/*
* 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.replay;
import java.net.SocketAddress;
import io.netty.buffer.ChannelBuffer;
import io.netty.buffer.ChannelBufferFactory;
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.Channels;
import io.netty.channel.ExceptionEvent;
import io.netty.channel.MessageEvent;
import io.netty.channel.SimpleChannelUpstreamHandler;
import io.netty.handler.codec.frame.FrameDecoder;
/**
* A specialized variation of {@link FrameDecoder} which enables implementation
* of a non-blocking decoder in the blocking I/O paradigm.
* <p>
* The biggest difference between {@link ReplayingDecoder} and
* {@link FrameDecoder} is that {@link ReplayingDecoder} allows you to
* implement the {@code decode()} and {@code decodeLast()} methods just like
* all required bytes were received already, rather than checking the
* availability of the required bytes. For example, the following
* {@link FrameDecoder} implementation:
* <pre>
* public class IntegerHeaderFrameDecoder extends {@link FrameDecoder} {
*
* {@code @Override}
* protected Object decode({@link ChannelHandlerContext} ctx,
* {@link Channel} channel,
* {@link ChannelBuffer} buf) throws Exception {
*
* if (buf.readableBytes() < 4) {
* return <strong>null</strong>;
* }
*
* buf.markReaderIndex();
* int length = buf.readInt();
*
* if (buf.readableBytes() < length) {
* buf.resetReaderIndex();
* return <strong>null</strong>;
* }
*
* return buf.readBytes(length);
* }
* }
* </pre>
* is simplified like the following with {@link ReplayingDecoder}:
* <pre>
* public class IntegerHeaderFrameDecoder
* extends {@link ReplayingDecoder}<{@link VoidEnum}> {
*
* protected Object decode({@link ChannelHandlerContext} ctx,
* {@link Channel} channel,
* {@link ChannelBuffer} buf,
* {@link VoidEnum} state) throws Exception {
*
* return buf.readBytes(buf.readInt());
* }
* }
* </pre>
*
* <h3>How does this work?</h3>
* <p>
* {@link ReplayingDecoder} passes a specialized {@link ChannelBuffer}
* implementation which throws an {@link Error} of certain type when there's not
* enough data in the buffer. In the {@code IntegerHeaderFrameDecoder} above,
* you just assumed that there will be 4 or more bytes in the buffer when
* you call {@code buf.readInt()}. If there's really 4 bytes in the buffer,
* it will return the integer header as you expected. Otherwise, the
* {@link Error} will be raised and the control will be returned to
* {@link ReplayingDecoder}. If {@link ReplayingDecoder} catches the
* {@link Error}, then it will rewind the {@code readerIndex} of the buffer
* back to the 'initial' position (i.e. the beginning of the buffer) and call
* the {@code decode(..)} method again when more data is received into the
* buffer.
* <p>
* Please note that {@link ReplayingDecoder} always throws the same cached
* {@link Error} instance to avoid the overhead of creating a new {@link Error}
* and filling its stack trace for every throw.
*
* <h3>Limitations</h3>
* <p>
* At the cost of the simplicity, {@link ReplayingDecoder} enforces you a few
* limitations:
* <ul>
* <li>Some buffer operations are prohibited.</li>
* <li>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.</li>
* <li>You must keep in mind that {@code decode(..)} method can be called many
* times to decode a single message. For example, the following code will
* not work:
* <pre> public class MyDecoder extends {@link ReplayingDecoder}<{@link VoidEnum}> {
*
* private final Queue<Integer> values = new LinkedList<Integer>();
*
* {@code @Override}
* public Object decode(.., {@link 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();
* }
* }</pre>
* The correct implementation looks like the following, and you can also
* utilize the 'checkpoint' feature which is explained in detail in the
* next section.
* <pre> public class MyDecoder extends {@link ReplayingDecoder}<{@link VoidEnum}> {
*
* private final Queue<Integer> values = new LinkedList<Integer>();
*
* {@code @Override}
* public Object decode(.., {@link 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();
* }
* }</pre>
* </li>
* </ul>
*
* <h3>Improving the performance</h3>
* <p>
* Fortunately, the performance of a complex decoder implementation can be
* improved significantly with the {@code checkpoint()} method. The
* {@code checkpoint()} method updates the 'initial' position of the buffer so
* that {@link ReplayingDecoder} rewinds the {@code readerIndex} of the buffer
* to the last position where you called the {@code checkpoint()} method.
