org.jboss.netty.handler.codec.frame

Class FrameDecoder

java.lang.Object

org.jboss.netty.channel.SimpleChannelUpstreamHandler

org.jboss.netty.handler.codec.frame.FrameDecoder

All Implemented Interfaces:

ChannelHandler, ChannelUpstreamHandler, LifeCycleAwareChannelHandler

Direct Known Subclasses:

DelimiterBasedFrameDecoder, FixedLengthFrameDecoder, LengthFieldBasedFrameDecoder, ProtobufVarint32FrameDecoder, ReplayingDecoder, SpdyFrameDecoder, SslHandler

public abstract class FrameDecoder
extends SimpleChannelUpstreamHandler
implements LifeCycleAwareChannelHandler

Decodes the received ChannelBuffers into a meaningful frame object.

In a stream-based transport such as TCP/IP, packets can be fragmented and reassembled during transmission even in a LAN environment. For example, let us assume you have received three packets:

 +-----+-----+-----+
 | ABC | DEF | GHI |
 +-----+-----+-----+
 

because of the packet fragmentation, a server can receive them like the following:

 +----+-------+---+---+
 | AB | CDEFG | H | I |
 +----+-------+---+---+
 

FrameDecoder helps you defrag the received packets into one or more meaningful frames that could be easily understood by the application logic. In case of the example above, your FrameDecoder implementation could defrag the received packets like the following:

 +-----+-----+-----+
 | ABC | DEF | GHI |
 +-----+-----+-----+
 

The following code shows an example handler which decodes a frame whose first 4 bytes header represents the length of the frame, excluding the header.

 MESSAGE FORMAT
 ==============

 Offset:  0        4                   (Length + 4)
          +--------+------------------------+
 Fields:  | Length | Actual message content |
          +--------+------------------------+

 DECODER IMPLEMENTATION
 ======================

 public class IntegerHeaderFrameDecoder extends FrameDecoder {

   @Override
   protected Object decode(ChannelHandlerContext ctx,
                           channel,
                           ChannelBuffer buf) throws Exception {

     // Make sure if the length field was received.
     if (buf.readableBytes() < 4) {
        // The length field was not received yet - return null.
        // This method will be invoked again when more packets are
        // received and appended to the buffer.
        return null;
     }

     // The length field is in the buffer.

     // Mark the current buffer position before reading the length field
     // because the whole frame might not be in the buffer yet.
     // We will reset the buffer position to the marked position if
     // there's not enough bytes in the buffer.
     buf.markReaderIndex();

     // Read the length field.
     int length = buf.readInt();

     // Make sure if there's enough bytes in the buffer.
     if (buf.readableBytes() < length) {
        // The whole bytes were not received yet - return null.
        // This method will be invoked again when more packets are
        // received and appended to the buffer.

        // Reset to the marked position to read the length field again
        // next time.
        buf.resetReaderIndex();

        return null;
     }

     // There's enough bytes in the buffer. Read it.
     ChannelBuffer frame = buf.readBytes(length);

     // Successfully decoded a frame.  Return the decoded frame.
     return frame;
   }
 }
 

Returning a POJO rather than a ChannelBuffer

Please note that you can return an object of a different type than ChannelBuffer in your decode() and decodeLast() implementation. For example, you could return a POJO so that the next ChannelUpstreamHandler receives a MessageEvent which contains a POJO rather than a ChannelBuffer.

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 FrameDecoder (protocol detector) with another FrameDecoder or ReplayingDecoder (actual protocol decoder). It is not possible to achieve this simply by calling ChannelPipeline.replace(ChannelHandler, String, ChannelHandler), but some additional steps are required:

 public class FirstDecoder extends FrameDecoder {

     public FirstDecoder() {
         super(true); // Enable unfold
     }

     @Override
     protected Object decode(ChannelHandlerContext ctx,
                             Channel channel,
                             ChannelBuffer buf) {
         ...
         // Decode the first message
         Object firstMessage = ...;

         // Add the second decoder
         ctx.getPipeline().addLast("second", new SecondDecoder());

         // Remove the first decoder (me)
         ctx.getPipeline().remove(this);

         if (buf.readable()) {
             // Hand off the remaining data to the second decoder
             return new Object[] { firstMessage, buf.readBytes(buf.readableBytes()) };
         } else {
             // Nothing to hand off
             return firstMessage;
         }
     }
 }
 

Nested Class Summary

Nested classes/interfaces inherited from interface org.jboss.netty.channel.ChannelHandler

ChannelHandler.Sharable

Field Summary

Fields

Modifier and Type	Field and Description
protected ChannelBuffer	cumulation
static int	DEFAULT_MAX_COMPOSITEBUFFER_COMPONENTS

Constructor Summary

Constructors

Modifier	Constructor and Description
protected	FrameDecoder()
protected	FrameDecoder(boolean unfold)

