/*
* 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.channel.ChannelHandler;
import io.netty.channel.ChannelHandlerAdapter;
import io.netty.channel.ChannelHandlerContext;
import io.netty.channel.ChannelPipeline;
import io.netty.channel.ChannelPromise;
import io.netty.util.ReferenceCountUtil;
import io.netty.util.ReferenceCounted;
import io.netty.util.internal.RecyclableArrayList;
import io.netty.util.internal.StringUtil;
import io.netty.util.internal.TypeParameterMatcher;
import java.util.List;
/**
* A {@link ChannelHandler} which encodes from one message to an other message
*
* For example here is an implementation which decodes an {@link Integer} to an {@link String}.
*
* <pre>
* public class IntegerToStringEncoder extends
* {@link MessageToMessageEncoder}<{@link Integer}> {
*
* {@code @Override}
* public void encode({@link ChannelHandlerContext} ctx, {@link Integer} message, List<Object> out)
* throws {@link Exception} {
* out.add(message.toString());
* }
* }
* </pre>
*
* Be aware that you need to call {@link ReferenceCounted#retain()} on messages that are just passed through if they
* are of type {@link ReferenceCounted}. This is needed as the {@link MessageToMessageEncoder} will call
* {@link ReferenceCounted#release()} on encoded messages.
*/
public abstract class MessageToMessageEncoder<I> extends ChannelHandlerAdapter {
private final TypeParameterMatcher matcher;
/**
* Create a new instance which will try to detect the types to match out of the type parameter of the class.
*/
protected MessageToMessageEncoder() {
matcher = TypeParameterMatcher.find(this, MessageToMessageEncoder.class, "I");
}
/**
* Create a new instance
*
* @param outboundMessageType The type of messages to match and so encode
*/
protected MessageToMessageEncoder(Class<? extends I> outboundMessageType) {
matcher = TypeParameterMatcher.get(outboundMessageType);
}
/**
* Returns {@code true} if the given message should be handled. If {@code false} it will be passed to the next
* {@link ChannelHandler} in the {@link ChannelPipeline}.
*/
public boolean acceptOutboundMessage(Object msg) throws Exception {
return matcher.match(msg);
}
@Override
public void write(ChannelHandlerContext ctx, Object msg, ChannelPromise promise) throws Exception {
RecyclableArrayList out = null;
try {
if (acceptOutboundMessage(msg)) {
out = RecyclableArrayList.newInstance();
@SuppressWarnings("unchecked")
I cast = (I) msg;
try {
encode(ctx, cast, out);
} finally {
ReferenceCountUtil.release(cast);
}
if (out.isEmpty()) {
out.recycle();
out = null;
throw new EncoderException(
StringUtil.simpleClassName(this) + " must produce at least one message.");
}
} else {
ctx.write(msg, promise);
}
} catch (EncoderException e) {
throw e;
} catch (Throwable t) {
throw new EncoderException(t);
} finally {
if (out != null) {
final int sizeMinusOne = out.size() - 1;
if (sizeMinusOne == 0) {
ctx.write(out.get(0), promise);
} else if (sizeMinusOne > 0) {
// Check if we can use a voidPromise for our extra writes to reduce GC-Pressure
// See https://github.com/netty/netty/issues/2525
ChannelPromise voidPromise = ctx.voidPromise();
boolean isVoidPromise = promise == voidPromise;
for (int i = 0; i < sizeMinusOne; i ++) {
ChannelPromise p;
if (isVoidPromise) {
p = voidPromise;
} else {
p = ctx.newPromise();
}
ctx.write(out.get(i), p);
}
ctx.write(out.get(sizeMinusOne), promise);
}
out.recycle();
}
}
}
/**
* Encode from one message to an other. This method will be called for each written message that can be handled
* by this encoder.
*
* @param ctx the {@link ChannelHandlerContext} which this {@link MessageToMessageEncoder} belongs to
* @param msg the message to encode to an other one
* @param out the {@link List} into which the encoded msg should be added
* needs to do some kind of aggragation
* @throws Exception is thrown if an error accour
*/
protected abstract void encode(ChannelHandlerContext ctx, I msg, List<Object> out) throws Exception;
}