Package io.netty.handler.codec

Class ByteToMessageDecoder

java.lang.Object

io.netty.channel.ChannelHandlerAdapter

io.netty.channel.ChannelInboundHandlerAdapter

io.netty.handler.codec.ByteToMessageDecoder

All Implemented Interfaces:

io.netty.channel.ChannelHandler, io.netty.channel.ChannelInboundHandler

Direct Known Subclasses:

BrotliDecoder, Bzip2Decoder, DelimiterBasedFrameDecoder, FastLzFrameDecoder, FixedLengthFrameDecoder, JsonObjectDecoder, LengthFieldBasedFrameDecoder, LineBasedFrameDecoder, Lz4FrameDecoder, LzfDecoder, ProtobufVarint32FrameDecoder, ReplayingDecoder, SnappyFrameDecoder, XmlFrameDecoder, ZlibDecoder, ZstdDecoder

public abstract class ByteToMessageDecoder
extends io.netty.channel.ChannelInboundHandlerAdapter

ChannelInboundHandlerAdapter which decodes bytes in a stream-like fashion from one ByteBuf to an other 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 sub-classes of ByteToMessageDecoder MUST NOT annotated with {@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.

Nested Class Summary

Nested Classes

Modifier and Type	Class	Description
static interface	ByteToMessageDecoder.Cumulator	Cumulate ByteBufs.

Nested classes/interfaces inherited from interface io.netty.channel.ChannelHandler

io.netty.channel.ChannelHandler.Sharable

Field Summary

Fields

Modifier and Type	Field	Description
static ByteToMessageDecoder.Cumulator	COMPOSITE_CUMULATOR	Cumulate ByteBufs by add them to a CompositeByteBuf and so do no memory copy whenever possible.
static ByteToMessageDecoder.Cumulator	MERGE_CUMULATOR	Cumulate ByteBufs by merge them into one ByteBuf's, using memory copies.

Constructor Summary

Constructors

Modifier	Constructor	Description
protected	ByteToMessageDecoder()

Method Summary

Modifier and Type	Method	Description
protected int	actualReadableBytes()	Returns the actual number of readable bytes in the internal cumulative buffer of this decoder.
protected void	callDecode(io.netty.channel.ChannelHandlerContext ctx, io.netty.buffer.ByteBuf in, List<Object> out)	Called once data should be decoded from the given ByteBuf.
void	channelInactive(io.netty.channel.ChannelHandlerContext ctx)
void	channelRead(io.netty.channel.ChannelHandlerContext ctx, Object msg)
void	channelReadComplete(io.netty.channel.ChannelHandlerContext ctx)
protected abstract void	decode(io.netty.channel.ChannelHandlerContext ctx, io.netty.buffer.ByteBuf in, List<Object> out)	Decode the from one ByteBuf to an other.
protected void	decodeLast(io.netty.channel.ChannelHandlerContext ctx, io.netty.buffer.ByteBuf in, List<Object> out)	Is called one last time when the ChannelHandlerContext goes in-active.
protected void	discardSomeReadBytes()
void	handlerRemoved(io.netty.channel.ChannelHandlerContext ctx)
protected void	handlerRemoved0(io.netty.channel.ChannelHandlerContext ctx)	Gets called after the ByteToMessageDecoder was removed from the actual context and it doesn't handle events anymore.
protected io.netty.buffer.ByteBuf	internalBuffer()	Returns the internal cumulative buffer of this decoder.
boolean	isSingleDecode()	If true then only one message is decoded on each channelRead(ChannelHandlerContext, Object) call.
void	setCumulator(ByteToMessageDecoder.Cumulator cumulator)	Set the ByteToMessageDecoder.Cumulator to use for cumulate the received ByteBufs.
void	setDiscardAfterReads(int discardAfterReads)	Set the number of reads after which ByteBuf.discardSomeReadBytes() are called and so free up memory.
void	setSingleDecode(boolean singleDecode)	If set then only one message is decoded on each channelRead(ChannelHandlerContext, Object) call.
void	userEventTriggered(io.netty.channel.ChannelHandlerContext ctx, Object evt)

Methods inherited from class io.netty.channel.ChannelInboundHandlerAdapter

channelActive, channelRegistered, channelUnregistered, channelWritabilityChanged, exceptionCaught

Methods inherited from class io.netty.channel.ChannelHandlerAdapter

ensureNotSharable, handlerAdded, isSharable

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Methods inherited from interface io.netty.channel.ChannelHandler

handlerAdded

Field Detail

MERGE_CUMULATOR

public static final ByteToMessageDecoder.Cumulator MERGE_CUMULATOR

Cumulate ByteBufs by merge them into one ByteBuf's, using memory copies.

COMPOSITE_CUMULATOR

public static final ByteToMessageDecoder.Cumulator COMPOSITE_CUMULATOR

Cumulate ByteBufs by add them to a CompositeByteBuf and so do no memory copy whenever possible. Be aware that CompositeByteBuf use a more complex indexing implementation so depending on your use-case and the decoder implementation this may be slower than just use the MERGE_CUMULATOR.

