Netty源碼分析-MessageToMessageDecoder

 

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;

//消息到消息的解碼器
public abstract class MessageToMessageDecoder<I> extends ChannelInboundHandlerAdapter {
	
	//類型匹配器
    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<? extends I> 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}.
     */
	//匹配定義類時指定的數據類型<I>
    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 {
			//判斷msg是否是《I》類型
            if (acceptInboundMessage(msg)) {
                @SuppressWarnings("unchecked")
                I cast = (I) msg;
                try {
					//子類解碼邏輯
                    decode(ctx, cast, out);
                } finally {
					//釋放msg
                    ReferenceCountUtil.release(cast);
                }
            } else {
				//不是I類型則傳遞msg
                out.add(msg);
            }
        } catch (DecoderException e) {
            throw e;
        } catch (Exception e) {
            throw new DecoderException(e);
        } finally {
			//傳遞給PPLine後續解碼器
            int size = out.size();
            for (int i = 0; i < size; i ++) {
                ctx.fireChannelRead(out.getUnsafe(i));
            }
            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<Object> out) throws Exception;
}

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