Package org.jboss.netty.handler.codec.replay

Class ReplayingDecoder<T extends java.lang.Enum<T>>

  • Type Parameters:
    T - the state type; use VoidEnum if state management is unused
    All Implemented Interfaces:
    ChannelHandler, ChannelUpstreamHandler, LifeCycleAwareChannelHandler
    Direct Known Subclasses:
    CompatibleMarshallingDecoder, HttpMessageDecoder, SocksAuthRequestDecoder, SocksAuthResponseDecoder, SocksCmdRequestDecoder, SocksCmdResponseDecoder, SocksInitRequestDecoder, SocksInitResponseDecoder, WebSocket00FrameDecoder, WebSocket08FrameDecoder
    public abstract class ReplayingDecoder<T extends java.lang.Enum<T>>
extends FrameDecoder
    A specialized variation of FrameDecoder which enables implementation of a non-blocking decoder in the blocking I/O paradigm.

    The biggest difference between ReplayingDecoder and FrameDecoder is that ReplayingDecoder allows you to implement the decode() and decodeLast() methods just like all required bytes were received already, rather than checking the availability of the required bytes. For example, the following FrameDecoder implementation: 

     public class IntegerHeaderFrameDecoder extends FrameDecoder {

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

     if (buf.readableBytes() < 4) {
        return null;
     }

     buf.markReaderIndex();
     int length = buf.readInt();

     if (buf.readableBytes() < length) {
        buf.resetReaderIndex();
        return null;
     }

     return buf.readBytes(length);
   }
 }
    is simplified like the following with ReplayingDecoder: 
     public class IntegerHeaderFrameDecoder
      extends ReplayingDecoder<VoidEnum> {

   protected Object decode(ChannelHandlerContext ctx,
                           Channel channel,
                           ChannelBuffer buf,
                           VoidEnum state) throws Exception {

     return buf.readBytes(buf.readInt());
   }
 }

    How does this work?

    ReplayingDecoder passes a specialized ChannelBuffer implementation which throws an Error of certain type when there's not enough data in the buffer. In the IntegerHeaderFrameDecoder above, you just assumed that there will be 4 or more bytes in the buffer when you call buf.readInt(). If there's really 4 bytes in the buffer, it will return the integer header as you expected. Otherwise, the Error will be raised and the control will be returned to ReplayingDecoder. If ReplayingDecoder catches the Error, then it will rewind the readerIndex of the buffer back to the 'initial' position (i.e. the beginning of the buffer) and call the decode(..) method again when more data is received into the buffer.

    Please note that ReplayingDecoder always throws the same cached Error instance to avoid the overhead of creating a new Error 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 ReplayingDecoder<VoidEnum> {

   private final Queue<Integer> values = new LinkedList<Integer>();

   @Override
   public Object decode(.., ChannelBuffer buffer, ..) throws Exception {

     // A message contains 2 integers.
     values.offer(buffer.readInt());
     values.offer(buffer.readInt());

     // This assertion will fail intermittently since values.offer()
     // can be called more than two times!
     assert values.size() == 2;
     return values.poll() + values.poll();
   }
 }
      The correct implementation looks like the following, and you can also utilize the 'checkpoint' feature which is explained in detail in the next section. 
       public class MyDecoder extends ReplayingDecoder<VoidEnum> {

   private final Queue<Integer> values = new LinkedList<Integer>();

   @Override
   public Object decode(.., ChannelBuffer buffer, ..) 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(buffer.readInt());
     values.offer(buffer.readInt());

     // Now we know this assertion will never fail.
     assert values.size() == 2;
     return values.poll() + values.poll();
   }
 }

    Improving the performance

    Fortunately, the performance of a complex decoder implementation can be improved significantly with the checkpoint() method. The checkpoint() method updates the 'initial' position of the buffer so that ReplayingDecoder rewinds the readerIndex of the buffer to the last position where you called the checkpoint() method.

