Class ByteToMessageDecoder

java.lang.Object
io.netty.channel.ChannelHandlerAdapter
io.netty.channel.ChannelInboundHandlerAdapter
io.netty.handler.codec.ByteToMessageDecoder
All Implemented Interfaces:
ChannelHandler, ChannelInboundHandler
Direct Known Subclasses:
AbstractMemcacheObjectDecoder, BrotliDecoder, Bzip2Decoder, CleartextHttp2ServerUpgradeHandler, DelimiterBasedFrameDecoder, FastLzFrameDecoder, FixedLengthFrameDecoder, HAProxyMessageDecoder, Http2ConnectionHandler, HttpObjectDecoder, JsonObjectDecoder, LengthFieldBasedFrameDecoder, LineBasedFrameDecoder, Lz4FrameDecoder, LzfDecoder, OptionalSslHandler, ProtobufVarint32FrameDecoder, RedisDecoder, ReplayingDecoder, SnappyFrameDecoder, Socks5PrivateAuthRequestDecoder, Socks5PrivateAuthResponseDecoder, SocksPortUnificationServerHandler, SpdyFrameCodec, SslClientHelloHandler, SslHandler, WebSocket08FrameDecoder, XmlDecoder, XmlFrameDecoder, ZlibDecoder, ZstdDecoder
public abstract class ByteToMessageDecoder extends ChannelInboundHandlerAdapter
ChannelInboundHandlerAdapter which decodes bytes in a stream-like fashion from one ByteBuf to another Message type.

For example here is an implementation which reads all readable bytes from the input ByteBuf and create a new ByteBuf. 

 public class SquareDecoder extends ByteToMessageDecoder {
 @Override
 public void decode(ChannelHandlerContext ctx, ByteBuf in, List<Object> out)
 throws Exception {
 out.add(in.readBytes(in.readableBytes()));
 }
 }

Frame detection

Generally frame detection should be handled earlier in the pipeline by adding a DelimiterBasedFrameDecoder, FixedLengthFrameDecoder, LengthFieldBasedFrameDecoder, or LineBasedFrameDecoder.

If a custom frame decoder is required, then one needs to be careful when implementing one with ByteToMessageDecoder. Ensure there are enough bytes in the buffer for a complete frame by checking ByteBuf.readableBytes(). If there are not enough bytes for a complete frame, return without modifying the reader index to allow more bytes to arrive.

To check for complete frames without modifying the reader index, use methods like ByteBuf.getInt(int). One MUST use the reader index when using methods like ByteBuf.getInt(int). For example calling in.getInt(0) is assuming the frame starts at the beginning of the buffer, which is not always the case. Use in.getInt(in.readerIndex()) instead.

Pitfalls

Be aware that subclasses of ByteToMessageDecoder MUST NOT annotated with

invalid @link 
{@link @Sharable
}.

Some methods such as ByteBuf.readBytes(int) will cause a memory leak if the returned buffer is not released or added to the out List. Use derived buffers like ByteBuf.readSlice(int) to avoid leaking memory.