Class ReplayingDecoder<S>
- java.lang.Object
-
- io.netty.channel.ChannelHandlerAdapter
-
- io.netty.channel.ChannelInboundHandlerAdapter
-
- io.netty.handler.codec.ByteToMessageDecoder
-
- io.netty.handler.codec.ReplayingDecoder<S>
-
- Type Parameters:
S
- the state type which is usually anEnum
; useVoid
if state management is unused
- All Implemented Interfaces:
ChannelHandler
,ChannelInboundHandler
- Direct Known Subclasses:
CompatibleMarshallingDecoder
,MqttDecoder
,Socks4ClientDecoder
,Socks4ServerDecoder
,Socks5CommandRequestDecoder
,Socks5CommandResponseDecoder
,Socks5InitialRequestDecoder
,Socks5InitialResponseDecoder
,Socks5PasswordAuthRequestDecoder
,Socks5PasswordAuthResponseDecoder
,SocksAuthRequestDecoder
,SocksAuthResponseDecoder
,SocksCmdRequestDecoder
,SocksCmdResponseDecoder
,SocksInitRequestDecoder
,SocksInitResponseDecoder
,StompSubframeDecoder
,WebSocket00FrameDecoder
public abstract class ReplayingDecoder<S> extends ByteToMessageDecoder
A specialized variation ofByteToMessageDecoder
which enables implementation of a non-blocking decoder in the blocking I/O paradigm.The biggest difference between
ReplayingDecoder
andByteToMessageDecoder
is thatReplayingDecoder
allows you to implement thedecode()
anddecodeLast()
methods just like all required bytes were received already, rather than checking the availability of the required bytes. For example, the followingByteToMessageDecoder
implementation:public class IntegerHeaderFrameDecoder extends
is simplified like the following withByteToMessageDecoder
{@Override
protected void decode(ChannelHandlerContext
ctx,ByteBuf
buf, List<Object> out) throws Exception { if (buf.readableBytes() < 4) { return; } buf.markReaderIndex(); int length = buf.readInt(); if (buf.readableBytes() < length) { buf.resetReaderIndex(); return; } out.add(buf.readBytes(length)); } }ReplayingDecoder
:public class IntegerHeaderFrameDecoder extends
ReplayingDecoder
<Void
> { protected void decode(ChannelHandlerContext
ctx,ByteBuf
buf, List<Object> out) throws Exception { out.add(buf.readBytes(buf.readInt())); } }How does this work?
ReplayingDecoder
passes a specializedByteBuf
implementation which throws anError
of certain type when there's not enough data in the buffer. In theIntegerHeaderFrameDecoder
above, you just assumed that there will be 4 or more bytes in the buffer when you callbuf.readInt()
. If there's really 4 bytes in the buffer, it will return the integer header as you expected. Otherwise, theError
will be raised and the control will be returned toReplayingDecoder
. IfReplayingDecoder
catches theError
, then it will rewind thereaderIndex
of the buffer back to the 'initial' position (i.e. the beginning of the buffer) and call thedecode(..)
method again when more data is received into the buffer.Please note that
ReplayingDecoder
always throws the same cachedError
instance to avoid the overhead of creating a newError
and filling its stack trace for every throw.Limitations
At the cost of the simplicity,
ReplayingDecoder
enforces you a few limitations:- Some buffer operations are prohibited.
- Performance can be worse if the network is slow and the message format is complicated unlike the example above. In this case, your decoder might have to decode the same part of the message over and over again.
- You must keep in mind that
decode(..)
method can be called many times to decode a single message. For example, the following code will not work:public class MyDecoder extends
The correct implementation looks like the following, and you can also utilize the 'checkpoint' feature which is explained in detail in the next section.ReplayingDecoder
<Void
> { private final Queue<Integer> values = new LinkedList<Integer>();@Override
public void decode(..,ByteBuf
buf, List<Object> out) throws Exception { // A message contains 2 integers. values.offer(buf.readInt()); values.offer(buf.readInt()); // This assertion will fail intermittently since values.offer() // can be called more than two times! assert values.size() == 2; out.add(values.poll() + values.poll()); } }public class MyDecoder extends
ReplayingDecoder
<Void
> { private final Queue<Integer> values = new LinkedList<Integer>();@Override
public void decode(..,ByteBuf
buf, List<Object> out) throws Exception { // Revert the state of the variable that might have been changed // since the last partial decode. values.clear(); // A message contains 2 integers. values.offer(buf.readInt()); values.offer(buf.readInt()); // Now we know this assertion will never fail. assert values.size() == 2; out.add(values.poll() + values.poll()); } }
Improving the performance
Fortunately, the performance of a complex decoder implementation can be improved significantly with the
checkpoint()
method. Thecheckpoint()
method updates the 'initial' position of the buffer so thatReplayingDecoder
rewinds thereaderIndex
of the buffer to the last position where you called thecheckpoint()
method.Calling
checkpoint(T)
with anEnum
Although you can just use
checkpoint()
method and manage the state of the decoder by yourself, the easiest way to manage the state of the decoder is to create anEnum
type which represents the current state of the decoder and to callcheckpoint(T)
method whenever the state changes. You can have as many states as you want depending on the complexity of the message you want to decode:public enum MyDecoderState { READ_LENGTH, READ_CONTENT; } public class IntegerHeaderFrameDecoder extends
ReplayingDecoder
<MyDecoderState> { private int length; public IntegerHeaderFrameDecoder() { // Set the initial state. super(MyDecoderState.READ_LENGTH); }@Override
protected void decode(ChannelHandlerContext
ctx,ByteBuf
buf, List<Object> out) throws Exception { switch (state()) { case READ_LENGTH: length = buf.readInt(); checkpoint(MyDecoderState.READ_CONTENT); case READ_CONTENT: ByteBuf frame = buf.readBytes(length); checkpoint(MyDecoderState.READ_LENGTH); out.add(frame); break; default: throw new Error("Shouldn't reach here."); } } }Calling
checkpoint()
with no parameterAn alternative way to manage the decoder state is to manage it by yourself.
