Netty源码学习-ObjectEncoder和ObjectDecoder
Netty中传递对象的思路很直观：
Netty中数据的传递是基于ChannelBuffer（也就是byte[]）；
那把对象序列化为字节流，就可以在Netty中传递对象了
相应的从ChannelBuffer恢复对象，就是反序列化的过程

Netty已经封装好ObjectEncoder和ObjectDecoder

先看ObjectEncoder
ObjectEncoder是往外发送对象，因此ObjectEncoder肯定是一个ChannelDownstreamHandler
ObjectEncoder extends OneToOneEncoder，而OneToOneEncoder implements ChannelDownstreamHandler
OneToOneEncoder主要是做两件事情
一是encode
二是把消息发送到紧接着的下一个ChannelDownstreamHandler
其中第一个操作交由子类实现，是一个抽象方法
关键源码如下：

public void handleDownstream(            ChannelHandlerContext ctx, ChannelEvent evt) throws Exception {        if (!(evt instanceof MessageEvent)) {            ctx.sendDownstream(evt);            return;        }        MessageEvent e = (MessageEvent) evt;        Object originalMessage = e.getMessage();        Object encodedMessage = encode(ctx, e.getChannel(), originalMessage);        if (originalMessage == encodedMessage) {            ctx.sendDownstream(evt);        } else if (encodedMessage != null) {            write(ctx, e.getFuture(), encodedMessage, e.getRemoteAddress());        }}

ObjectEncoder当中实现了encode方法：

protected Object encode(ChannelHandlerContext ctx, Channel channel, Object msg) throws Exception {        ChannelBufferOutputStream bout =            new ChannelBufferOutputStream(dynamicBuffer(                    estimatedLength, ctx.getChannel().getConfig().getBufferFactory()));        bout.write(LENGTH_PLACEHOLDER);        ObjectOutputStream oout = new CompactObjectOutputStream(bout);        oout.writeObject(msg);        oout.flush();        oout.close();        ChannelBuffer encoded = bout.buffer();        encoded.setInt(0, encoded.writerIndex() - 4);        return encoded;    }

简单地说就是把对象序列化后，转为ChannelBuffer并返回
注意到，返回的ChannelBuffer的最前面4个字节（int），这个数字，代表了该对象转为字节流后，字节流的长度
因此decode时，就要用到LengthFieldBasedFrameDecoder
所以，ObjectDecoder extends LengthFieldBasedFrameDecoder 就一点也不奇怪了：

public class ObjectDecoder extends LengthFieldBasedFrameDecoder {               /*lengthFieldLength = 4，initialBytesToStrip = 4（去掉前面4个字节）*/public ObjectDecoder(int maxObjectSize, ClassResolver classResolver) {super(maxObjectSize, 0, 4, 0, 4);this.classResolver = classResolver;}@Overrideprotected Object decode(ChannelHandlerContext ctx, Channel channel, ChannelBuffer buffer) throws Exception {//根据header（length） + content的格式，取得contentChannelBuffer frame = (ChannelBuffer) super.decode(ctx, channel, buffer);if (frame == null) {return null;}//反序列化return new CompactObjectInputStream(new ChannelBufferInputStream(frame), classResolver).readObject();}/*重写了父类的这个方法，避免了memory copy。本方法被decode方法调用在LengthFieldBasedFrameDecoder写道：If you are sure that the frame and its content are not accessed afterthe current {@link #decode(ChannelHandlerContext, Channel, ChannelBuffer)}call returns, you can even avoid memory copy by returning the sliced*/@Overrideprotected ChannelBuffer extractFrame(ChannelBuffer buffer, int index, int length) {return buffer.slice(index, length);}}

分析完毕

如果要自己实现ObjectEncoder和ObjectDecoder，可参考下面这篇文章：
http://www.coderli.com/netty-custom-object-codec

最后看一下Netty里面常用的StringEncoder和StringEncoder

由于String太常用了，因此ChannelBuffers这个工具类，
提供了String和ChannelBuffer的相互转化：

static String decodeString(ByteBuffer src, Charset charset) static ByteBuffer encodeString(CharBuffer src, Charset charset)

StringDecoder的decode很简单，ChannelBuffer直接转化为String：

@Override    protected Object decode(            ChannelHandlerContext ctx, Channel channel, Object msg) throws Exception {        if (!(msg instanceof ChannelBuffer)) {            return msg;        }        return ((ChannelBuffer) msg).toString(charset);    }

而((ChannelBuffer) msg).toString(charset)方法最终调用的是：

return ChannelBuffers.decodeString(                toByteBuffer(index, length), charset);

StringEncoder的encode就麻烦一点
encode最终会调用ChannelsBuffer的copiedBuffer方法：

    private static ChannelBuffer copiedBuffer(ByteOrder endianness, CharBuffer buffer, Charset charset) {        CharBuffer src = buffer;        ByteBuffer dst = ChannelBuffers.encodeString(src, charset);        ChannelBuffer result = wrappedBuffer(endianness, dst.array());        result.writerIndex(dst.remaining());        return result;    }

注意到，调用ChannelBuffers.encodeString返回的ByteBuffer还要转化为ChannelBuffer