*
* <h4>Calling {@code checkpoint(T)} with an {@link Enum}</h4>
* <p>
* Although you can just use {@code 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 {@link Enum} type which represents the current state
* of the decoder and to call {@code 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:
*
* <pre>
* public enum MyDecoderState {
* READ_LENGTH,
* READ_CONTENT;
* }
*
* public class IntegerHeaderFrameDecoder
* extends {@link ReplayingDecoder}<<strong>MyDecoderState</strong>> {
*
* private int length;
*
* public IntegerHeaderFrameDecoder() {
* // Set the initial state.
* <strong>super(MyDecoderState.READ_LENGTH);</strong>
* }
*
* {@code @Override}
* protected Object decode({@link ChannelHandlerContext} ctx,
* {@link Channel} channel,
* {@link ChannelBuffer} buf,
* <b>MyDecoderState</b> state) throws Exception {
* switch (state) {
* case READ_LENGTH:
* length = buf.readInt();
* <strong>checkpoint(MyDecoderState.READ_CONTENT);</strong>
* case READ_CONTENT:
* ChannelBuffer frame = buf.readBytes(length);
* <strong>checkpoint(MyDecoderState.READ_LENGTH);</strong>
* return frame;
* default:
* throw new Error("Shouldn't reach here.");
* }
* }
* }
* </pre>
*
* <h4>Calling {@code checkpoint()} with no parameter</h4>
* <p>
* An alternative way to manage the decoder state is to manage it by yourself.
* <pre>
* public class IntegerHeaderFrameDecoder
* extends {@link ReplayingDecoder}<<strong>{@link VoidEnum}</strong>> {
*
* <strong>private boolean readLength;</strong>
* private int length;
*
* {@code @Override}
* protected Object decode({@link ChannelHandlerContext} ctx,
* {@link Channel} channel,
* {@link ChannelBuffer} buf,
* {@link VoidEnum} state) throws Exception {
* if (!readLength) {
* length = buf.readInt();
* <strong>readLength = true;</strong>
* <strong>checkpoint();</strong>
* }
*
* if (readLength) {
* ChannelBuffer frame = buf.readBytes(length);
* <strong>readLength = false;</strong>
* <strong>checkpoint();</strong>
* return frame;
* }
* }
* }
* </pre>
*
* <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 ReplayingDecoder} (protocol detector) with another
* {@link ReplayingDecoder} or {@link FrameDecoder} (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 ReplayingDecoder}<{@link VoidEnum}> {
*
* public FirstDecoder() {
* super(true); // Enable unfold
* }
*
* {@code @Override}
* protected Object decode({@link ChannelHandlerContext} ctx,
* {@link Channel} ch,
* {@link ChannelBuffer} buf,
* {@link 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(<b>super.actualReadableBytes()</b>) };
* } else {
* // Nothing to hand off
* return firstMessage;
* }
* }
* </pre>
* @param <T>
* the state type; use {@link VoidEnum} if state management is unused
*
* @apiviz.landmark
* @apiviz.has io.netty.handler.codec.replay.UnreplayableOperationException oneway - - throws
*/
public abstract class ReplayingDecoder<T extends Enum<T>>
extends SimpleChannelUpstreamHandler {
private ChannelBuffer cumulation;
private boolean needsCleanup;
private final boolean unfold;
private ReplayingDecoderBuffer replayable;
private T state;
private int checkpoint;
/**
* Creates a new instance with no initial state (i.e: {@code null}).
*/
protected ReplayingDecoder() {
this(null);
}
protected ReplayingDecoder(boolean unfold) {
this(null, unfold);
}
/**
* Creates a new instance with the specified initial state.
*/
protected ReplayingDecoder(T initialState) {
this(initialState, false);
}
protected ReplayingDecoder(T initialState, boolean unfold) {
this.state = initialState;
this.unfold = unfold;
}
/**
* Stores the internal cumulative buffer's reader position.
*/
protected void checkpoint() {
ChannelBuffer cumulation = this.cumulation;
if (cumulation != null) {
checkpoint = cumulation.readerIndex();
} else {
checkpoint = -1; // buffer not available (already cleaned up)
}
}
/**
* Stores the internal cumulative buffer's reader position and updates
* the current decoder state.
