Netty源码分析-MessageToMessageDecoder

Posted 2022-07-30 征服.刘华强

 

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;

//消息到消息的解码器
public abstract class MessageToMessageDecoder<I> extends ChannelInboundHandlerAdapter 
	
	//类型匹配器
    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.
     */
	//匹配定义类时指定的数据类型<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 
			//判断msg是否是《I》类型
            if (acceptInboundMessage(msg)) 
                @SuppressWarnings("unchecked")
                I cast = (I) msg;
                try 
					//子类解码逻辑
                    decode(ctx, cast, out);
                 finally 
					//释放msg
                    ReferenceCountUtil.release(cast);
                
             else 
				//不是I类型则传递msg
                out.add(msg);
            
         catch (DecoderException e) 
            throw e;
         catch (Exception e) 
            throw new DecoderException(e);
         finally 
			//传递给PPLine后续解码器
            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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