
@ChannelPipelineCoverage(value="one") public abstract class FrameDecoder extends SimpleChannelUpstreamHandler
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 {
protected Object decode(ChannelHandlerContext ctx,
Channel 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;
}
}
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.
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
}
protected Object decode(ChannelHandlerContext ctx, Channel ch, 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;
}
}
}
| Modifier | Constructor and Description |
|---|---|
protected |
FrameDecoder() |
protected |
FrameDecoder(boolean unfold) |
| Modifier and Type | Method and Description |
|---|---|
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 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. |
void |
messageReceived(ChannelHandlerContext ctx,
MessageEvent e)
Invoked when a message object (e.g:
ChannelBuffer) was received
from a remote peer. |
channelBound, channelConnected, channelInterestChanged, channelOpen, channelUnbound, childChannelClosed, childChannelOpen, handleUpstream, writeCompleteprotected FrameDecoder()
protected FrameDecoder(boolean unfold)
public void messageReceived(ChannelHandlerContext ctx, MessageEvent e) throws Exception
SimpleChannelUpstreamHandlerChannelBuffer) was received
from a remote peer.messageReceived in class SimpleChannelUpstreamHandlerExceptionpublic void channelDisconnected(ChannelHandlerContext ctx, ChannelStateEvent e) throws Exception
SimpleChannelUpstreamHandlerChannel was disconnected from its remote peer.channelDisconnected in class SimpleChannelUpstreamHandlerExceptionpublic void channelClosed(ChannelHandlerContext ctx, ChannelStateEvent e) throws Exception
SimpleChannelUpstreamHandlerChannel was closed and all its related resources
were released.channelClosed in class SimpleChannelUpstreamHandlerExceptionpublic void exceptionCaught(ChannelHandlerContext ctx, ExceptionEvent e) throws Exception
SimpleChannelUpstreamHandlerChannelHandler.exceptionCaught in class SimpleChannelUpstreamHandlerExceptionprotected abstract Object decode(ChannelHandlerContext ctx, Channel channel, ChannelBuffer buffer) throws Exception
ctx - the context of this handlerchannel - the current channelbuffer - 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.null if there's not enough data in the buffer to decode a frame.Exceptionprotected Object decodeLast(ChannelHandlerContext ctx, Channel channel, ChannelBuffer buffer) throws Exception
ctx - the context of this handlerchannel - the current channelbuffer - 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.null if there's not enough data in the buffer to decode a frame.ExceptionCopyright © 2008-2013 JBoss, by Red Hat. All Rights Reserved.