/*
* Copyright 2011 The Netty Project
*
* The Netty Project licenses this file to you under the Apache License,
* version 2.0 (the "License"); you may not use this file except in compliance
* with the License. You may obtain a copy of the License at:
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
* WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
* License for the specific language governing permissions and limitations
* under the License.
*/
package io.netty.handler.codec.frame;
import java.net.SocketAddress;
import io.netty.buffer.ChannelBuffer;
import io.netty.buffer.ChannelBuffers;
import io.netty.channel.Channel;
import io.netty.channel.ChannelHandler;
import io.netty.channel.ChannelHandlerContext;
import io.netty.channel.ChannelPipeline;
import io.netty.channel.ChannelStateEvent;
import io.netty.channel.ChannelUpstreamHandler;
import io.netty.channel.Channels;
import io.netty.channel.ExceptionEvent;
import io.netty.channel.MessageEvent;
import io.netty.channel.SimpleChannelUpstreamHandler;
import io.netty.handler.codec.replay.ReplayingDecoder;
/**
* Decodes the received {@link ChannelBuffer}s into a meaningful frame object.
* <p>
* 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:
* <pre>
* +-----+-----+-----+
* | ABC | DEF | GHI |
* +-----+-----+-----+
* </pre>
* because of the packet fragmentation, a server can receive them like the
* following:
* <pre>
* +----+-------+---+---+
* | AB | CDEFG | H | I |
* +----+-------+---+---+
* </pre>
* <p>
* {@link FrameDecoder} helps you defrag the received packets into one or more
* meaningful <strong>frames</strong> that could be easily understood by the
* application logic. In case of the example above, your {@link FrameDecoder}
* implementation could defrag the received packets like the following:
* <pre>
* +-----+-----+-----+
* | ABC | DEF | GHI |
* +-----+-----+-----+
* </pre>
* <p>
* 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.
* <pre>
* MESSAGE FORMAT
* ==============
*
* Offset: 0 4 (Length + 4)
* +--------+------------------------+
* Fields: | Length | Actual message content |
* +--------+------------------------+
*
* DECODER IMPLEMENTATION
* ======================
*
* public class IntegerHeaderFrameDecoder extends {@link FrameDecoder} {
*
* {@code @Override}
* protected Object decode({@link ChannelHandlerContext} ctx,
* {@link Channel channel},
* {@link 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 <strong>null</strong>;
* }
*
* // 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 <strong>null</strong>;
* }
*
* // There's enough bytes in the buffer. Read it.
* {@link ChannelBuffer} frame = buf.readBytes(length);
*
* // Successfully decoded a frame. Return the decoded frame.
* return <strong>frame</strong>;
* }
* }
* </pre>
*
* <h3>Returning a POJO rather than a {@link ChannelBuffer}</h3>
* <p>
* Please note that you can return an object of a different type than
* {@link ChannelBuffer} in your {@code decode()} and {@code decodeLast()}
* implementation. For example, you could return a
* <a href="http://en.wikipedia.org/wiki/POJO">POJO</a> so that the next
* {@link ChannelUpstreamHandler} receives a {@link MessageEvent} which
* contains a POJO rather than a {@link ChannelBuffer}.
*
* <h3>Replacing a decoder with another decoder in a pipeline</h3>
* <p>
* If you are going to write a protocol multiplexer, you will probably want to
* replace a {@link FrameDecoder} (protocol detector) with another
* {@link FrameDecoder} or {@link ReplayingDecoder} (actual protocol decoder).
* It is not possible to achieve this simply by calling
* {@link ChannelPipeline#replace(ChannelHandler, String, ChannelHandler)}, but
* some additional steps are required:
* <pre>
* public class FirstDecoder extends {@link FrameDecoder} {
*
* public FirstDecoder() {
* super(true); // Enable unfold
* }
*
* {@code @Override}
* protected Object decode({@link ChannelHandlerContext} ctx,
* {@link Channel} channel,
* {@link 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;
* }
* }
* }
* </pre>
* @apiviz.landmark
*/
public abstract class FrameDecoder extends SimpleChannelUpstreamHandler {
private final boolean unfold;
private ChannelBuffer cumulation;
protected FrameDecoder() {
this(false);
}
protected FrameDecoder(boolean unfold) {
this.unfold = unfold;
}
@Override
public void messageReceived(
ChannelHandlerContext ctx, MessageEvent e) throws Exception {
Object m = e.getMessage();
if (!(m instanceof ChannelBuffer)) {
ctx.sendUpstream(e);
return;
}
ChannelBuffer input = (ChannelBuffer) m;
if (!input.readable()) {
return;
}
if (cumulation == null) {
// the cumulation buffer is not created yet so just pass the input to callDecode(...) method
callDecode(ctx, e.getChannel(), input, e.getRemoteAddress());
if (input.readable()) {
// seems like there is something readable left in the input buffer. So create the cumulation buffer and copy the input into it
ChannelBuffer cumulation = cumulation(ctx);
cumulation.writeBytes(input);
}
} else {
ChannelBuffer cumulation = cumulation(ctx);
if (cumulation.readable()) {
cumulation.discardReadBytes();
cumulation.writeBytes(input);
callDecode(ctx, e.getChannel(), cumulation, e.getRemoteAddress());
} else {
callDecode(ctx, e.getChannel(), input, e.getRemoteAddress());
if (input.readable()) {
cumulation.writeBytes(input);
}
}
}
}
@Override
public void channelDisconnected(
ChannelHandlerContext ctx, ChannelStateEvent e) throws Exception {
cleanup(ctx, e);
}
@Override
public void channelClosed(
ChannelHandlerContext ctx, ChannelStateEvent e) throws Exception {
cleanup(ctx, e);
}
@Override
public void exceptionCaught(
ChannelHandlerContext ctx, ExceptionEvent e) throws Exception {
ctx.sendUpstream(e);
}
/**
* Decodes the received packets so far into a frame.