Method Summary

Methods

Modifier and Type	Method and Description
protected int	actualReadableBytes() Returns the actual number of readable bytes in the internal cumulative buffer of this decoder.
void	afterAdd(ChannelHandlerContext ctx)
void	afterRemove(ChannelHandlerContext ctx)
protected ChannelBuffer	appendToCumulation(ChannelBuffer input)
void	beforeAdd(ChannelHandlerContext ctx)
void	beforeRemove(ChannelHandlerContext ctx)
void	channelClosed(ChannelHandlerContext ctx, ChannelStateEvent e) Invoked when a Channel was closed and all its related resources were released.
void	channelDisconnected(ChannelHandlerContext ctx, ChannelStateEvent e) Invoked when a Channel was disconnected from its remote peer.
protected void	cleanup(ChannelHandlerContext ctx, ChannelStateEvent e) Gets called on channelDisconnected(ChannelHandlerContext, ChannelStateEvent) and channelClosed(ChannelHandlerContext, ChannelStateEvent)
protected abstract Object	decode(ChannelHandlerContext ctx, Channel channel, ChannelBuffer buffer) Decodes the received packets so far into a frame.
protected Object	decodeLast(ChannelHandlerContext ctx, Channel channel, ChannelBuffer buffer) Decodes the received data so far into a frame when the channel is disconnected.
void	exceptionCaught(ChannelHandlerContext ctx, ExceptionEvent e) Invoked when an exception was raised by an I/O thread or a ChannelHandler.
int	getMaxCumulationBufferCapacity() See setMaxCumulationBufferCapacity(int) for explaintation of this setting
int	getMaxCumulationBufferComponents() Returns the maximum number of components in the cumulation buffer.
protected ChannelBuffer	internalBuffer() Returns the internal cumulative buffer of this decoder.
boolean	isUnfold()
void	messageReceived(ChannelHandlerContext ctx, MessageEvent e) Invoked when a message object (e.g: ChannelBuffer) was received from a remote peer.
protected ChannelBuffer	newCumulationBuffer(ChannelHandlerContext ctx, int minimumCapacity) Create a new ChannelBuffer which is used for the cumulation.
void	replace(String handlerName, ChannelHandler handler) Replace this FrameDecoder in the ChannelPipeline with the given ChannelHandler.
void	setMaxCumulationBufferCapacity(int copyThreshold) Set the maximal capacity of the internal cumulation ChannelBuffer to use before the FrameDecoder tries to minimize the memory usage by "byte copy".
void	setMaxCumulationBufferComponents(int maxCumulationBufferComponents) Sets the maximum number of components in the cumulation buffer.
void	setUnfold(boolean unfold)
protected void	unfoldAndFireMessageReceived(ChannelHandlerContext context, SocketAddress remoteAddress, Object result)
protected ChannelBuffer	updateCumulation(ChannelHandlerContext ctx, ChannelBuffer input)

Methods inherited from class org.jboss.netty.channel.SimpleChannelUpstreamHandler

channelBound, channelConnected, channelInterestChanged, channelOpen, channelUnbound, childChannelClosed, childChannelOpen, handleUpstream, writeComplete

Methods inherited from class java.lang.Object

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

Field Detail

DEFAULT_MAX_COMPOSITEBUFFER_COMPONENTS

public static final int DEFAULT_MAX_COMPOSITEBUFFER_COMPONENTS

See Also:

Constant Field Values

cumulation

protected ChannelBuffer cumulation

Constructor Detail

FrameDecoder

protected FrameDecoder()

FrameDecoder

protected FrameDecoder(boolean unfold)

Method Detail

isUnfold

public final boolean isUnfold()

setUnfold

public final void setUnfold(boolean unfold)

getMaxCumulationBufferCapacity

public final int getMaxCumulationBufferCapacity()

See setMaxCumulationBufferCapacity(int) for explaintation of this setting

setMaxCumulationBufferCapacity

public final void setMaxCumulationBufferCapacity(int copyThreshold)

Set the maximal capacity of the internal cumulation ChannelBuffer to use before the FrameDecoder tries to minimize the memory usage by "byte copy". What you use here really depends on your application and need. Using Integer.MAX_VALUE will disable all byte copies but give you the cost of a higher memory usage if big ChannelBuffer's will be received. By default a threshold of 0 is used, which means it will always copy to try to reduce memory usage

Parameters:

copyThreshold - the threshold (in bytes) or Integer.MAX_VALUE to disable it. The value must be at least 0

Throws:

IllegalStateException - get thrown if someone tries to change this setting after the Decoder was added to the ChannelPipeline

getMaxCumulationBufferComponents

public final int getMaxCumulationBufferComponents()

Returns the maximum number of components in the cumulation buffer. If the number of the components in the cumulation buffer exceeds this value, the components of the cumulation buffer are consolidated into a single component, involving memory copies. The default value of this property DEFAULT_MAX_COMPOSITEBUFFER_COMPONENTS.

setMaxCumulationBufferComponents

public final void setMaxCumulationBufferComponents(int maxCumulationBufferComponents)

Sets the maximum number of components in the cumulation buffer. If the number of the components in the cumulation buffer exceeds this value, the components of the cumulation buffer are consolidated into a single component, involving memory copies. The default value of this property is DEFAULT_MAX_COMPOSITEBUFFER_COMPONENTS and its minimum allowed value is 2.

messageReceived

public void messageReceived(ChannelHandlerContext ctx,
                   MessageEvent e)
                     throws Exception

Description copied from class: SimpleChannelUpstreamHandler

Invoked when a message object (e.g: ChannelBuffer) was received from a remote peer.

