndex() > cumulation.maxCapacity() - in.readableBytes() || cumulation.refCnt() > 1) {
// 如果有就进行拓展
buffer = expandCumulation(alloc, cumulation, in.readableBytes());
} else {
buffer = cumulation;
}
// 将新来的字节流加入到buf中，然后释放
buffer.writeBytes(in);
in.release();
return buffer;
}
};
拓展的方法如下：
// 总的来说就是拿到一个新的加上旧的空间，然后将旧的写到新申请的buf中，然后释放旧的
static ByteBuf expandCumulation(ByteBufAllocator alloc, ByteBuf cumulation, int readable) {
ByteBuf oldCumulation = cumulation;
cumulation = alloc.buffer(oldCumulation.readableBytes() + readable);
cumulation.writeBytes(oldCumulation);
oldCumulation.release();
return cumulation;
}