*
* @param ctx the context of this handler
* @param channel the current channel
* @param 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.
*
* @return the decoded frame if a full frame was received and decoded.
* {@code null} if there's not enough data in the buffer to decode a frame.
*/
protected abstract Object decode(
ChannelHandlerContext ctx, Channel channel, ChannelBuffer buffer) throws Exception;
/**
* Decodes the received data so far into a frame when the channel is
* disconnected.
*
* @param ctx the context of this handler
* @param channel the current channel
* @param 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.
*
* @return the decoded frame if a full frame was received and decoded.
* {@code null} if there's not enough data in the buffer to decode a frame.
*/
protected Object decodeLast(
ChannelHandlerContext ctx, Channel channel, ChannelBuffer buffer) throws Exception {
return decode(ctx, channel, buffer);
}
private void callDecode(
ChannelHandlerContext context, Channel channel,
ChannelBuffer cumulation, SocketAddress remoteAddress) throws Exception {
while (cumulation.readable()) {
int oldReaderIndex = cumulation.readerIndex();
Object frame = decode(context, channel, cumulation);
if (frame == null) {
if (oldReaderIndex == cumulation.readerIndex()) {
// Seems like more data is required.
// Let us wait for the next notification.
break;
} else {
// Previous data has been discarded.
// Probably it is reading on.
continue;
}
} else if (oldReaderIndex == cumulation.readerIndex()) {
throw new IllegalStateException(
"decode() method must read at least one byte " +
"if it returned a frame (caused by: " + getClass() + ")");
}
unfoldAndFireMessageReceived(context, remoteAddress, frame);
}
if (!cumulation.readable()) {
this.cumulation = null;
}
}
private void unfoldAndFireMessageReceived(ChannelHandlerContext context, SocketAddress remoteAddress, Object result) {
if (unfold) {
if (result instanceof Object[]) {
for (Object r: (Object[]) result) {
Channels.fireMessageReceived(context, r, remoteAddress);
}
} else if (result instanceof Iterable<?>) {
for (Object r: (Iterable<?>) result) {
Channels.fireMessageReceived(context, r, remoteAddress);
}
} else {
Channels.fireMessageReceived(context, result, remoteAddress);
}
} else {
Channels.fireMessageReceived(context, result, remoteAddress);
}
}
private void cleanup(ChannelHandlerContext ctx, ChannelStateEvent e)
throws Exception {
try {
ChannelBuffer cumulation = this.cumulation;
if (cumulation == null) {
return;
} else {
this.cumulation = null;
}
if (cumulation.readable()) {
// Make sure all frames are read before notifying a closed channel.
callDecode(ctx, ctx.getChannel(), cumulation, null);
}
// Call decodeLast() finally. Please note that decodeLast() is
// called even if there's nothing more to read from the buffer to
// notify a user that the connection was closed explicitly.
Object partialFrame = decodeLast(ctx, ctx.getChannel(), cumulation);
if (partialFrame != null) {
unfoldAndFireMessageReceived(ctx, null, partialFrame);
}
} finally {
ctx.sendUpstream(e);
}
}
/**
* Get the currently used {@link ChannelBuffer} for cumulation or create one in a lazy fashion if none exist yet
*
* @param ctx the {@link ChannelHandlerContext} for this handler
* @return buffer the {@link ChannelBuffer} which is used for cumulation
*/
private ChannelBuffer cumulation(ChannelHandlerContext ctx) {
ChannelBuffer c = cumulation;
if (c == null) {
c = createCumulationDynamicBuffer(ctx);
cumulation = c;
}
return c;
}
/**
* Create a new {@link ChannelBuffer} which is used for the cumulation. Be aware that this MUST be a dynamic buffer. Sub-classes may override this to provide a
* dynamic {@link ChannelBuffer} which has some prelocated size that better fit their need.
*
* @param ctx {@link ChannelHandlerContext} for this handler
* @return buffer the {@link ChannelBuffer} which is used for cumulation
*/
protected ChannelBuffer createCumulationDynamicBuffer(ChannelHandlerContext ctx) {
return ChannelBuffers.dynamicBuffer(ctx.getChannel().getConfig().getBufferFactory());
}
}