固定长度解码器
For example, if you received the following four fragmented packets:
<pre>
+---+----+------+----+
| A | BC | DEFG | HI |
+---+----+------+----+
</pre>
A FixedLengthFrameDecoderwill decode them into the following three packets with the fixed length:
<pre>
+-----+-----+-----+
| ABC | DEF | GHI |
+-----+-----+-----+
解码
protected final void decode(ChannelHandlerContext ctx, ByteBuf in, List<Object> out) throws Exception {
// 解码
Object decoded = decode(ctx, in);
// 如果解码成功,那么将返回结果添加到out中
if (decoded != null) {
out.add(decoded);
}
}
protected Object decode(
@SuppressWarnings("UnusedParameters") ChannelHandlerContext ctx, ByteBuf in) throws Exception {
// 如果没有到达规定的定长,那么返回null
// 在ByteToMessageDecode那边会认为没有读取任何数据,直接结束循环,等待下次读取事件的到来
if (in.readableBytes() < frameLength) {
return null;
} else {
// 可见数据满足要求, 截取定长的数据返回.
return in.readRetainedSlice(frameLength);
}
}
readRetainedSlice
// 这里实际上是生成源Buffer的只读缓冲区,且retain
public ByteBuf readRetainedSlice(int length) {
ByteBuf slice = retainedSlice(readerIndex, length);
readerIndex += length;
return slice;
}
