/*
* Copyright 2012 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;
import io.netty.buffer.ByteBuf;
import io.netty.buffer.MessageBuf;
import io.netty.channel.ChannelHandler;
import io.netty.channel.ChannelHandlerContext;
import io.netty.channel.ChannelPipeline;
import io.netty.util.Signal;
/**
* A specialized variation of {@link ByteToMessageDecoder} which enables implementation
* of a non-blocking decoder in the blocking I/O paradigm.
* <p>
* The biggest difference between {@link ReplayingDecoder} and
* {@link ByteToMessageDecoder} 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 ByteToByteDecoder} implementation:
* <pre>
* public class IntegerHeaderFrameDecoder extends {@link ByteToMessageDecoder}<{@link ByteBuf}> {
*
* {@code @Override}
* protected ByteBuf decode({@link ChannelHandlerContext} ctx,
* {@link ByteBuf} in) throws Exception {
*
* if (in.readableBytes() < 4) {
* return <strong>null</strong>;
* }
*
* in.markReaderIndex();
* int length = in.readInt();
*
* if (in.readableBytes() < length) {
* in.resetReaderIndex();
* return <strong>null</strong>;
* }
*
* return in.readBytes(length);
* }
* }
* </pre>
* is simplified like the following with {@link ReplayingDecoder}:
* <pre>
* public class IntegerHeaderFrameDecoder
* extends {@link ReplayingDecoder}<{@link ByteBuf},{@link Void}> {
*
* protected Object decode({@link ChannelHandlerContext} ctx,
* {@link ByteBuf} buf) throws Exception {
*
* return buf.readBytes(buf.readInt());
* }
* }
* </pre>
*
* <h3>How does this work?</h3>
* <p>
* {@link ReplayingDecoder} passes a specialized {@link ByteBuf}
* 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 Integer}, {@link Void}> {
*
* private final Queue<Integer> values = new LinkedList<Integer>();
*
* {@code @Override}
* public {@link Integer} decode(.., {@link ByteBuf} in) 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 Integer}, {@link Void}> {
*
* private final Queue<Integer> values = new LinkedList<Integer>();
*
* {@code @Override}
* public {@link Integer} decode(.., {@link ByteBuf} 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 {@link Object} decode({@link ChannelHandlerContext} ctx,
* {@link ByteBuf} in) throws Exception {
* switch (state()) {
* case READ_LENGTH:
* length = buf.readInt();
* <strong>checkpoint(MyDecoderState.READ_CONTENT);</strong>
* case READ_CONTENT:
* ByteBuf 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 Void}</strong>> {
*
* <strong>private boolean readLength;</strong>
* private int length;
*
* {@code @Override}
* protected {@link Object} decode({@link ChannelHandlerContext} ctx,
* {@link ByteBuf} in) throws Exception {
* if (!readLength) {
* length = buf.readInt();
* <strong>readLength = true;</strong>
* <strong>checkpoint();</strong>
* }
*
* if (readLength) {
* ByteBuf 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}, {@link ByteToByteDecoder}, {@link ByteToMessageDecoder} or {@link MessageToMessageDecoder}
* (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 Void}> {
*
* {@code @Override}
* protected Object decode({@link ChannelHandlerContext} ctx,
* {@link ByteBuf} in) {
* ...
* // Decode the first message
* Object firstMessage = ...;
*
* // Add the second decoder
* ctx.pipeline().addLast("second", new SecondDecoder());
*
* // Remove the first decoder (me)
* ctx.pipeline().remove(this);
*
* if (buf.isReadable()) {
* // 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 <S>
* the state type which is usually an {@link Enum}; use {@link Void} if state management is
* unused
*/
public abstract class ReplayingDecoder<S> extends ByteToMessageDecoder {
static final Signal REPLAY = new Signal(ReplayingDecoder.class.getName() + ".REPLAY");
private ByteBuf cumulation;
private ReplayingDecoderBuffer replayable;
private S state;
private int checkpoint = -1;
private boolean decodeWasNull;
/**
* Creates a new instance with no initial state (i.e: {@code null}).
*/
protected ReplayingDecoder() {
this(null);
}
/**
* Creates a new instance with the specified initial state.
