LengthFieldBasedFrameDecoder 继承自 ByteToMessageDecoder
public abstract class ByteToMessageDecoder extends ChannelInboundHandlerAdapter
ByteToMessageDecoder 本身是一个 ChannelInboundHandler
ByteToMessageDecoder 中有 2 种数据积累器，一种拷贝式，一种组合式，默认使用拷贝式，组合式更省内存，更复杂，会慢点。
// io.netty.handler.codec.ByteToMessageDecoder#cumulator
private Cumulator cumulator = MERGE_CUMULATOR;
public static final Cumulator MERGE_CUMULATOR = new Cumulator() {
@Override
public ByteBuf cumulate(ByteBufAllocator alloc, ByteBuf cumulation, ByteBuf in) {
try {
final int required = in.readableBytes();
// 需要扩容
if (required > cumulation.maxWritableBytes() ||
(required > cumulation.maxFastWritableBytes() && cumulation.refCnt() > 1) ||
cumulation.isReadOnly()) {
// Expand cumulation (by replacing it) under the following conditions:
// - cumulation cannot be resized to accommodate the additional data
// - cumulation can be expanded with a reallocation operation to accommodate but the buffer is
// assumed to be shared (e.g. refCnt() > 1) and the reallocation may not be safe.
return expandCumulation(alloc, cumulation, in);
}
// 把新读到的数据写入积累器
return cumulation.writeBytes(in);
} finally {
// We must release in in all cases as otherwise it may produce a leak if writeBytes(...) throw
// for whatever release (for example because of OutOfMemoryError)
in.release();
}
}
};
入口在
// io.netty.handler.codec.ByteToMessageDecoder#channelRead
public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
if (msg instanceof ByteBuf) {
// 从对象池获取 CodecOutputList
CodecOutputList out = CodecOutputList.newInstance();
try {
// 从 socket 读到的数据
ByteBuf data = (ByteBuf) msg;
// 是否第一次接收数据
first = cumulation == null;
if (first) {
cumulation = data;
} else {
// 数据积累器是拷贝式
// 如果上一次由于拆包，没有解析出消息，则数据会存留在积累器中
// 积累器接收新的数据
cumulation = cumulator.cumulate(ctx.alloc(), cumulation, data);
}
// 对积累器的数据进行解码
callDecode(ctx, cumulation, out);
} catch (DecoderException e) {
throw e;
} catch (Exception e) {
throw new DecoderException(e);
} 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();
firedChannelRead |= out.insertSinceRecycled();
fireChannelRead(ctx, out, size);
out.recycle();
}
} else {
ctx.fireChannelRead(msg);
}
}
解码
// io.netty.handler.codec.ByteToMessageDecoder#callDecode
protected void callDecode(ChannelHandlerContext ctx, ByteBuf in, List<Object> out) {
try {
while (in.isReadable()) {
// 已解码的消息个数
int outSize = out.size();
if (outSize > 0) {
fireChannelRead(ctx, out, outSize);
out.clear();
// Check if this handler was removed before continuing with decoding.
// If it was removed, it is not safe to continue to operate on the buffer.
//
// See:
// - https://github.com/netty/netty/issues/4635
if (ctx.isRemoved()) {
break;
}
outSize = 0;
}
int oldInputLength = in.readableBytes();
// 调用子类的 decode 方法
decodeRemovalReentryProtection(ctx, in, out);
// Check if this handler was removed before continuing the loop.
// If it was removed, it is not safe to continue to operate on the buffer.
//
// See https://github.com/netty/netty/issues/1664
if (ctx.isRemoved()) {
break;
}
// 前后的消息个数相等，表示没有解码新的消息
if (outSize == out.size()) {
if (oldInputLength == in.readableBytes()) {
// 拆包情况，走这个分支，跳出循环，读取更多的数据
break;
} else {
continue;
}
}
if (oldInputLength == in.readableBytes()) {
throw new DecoderException(
StringUtil.simpleClassName(getClass()) +
".decode() did not read anything but decoded a message.");
}
if (isSingleDecode()) {
break;
}
}
} catch (DecoderException e) {
throw e;
} catch (Exception cause) {
throw new DecoderException(cause);
}
}
// io.netty.handler.codec.LengthFieldBasedFrameDecoder#decode
protected final void decode(ChannelHandlerContext ctx, ByteBuf in, List<Object> out) throws Exception {
Object decoded = decode(ctx, in);
if (decoded != null) {
out.add(decoded);
}
}
protected Object decode(ChannelHandlerContext ctx, ByteBuf in) throws Exception {
if (discardingTooLongFrame) {
discardingTooLongFrame(in);
}
if (in.readableBytes() < lengthFieldEndOffset) {
return null;
}
int actualLengthFieldOffset = in.readerIndex() + lengthFieldOffset;
// 读取报文的长度字段，解析出报文长度
// ByteBuf.getUnsignedByte 并不会移动 readerIndex
long frameLength = getUnadjustedFrameLength(in, actualLengthFieldOffset, lengthFieldLength, byteOrder);
if (frameLength < 0) {
failOnNegativeLengthField(in, frameLength, lengthFieldEndOffset);
}
frameLength += lengthAdjustment + lengthFieldEndOffset;
if (frameLength < lengthFieldEndOffset) {
failOnFrameLengthLessThanLengthFieldEndOffset(in, frameLength, lengthFieldEndOffset);
}
if (frameLength > maxFrameLength) {
exceededFrameLength(in, frameLength);
return null;
}
// never overflows because it's less than maxFrameLength
int frameLengthInt = (int) frameLength;
// 拆包情况，读到的数据小于报文的长度，无法解析，需要继续接收数据，进行第二次解析
if (in.readableBytes() < frameLengthInt) {
return null;
}
if (initialBytesToStrip > frameLengthInt) {
failOnFrameLengthLessThanInitialBytesToStrip(in, frameLength, initialBytesToStrip);
}
in.skipBytes(initialBytesToStrip);
// extract frame
int readerIndex = in.readerIndex();
int actualFrameLength = frameLengthInt - initialBytesToStrip;
ByteBuf frame = extractFrame(ctx, in, readerIndex, actualFrameLength);
in.readerIndex(readerIndex + actualFrameLength);
return frame;
}
处理拆包的流程：
1. 从 channel 读取的数据会先放到数据积累器中
2. 使用解码器对数据进行解码，拆包情况则是接收的数据小于报文的实际长度，因此解码失败
3. 重新从 channel 读取消息，放入积累器中
4. 再次解码消息，若成功则传播到 pipeline 中，失败则继续接收消息