Netty源码分析-MessageToByteEncoder

MessageToByteEncoder是一个抽象编码器，子类可重新encode方法把对象编码为ByteBuf输出。

 

源码分析

package io.netty.handler.codec;

import io.netty.buffer.ByteBuf;
import io.netty.buffer.Unpooled;
import io.netty.channel.ChannelHandlerContext;
import io.netty.channel.ChannelOutboundHandler;
import io.netty.channel.ChannelOutboundHandlerAdapter;
import io.netty.channel.ChannelPipeline;
import io.netty.channel.ChannelPromise;
import io.netty.util.ReferenceCountUtil;
import io.netty.util.internal.TypeParameterMatcher;


public abstract class MessageToByteEncoder<I> extends ChannelOutboundHandlerAdapter {

    private final TypeParameterMatcher matcher;
    private final boolean preferDirect;

   
    protected MessageToByteEncoder() {
        this(true);
    }

    //匹配msg的数据类型是否为I，也就是子类定义的泛型
    public boolean acceptOutboundMessage(Object msg) throws Exception {
        return matcher.match(msg);
    }

    @Override
    public void write(ChannelHandlerContext ctx, Object msg, ChannelPromise promise) throws Exception {
        ByteBuf buf = null;
        try {
            //msg是I这种类型就调用编码逻辑，否则向下传递
            if (acceptOutboundMessage(msg)) {
                @SuppressWarnings("unchecked")
                I cast = (I) msg; //类型转换
                //分配ByteBuf，默认堆外类型
                buf = allocateBuffer(ctx, cast, preferDirect);
                try {
                    //子类实现编码逻辑
                    encode(ctx, cast, buf);
                } finally {
                    //释放msg
                    ReferenceCountUtil.release(cast);
                }

                //如果buf可读，则写入ctx，实际就是交给netty调用底层socket输出
                if (buf.isReadable()) {
                    ctx.write(buf, promise);
                } else {
                    //否则释放buf
                    buf.release();
                    //向下传递空的buf对象
                    ctx.write(Unpooled.EMPTY_BUFFER, promise);
                }
                buf = null;
            } else {
                //如果不是I类型，则次编码器不做任何处理，直接传递msg对象给下一个编码器
                ctx.write(msg, promise);
            }
        } catch (EncoderException e) {
            throw e;
        } catch (Throwable e) {
            throw new EncoderException(e);
        } finally {
            if (buf != null) {
                buf.release();
            }
        }
    }

   
    protected ByteBuf allocateBuffer(ChannelHandlerContext ctx, @SuppressWarnings("unused") I msg,
                               boolean preferDirect) throws Exception {
        if (preferDirect) {
            return ctx.alloc().ioBuffer();
        } else {
            return ctx.alloc().heapBuffer();
        }
    }

    
    //需要子类实现，根据msg的内容，把数据写入ByteBuf中
    protected abstract void encode(ChannelHandlerContext ctx, I msg, ByteBuf out) throws Exception;

    //默认使用堆外内存
    protected boolean isPreferDirect() {
        return preferDirect;
    }
}

 

例子，一个Integer类型的编码，负责把一个Integer写入到ByteBuf当中。

public class IntegerEncoder extends MessageToByteEncoder<Integer> {
          @Override
         public void encode(ChannelHandlerContext ctx, Integer msg, ByteBuf out)
                 throws Exception {
             out.writeInt(msg);
         }
     }