*/
protected ReplayingDecoder(S initialState) {
state = initialState;
}
/**
* Stores the internal cumulative buffer's reader position.
*/
protected void checkpoint() {
checkpoint = cumulation.readerIndex();
}
/**
* Stores the internal cumulative buffer's reader position and updates
* the current decoder state.
*/
protected void checkpoint(S state) {
checkpoint();
state(state);
}
/**
* Returns the current state of this decoder.
* @return the current state of this decoder
*/
protected S state() {
return state;
}
/**
* Sets the current state of this decoder.
* @return the old state of this decoder
*/
protected S state(S newState) {
S 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 ByteBuf internalBuffer() {
return cumulation;
}
@Override
public final ByteBuf newInboundBuffer(ChannelHandlerContext ctx) throws Exception {
cumulation = newInboundBuffer0(ctx);
replayable = new ReplayingDecoderBuffer(cumulation);
return cumulation;
}
protected ByteBuf newInboundBuffer0(ChannelHandlerContext ctx) throws Exception {
return super.newInboundBuffer(ctx);
}
@Override
public final void discardInboundReadBytes(ChannelHandlerContext ctx) throws Exception {
ByteBuf in = ctx.inboundByteBuffer();
final int oldReaderIndex = in.readerIndex();
discardInboundReadBytes0(ctx);
final int newReaderIndex = in.readerIndex();
checkpoint -= oldReaderIndex - newReaderIndex;
}
protected void discardInboundReadBytes0(ChannelHandlerContext ctx) throws Exception {
super.discardInboundReadBytes(ctx);
}
@Override
public void channelInactive(ChannelHandlerContext ctx) throws Exception {
try {
replayable.terminate();
ByteBuf in = cumulation;
if (in.isReadable()) {
callDecode(ctx, in);
}
if (ctx.nextInboundMessageBuffer().unfoldAndAdd(decodeLast(ctx, replayable))) {
ctx.fireInboundBufferUpdated();
}
} catch (Signal replay) {
// Ignore
replay.expect(REPLAY);
} catch (Throwable t) {
if (t instanceof CodecException) {
ctx.fireExceptionCaught(t);
} else {
ctx.fireExceptionCaught(new DecoderException(t));
}
} finally {
ctx.fireChannelInactive();
}
}
@Override
protected void callDecode(ChannelHandlerContext ctx, ByteBuf buf) {
boolean wasNull = false;
ByteBuf in = cumulation;
MessageBuf<Object> out = ctx.nextInboundMessageBuffer();
boolean decoded = false;
while (in.isReadable()) {
try {
int oldReaderIndex = checkpoint = in.readerIndex();
Object result = null;
S oldState = state;
try {
result = decode(ctx, replayable);
if (result == null) {
if (oldReaderIndex == in.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 (Signal replay) {
replay.expect(REPLAY);
// Return to the checkpoint (or oldPosition) and retry.
int checkpoint = this.checkpoint;
if (checkpoint >= 0) {
in.readerIndex(checkpoint);
} else {
// Called by cleanup() - no need to maintain the readerIndex
// anymore because the buffer has been released already.
}
}
if (result == null) {
wasNull = true;
// Seems like more data is required.
// Let us wait for the next notification.
break;
}
wasNull = false;
if (oldReaderIndex == in.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
if (out.unfoldAndAdd(result)) {
decoded = true;
if (isSingleDecode()) {
break;
}
}
} catch (Throwable t) {
if (decoded) {
decoded = false;
ctx.fireInboundBufferUpdated();
}
if (t instanceof CodecException) {
ctx.fireExceptionCaught(t);
} else {
ctx.fireExceptionCaught(new DecoderException(t));
}
break;
}
}
if (decoded) {
decodeWasNull = false;
ctx.fireInboundBufferUpdated();
} else {
if (wasNull) {
decodeWasNull = true;
}
}
}
@Override
public void channelReadSuspended(ChannelHandlerContext ctx) throws Exception {
if (decodeWasNull) {
decodeWasNull = false;
if (!ctx.channel().config().isAutoRead()) {
ctx.read();
}
}
super.channelReadSuspended(ctx);
}
}