    Calling checkpoint(T) with an Enum

    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 an Enum type which represents the current state of the decoder and to call checkpoint(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 Object decode(ChannelHandlerContext ctx,
                           Channel channel,
                           ChannelBuffer buf,
                           MyDecoderState state) throws Exception {
     switch (state) {
     case READ_LENGTH:
       length = buf.readInt();
       checkpoint(MyDecoderState.READ_CONTENT);
     case READ_CONTENT:
       ChannelBuffer frame = buf.readBytes(length);
       checkpoint(MyDecoderState.READ_LENGTH);
       return frame;
     default:
       throw new Error("Shouldn't reach here.");
     }
   }
 }

    Calling checkpoint() with no parameter

    An alternative way to manage the decoder state is to manage it by yourself. 

     public class IntegerHeaderFrameDecoder
      extends ReplayingDecoder<VoidEnum> {

   private boolean readLength;
   private int length;

   @Override
   protected Object decode(ChannelHandlerContext ctx,
                           Channel channel,
                           ChannelBuffer buf,
                           VoidEnum state) throws Exception {
     if (!readLength) {
       length = buf.readInt();
       readLength = true;
       checkpoint();
     }

     if (readLength) {
       ChannelBuffer frame = buf.readBytes(length);
       readLength = false;
       checkpoint();
       return 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 another ReplayingDecoder or FrameDecoder (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 ReplayingDecoder<VoidEnum> {

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

     @Override
     protected Object decode(ChannelHandlerContext ctx,
                             Channel ch,
                             ChannelBuffer buf,
                             VoidEnum state) {
         ...
         // 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(super.actualReadableBytes()) };
         } else {
             // Nothing to hand off
             return firstMessage;
         }
     }

    • Field Detail

      • state

        private T extends java.lang.Enum<T> state

      • checkpoint

        private int checkpoint

      • needsCleanup

        private boolean needsCleanup

    • Constructor Detail

      • ReplayingDecoder

        protected ReplayingDecoder()
        Creates a new instance with no initial state (i.e: null).

      • ReplayingDecoder

        protected ReplayingDecoder​(boolean unfold)

      • ReplayingDecoder

        protected ReplayingDecoder​(T initialState)
        Creates a new instance with the specified initial state.

      • ReplayingDecoder

        protected ReplayingDecoder​(T initialState,
                           boolean unfold)

    • Method Detail

      • internalBuffer

        protected ChannelBuffer internalBuffer()
        Description copied from class: FrameDecoder
        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.
        Overrides:
        internalBuffer in class FrameDecoder

      • checkpoint

        protected void checkpoint()
        Stores the internal cumulative buffer's reader position.

      • checkpoint

        protected void checkpoint​(T state)
        Stores the internal cumulative buffer's reader position and updates the current decoder state.

      • getState

        protected T getState()
        Returns the current state of this decoder.
        Returns:
        the current state of this decoder

      • setState

        protected T setState​(T newState)
        Sets the current state of this decoder.
        Returns:
        the old state of this decoder

      • decode

        protected abstract java.lang.Object decode​(ChannelHandlerContext ctx,
                                           Channel channel,
                                           ChannelBuffer buffer,
                                           T state)
                                    throws java.lang.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.
        state - the current decoder state (null if unused)
        Returns:
        the decoded frame
        Throws:
        java.lang.Exception

      • decodeLast

        protected java.lang.Object decodeLast​(ChannelHandlerContext ctx,
                                      Channel channel,
                                      ChannelBuffer buffer,
                                      T state)
                               throws java.lang.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.
        state - the current decoder state (null if unused)
        Returns:
        the decoded frame
        Throws:
        java.lang.Exception

      • decode

        protected final java.lang.Object decode​(ChannelHandlerContext ctx,
                                        Channel channel,
                                        ChannelBuffer buffer)
                                 throws java.lang.Exception
        Calls decode(ChannelHandlerContext, Channel, ChannelBuffer, Enum). This method should be never used by ReplayingDecoder itself. But to be safe we should handle it anyway
        Specified by:
        decode in class FrameDecoder
        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:
        java.lang.Exception

      • decodeLast

        protected final java.lang.Object decodeLast​(ChannelHandlerContext ctx,
                                            Channel channel,
                                            ChannelBuffer buffer)
                                     throws java.lang.Exception
        Description copied from class: FrameDecoder
        Decodes the received data so far into a frame when the channel is disconnected.
        Overrides:
        decodeLast in class FrameDecoder
        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:
        java.lang.Exception