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/*
 * 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:
 *
 *   https://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.ChannelHandlerContext;
import io.netty.channel.ChannelInboundHandler;
import io.netty.channel.ChannelInboundHandlerAdapter;
import io.netty.channel.ChannelPipeline;
import io.netty.util.ReferenceCountUtil;
import io.netty.util.ReferenceCounted;
import io.netty.util.internal.TypeParameterMatcher;

import java.util.List;

/**
 * {@link ChannelInboundHandlerAdapter} which decodes from one message to an other message.
 *
 *
 * For example here is an implementation which decodes a {@link String} to an {@link Integer} which represent
 * the length of the {@link String}.
 *
 * 
 *     public class StringToIntegerDecoder extends
 *             {@link MessageToMessageDecoder}<{@link String}> {
 *
 *         {@code @Override}
 *         public void decode({@link ChannelHandlerContext} ctx, {@link String} message,
 *                            List<Object> out) throws {@link Exception} {
 *             out.add(message.length());
 *         }
 *     }
 * 
* * 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 MessageToMessageDecoder} will call * {@link ReferenceCounted#release()} on decoded messages. * */ public abstract class MessageToMessageDecoder extends ChannelInboundHandlerAdapter { 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 MessageToMessageDecoder() { matcher = TypeParameterMatcher.find(this, MessageToMessageDecoder.class, "I"); } /** * Create a new instance * * @param inboundMessageType The type of messages to match and so decode */ protected MessageToMessageDecoder(Class inboundMessageType) { matcher = TypeParameterMatcher.get(inboundMessageType); } /** * Returns {@code true} if the given message should be handled. If {@code false} it will be passed to the next * {@link ChannelInboundHandler} in the {@link ChannelPipeline}. */ public boolean acceptInboundMessage(Object msg) throws Exception { return matcher.match(msg); } @Override public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception { CodecOutputList out = CodecOutputList.newInstance(); try { if (acceptInboundMessage(msg)) { @SuppressWarnings("unchecked") I cast = (I) msg; try { decode(ctx, cast, out); } finally { ReferenceCountUtil.release(cast); } } else { out.add(msg); } } catch (DecoderException e) { throw e; } catch (Exception e) { throw new DecoderException(e); } finally { try { int size = out.size(); for (int i = 0; i < size; i++) { ctx.fireChannelRead(out.getUnsafe(i)); } } finally { out.recycle(); } } } /** * Decode from one message to an other. This method will be called for each written message that can be handled * by this decoder. * * @param ctx the {@link ChannelHandlerContext} which this {@link MessageToMessageDecoder} belongs to * @param msg the message to decode to an other one * @param out the {@link List} to which decoded messages should be added * @throws Exception is thrown if an error occurs */ protected abstract void decode(ChannelHandlerContext ctx, I msg, List out) throws Exception; }