public class IntegerHeaderFrameDecoder extends
ReplayingDecoder
<Void
> { private boolean readLength; private int length;@Override
protected void decode(ChannelHandlerContext
ctx,ByteBuf
buf, List<Object> out) throws Exception { if (!readLength) { length = buf.readInt(); readLength = true; checkpoint(); } if (readLength) { ByteBuf frame = buf.readBytes(length); readLength = false; checkpoint(); out.add(frame); } } }Replacing a decoder with another decoder in a pipeline
If you are going to write a protocol multiplexer, you will probably want to replace a
ReplayingDecoder
(protocol detector) with anotherReplayingDecoder
,ByteToMessageDecoder
orMessageToMessageDecoder
(actual protocol decoder). It is not possible to achieve this simply by callingChannelPipeline.replace(ChannelHandler, String, ChannelHandler)
, but some additional steps are required:public class FirstDecoder extends
ReplayingDecoder
<Void
> {@Override
protected void decode(ChannelHandlerContext
ctx,ByteBuf
buf, List<Object> out) { ... // Decode the first message Object firstMessage = ...; // Add the second decoder ctx.pipeline().addLast("second", new SecondDecoder()); if (buf.isReadable()) { // Hand off the remaining data to the second decoder out.add(firstMessage); out.add(buf.readBytes(super.actualReadableBytes())); } else { // Nothing to hand off out.add(firstMessage); } // Remove the first decoder (me) ctx.pipeline().remove(this); }
-
-
Nested Class Summary
-
Nested classes/interfaces inherited from class io.netty.handler.codec.ByteToMessageDecoder
ByteToMessageDecoder.Cumulator
-
Nested classes/interfaces inherited from interface io.netty.channel.ChannelHandler
ChannelHandler.Sharable
-
-
Field Summary
-
Fields inherited from class io.netty.handler.codec.ByteToMessageDecoder
COMPOSITE_CUMULATOR, MERGE_CUMULATOR
-
-
Constructor Summary
Constructors Modifier Constructor Description protected
ReplayingDecoder()
Creates a new instance with no initial state (i.e:null
).protected
ReplayingDecoder(S initialState)
Creates a new instance with the specified initial state.
-
Method Summary
All Methods Instance Methods Concrete Methods Modifier and Type Method Description protected void
callDecode(ChannelHandlerContext ctx, ByteBuf in, java.util.List<java.lang.Object> out)
Called once data should be decoded from the givenByteBuf
.protected void
checkpoint()
Stores the internal cumulative buffer's reader position.protected void
checkpoint(S state)
Stores the internal cumulative buffer's reader position and updates the current decoder state.protected S
state()
Returns the current state of this decoder.protected S
state(S newState)
Sets the current state of this decoder.-
Methods inherited from class io.netty.handler.codec.ByteToMessageDecoder
actualReadableBytes, channelInactive, channelRead, channelReadComplete, decode, decodeLast, discardSomeReadBytes, handlerRemoved, handlerRemoved0, internalBuffer, isSingleDecode, setCumulator, setDiscardAfterReads, setSingleDecode, userEventTriggered
-
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
-
-
-
-
Constructor Detail
-
ReplayingDecoder
protected ReplayingDecoder()
Creates a new instance with no initial state (i.e:null
).
-
ReplayingDecoder
protected ReplayingDecoder(S initialState)
Creates a new instance with the specified initial state.
-
-
Method Detail
-
checkpoint
protected void checkpoint()
Stores the internal cumulative buffer's reader position.
-
checkpoint
protected void checkpoint(S state)
Stores the internal cumulative buffer's reader position and updates the current decoder state.
-
state
protected S state()
Returns the current state of this decoder.- Returns:
- the current state of this decoder
-
state
protected S state(S newState)
Sets the current state of this decoder.- Returns:
- the old state of this decoder
-
callDecode
protected void callDecode(ChannelHandlerContext ctx, ByteBuf in, java.util.List<java.lang.Object> out)
Description copied from class:ByteToMessageDecoder
Called once data should be decoded from the givenByteBuf
. This method will callByteToMessageDecoder.decode(ChannelHandlerContext, ByteBuf, List)
as long as decoding should take place.- Overrides:
callDecode
in classByteToMessageDecoder
- Parameters:
ctx
- theChannelHandlerContext
which thisByteToMessageDecoder
belongs toin
- theByteBuf
from which to read dataout
- theList
to which decoded messages should be added
-
-