Constructor Detail

ByteToMessageDecoder

protected ByteToMessageDecoder()

Method Detail

setSingleDecode

public void setSingleDecode(boolean singleDecode)

If set then only one message is decoded on each channelRead(ChannelHandlerContext, Object) call. This may be useful if you need to do some protocol upgrade and want to make sure nothing is mixed up. Default is false as this has performance impacts.

isSingleDecode

public boolean isSingleDecode()

If true then only one message is decoded on each channelRead(ChannelHandlerContext, Object) call. Default is false as this has performance impacts.

setCumulator

public void setCumulator(ByteToMessageDecoder.Cumulator cumulator)

Set the ByteToMessageDecoder.Cumulator to use for cumulate the received ByteBufs.

setDiscardAfterReads

public void setDiscardAfterReads(int discardAfterReads)

Set the number of reads after which ByteBuf.discardSomeReadBytes() are called and so free up memory. The default is 16.

actualReadableBytes

protected int actualReadableBytes()

Returns the actual number of readable bytes in the internal cumulative buffer of this decoder. You usually do not need to rely on this value to write a decoder. Use it only when you must use it at your own risk. This method is a shortcut to internalBuffer().readableBytes().

internalBuffer

protected io.netty.buffer.ByteBuf internalBuffer()

Returns the internal cumulative buffer of this decoder. You usually do not need to access the internal buffer directly to write a decoder. Use it only when you must use it at your own risk.

handlerRemoved

public final void handlerRemoved(io.netty.channel.ChannelHandlerContext ctx)
                          throws Exception

Specified by:

handlerRemoved in interface io.netty.channel.ChannelHandler

Overrides:

handlerRemoved in class io.netty.channel.ChannelHandlerAdapter

Throws:

Exception

handlerRemoved0

protected void handlerRemoved0(io.netty.channel.ChannelHandlerContext ctx)
                        throws Exception

Gets called after the ByteToMessageDecoder was removed from the actual context and it doesn't handle events anymore.

Throws:

Exception

channelRead

public void channelRead(io.netty.channel.ChannelHandlerContext ctx,
                        Object msg)
                 throws Exception

Specified by:

channelRead in interface io.netty.channel.ChannelInboundHandler

Overrides:

channelRead in class io.netty.channel.ChannelInboundHandlerAdapter

Throws:

Exception

channelReadComplete

public void channelReadComplete(io.netty.channel.ChannelHandlerContext ctx)
                         throws Exception

Specified by:

channelReadComplete in interface io.netty.channel.ChannelInboundHandler

Overrides:

channelReadComplete in class io.netty.channel.ChannelInboundHandlerAdapter

Throws:

Exception

discardSomeReadBytes

protected final void discardSomeReadBytes()

channelInactive

public void channelInactive(io.netty.channel.ChannelHandlerContext ctx)
                     throws Exception

Specified by:

channelInactive in interface io.netty.channel.ChannelInboundHandler

Overrides:

channelInactive in class io.netty.channel.ChannelInboundHandlerAdapter

Throws:

Exception

userEventTriggered

public void userEventTriggered(io.netty.channel.ChannelHandlerContext ctx,
                               Object evt)
                        throws Exception

Specified by:

userEventTriggered in interface io.netty.channel.ChannelInboundHandler

Overrides:

userEventTriggered in class io.netty.channel.ChannelInboundHandlerAdapter

Throws:

Exception

callDecode

protected void callDecode(io.netty.channel.ChannelHandlerContext ctx,
                          io.netty.buffer.ByteBuf in,
                          List<Object> out)

Called once data should be decoded from the given ByteBuf. This method will call decode(ChannelHandlerContext, ByteBuf, List) as long as decoding should take place.

Parameters:

ctx - the ChannelHandlerContext which this ByteToMessageDecoder belongs to

in - the ByteBuf from which to read data

out - the List to which decoded messages should be added

decode

protected abstract void decode(io.netty.channel.ChannelHandlerContext ctx,
                               io.netty.buffer.ByteBuf in,
                               List<Object> out)
                        throws Exception

Decode the from one ByteBuf to an other. This method will be called till either the input ByteBuf has nothing to read when return from this method or till nothing was read from the input ByteBuf.

Parameters:

ctx - the ChannelHandlerContext which this ByteToMessageDecoder belongs to

in - the ByteBuf from which to read data

out - the List to which decoded messages should be added

Throws:

Exception - is thrown if an error occurs

decodeLast

protected void decodeLast(io.netty.channel.ChannelHandlerContext ctx,
                          io.netty.buffer.ByteBuf in,
                          List<Object> out)
                   throws Exception

Is called one last time when the ChannelHandlerContext goes in-active. Which means the channelInactive(ChannelHandlerContext) was triggered. By default, this will just call decode(ChannelHandlerContext, ByteBuf, List) but sub-classes may override this for some special cleanup operation.

Throws:

Exception