Overrides:

messageReceived in class SimpleChannelUpstreamHandler

Throws:

Exception

appendToCumulation

protected ChannelBuffer appendToCumulation(ChannelBuffer input)

updateCumulation

protected ChannelBuffer updateCumulation(ChannelHandlerContext ctx,
                             ChannelBuffer input)

channelDisconnected

public void channelDisconnected(ChannelHandlerContext ctx,
                       ChannelStateEvent e)
                         throws Exception

Description copied from class: SimpleChannelUpstreamHandler

Invoked when a Channel was disconnected from its remote peer.

Overrides:

channelDisconnected in class SimpleChannelUpstreamHandler

Throws:

Exception

channelClosed

public void channelClosed(ChannelHandlerContext ctx,
                 ChannelStateEvent e)
                   throws Exception

Description copied from class: SimpleChannelUpstreamHandler

Invoked when a Channel was closed and all its related resources were released.

Overrides:

channelClosed in class SimpleChannelUpstreamHandler

Throws:

Exception

exceptionCaught

public void exceptionCaught(ChannelHandlerContext ctx,
                   ExceptionEvent e)
                     throws Exception

Description copied from class: SimpleChannelUpstreamHandler

Invoked when an exception was raised by an I/O thread or a ChannelHandler.

Overrides:

exceptionCaught in class SimpleChannelUpstreamHandler

Throws:

Exception

decode

protected abstract Object decode(ChannelHandlerContext ctx,
            Channel channel,
            ChannelBuffer buffer)
                          throws Exception

Decodes the received packets so far into a frame.

Parameters:

ctx - the context of this handler

channel - the current channel

buffer - the cumulative buffer of received packets so far. Note that the buffer might be empty, which means you should not make an assumption that the buffer contains at least one byte in your decoder implementation.

Returns:

the decoded frame if a full frame was received and decoded. null if there's not enough data in the buffer to decode a frame.

Throws:

Exception

decodeLast

protected Object decodeLast(ChannelHandlerContext ctx,
                Channel channel,
                ChannelBuffer buffer)
                     throws Exception

Decodes the received data so far into a frame when the channel is disconnected.

Parameters:

ctx - the context of this handler

channel - the current channel

buffer - the cumulative buffer of received packets so far. Note that the buffer might be empty, which means you should not make an assumption that the buffer contains at least one byte in your decoder implementation.

Returns:

the decoded frame if a full frame was received and decoded. null if there's not enough data in the buffer to decode a frame.

Throws:

Exception

unfoldAndFireMessageReceived

protected final void unfoldAndFireMessageReceived(ChannelHandlerContext context,
                                SocketAddress remoteAddress,
                                Object result)

cleanup

protected void cleanup(ChannelHandlerContext ctx,
           ChannelStateEvent e)
                throws Exception

Gets called on channelDisconnected(ChannelHandlerContext, ChannelStateEvent) and channelClosed(ChannelHandlerContext, ChannelStateEvent)

Throws:

Exception

newCumulationBuffer

protected ChannelBuffer newCumulationBuffer(ChannelHandlerContext ctx,
                                int minimumCapacity)

Create a new ChannelBuffer which is used for the cumulation. Sub-classes may override this.

Parameters:

ctx - ChannelHandlerContext for this handler

Returns:

buffer the ChannelBuffer which is used for cumulation

replace

public void replace(String handlerName,
           ChannelHandler handler)

Replace this FrameDecoder in the ChannelPipeline with the given ChannelHandler. All remaining bytes in the ChannelBuffer will get send to the new ChannelHandler that was used as replacement

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 muse use it at your own risk. This method is a shortcut to internalBuffer().readableBytes().

internalBuffer

protected ChannelBuffer 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.

beforeAdd

public void beforeAdd(ChannelHandlerContext ctx)
               throws Exception

Specified by:

beforeAdd in interface LifeCycleAwareChannelHandler

Throws:

Exception

afterAdd

public void afterAdd(ChannelHandlerContext ctx)
              throws Exception

Specified by:

afterAdd in interface LifeCycleAwareChannelHandler

Throws:

Exception

beforeRemove

public void beforeRemove(ChannelHandlerContext ctx)
                  throws Exception

Specified by:

beforeRemove in interface LifeCycleAwareChannelHandler

Throws:

Exception

afterRemove

public void afterRemove(ChannelHandlerContext ctx)
                 throws Exception

Specified by:

afterRemove in interface LifeCycleAwareChannelHandler

Throws:

Exception