*/
protected void checkpoint(T state) {
checkpoint();
setState(state);
}
/**
* Returns the current state of this decoder.
* @return the current state of this decoder
*/
protected T getState() {
return state;
}
/**
* Sets the current state of this decoder.
* @return the old state of this decoder
*/
protected T setState(T newState) {
T oldState = state;
state = newState;
return oldState;
}
/**
* 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 {@link #internalBuffer() internalBuffer().readableBytes()}.
*/
protected int actualReadableBytes() {
return internalBuffer().readableBytes();
}
/**
* 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.
*/
protected ChannelBuffer internalBuffer() {
ChannelBuffer buf = this.cumulation;
if (buf == null) {
return ChannelBuffers.EMPTY_BUFFER;
}
return buf;
}
/**
* 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.
* @param state the current decoder state ({@code null} if unused)
*
* @return the decoded frame
*/
protected abstract Object decode(ChannelHandlerContext ctx,
Channel channel, ChannelBuffer buffer, T state) 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.
* @param state the current decoder state ({@code null} if unused)
*
* @return the decoded frame
*/
protected Object decodeLast(
ChannelHandlerContext ctx, Channel channel, ChannelBuffer buffer, T state) throws Exception {
return decode(ctx, channel, buffer, state);
}
@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;
}
ChannelBuffer cumulation = cumulation(ctx);
needsCleanup = true;
cumulation.discardReadBytes();
cumulation.writeBytes(input);
callDecode(ctx, e.getChannel(), cumulation, e.getRemoteAddress());
}
@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);
}
private void callDecode(ChannelHandlerContext context, Channel channel, ChannelBuffer cumulation, SocketAddress remoteAddress) throws Exception {
while (cumulation.readable()) {
int oldReaderIndex = checkpoint = cumulation.readerIndex();
Object result = null;
T oldState = state;
try {
result = decode(context, channel, replayable, state);
if (result == null) {
if (oldReaderIndex == cumulation.readerIndex() && oldState == state) {
throw new IllegalStateException(
"null cannot be returned if no data is consumed and state didn't change.");
} else {
// Previous data has been discarded or caused state transition.
// Probably it is reading on.
continue;
}
}
} catch (ReplayError replay) {
// Return to the checkpoint (or oldPosition) and retry.
int checkpoint = this.checkpoint;
if (checkpoint >= 0) {
cumulation.readerIndex(checkpoint);
} else {
// Called by cleanup() - no need to maintain the readerIndex
// anymore because the buffer has been released already.
}
}
if (result == null) {
// Seems like more data is required.
// Let us wait for the next notification.
break;
}
if (oldReaderIndex == cumulation.readerIndex() && oldState == state) {
throw new IllegalStateException(
"decode() method must consume at least one byte " +
"if it returned a decoded message (caused by: " +
getClass() + ")");
}
// A successful decode
unfoldAndFireMessageReceived(context, result, remoteAddress);
if (!cumulation.readable()) {
this.cumulation = null;
replayable = ReplayingDecoderBuffer.EMPTY_BUFFER;
}
}
}
private void unfoldAndFireMessageReceived(
ChannelHandlerContext context, Object result, SocketAddress remoteAddress) {
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 {
if (!needsCleanup) {
return;
} else {
needsCleanup = false;
}
ChannelBuffer cumulation = this.cumulation;
this.cumulation = null;
replayable.terminate();
if (cumulation != null && cumulation.readable()) {
// Make sure all data was read before notifying a closed channel.
callDecode(ctx, e.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 partiallyDecoded = decodeLast(ctx, e.getChannel(), replayable, state);
if (partiallyDecoded != null) {
unfoldAndFireMessageReceived(ctx, partiallyDecoded, null);
}
} catch (ReplayError replay) {
// Ignore
} finally {
replayable = ReplayingDecoderBuffer.EMPTY_BUFFER;
ctx.sendUpstream(e);
}
}
private ChannelBuffer cumulation(ChannelHandlerContext ctx) {
ChannelBuffer buf = this.cumulation;
if (buf == null) {
if (cumulation == null) {
ChannelBufferFactory factory = ctx.getChannel().getConfig().getBufferFactory();
buf = new UnsafeDynamicChannelBuffer(factory);
cumulation = buf;
replayable = new ReplayingDecoderBuffer(buf);
} else {
buf = cumulation;
}
}
return buf;
}
}