Netty源码学习-FrameDecoder

Netty 3.x的user guide里FrameDecoder的例子,有几个疑问:
1.文档说:FrameDecoder calls decode method with an internally maintained cumulative buffer whenever new data is received.
为什么每次有新数据到达时,都会调用decode方法?
2.Decoder与其他handler在pipeline的顺序是怎样的?谁先谁后?
3.所要接收的数据可能要经过多次才能接收完全,那这之前数据是如何保留?

先说结论:
1.因为每一次消息到达时都会触发pipeline的Upstream处理流程,最终会调用handler的messageReceived方法,而FrameDecoder的messageRecieved方法会调用decode方法
2.Decoder在前
3.FrameDecoder维护了一个ChannelBuffer(作为它的field)

文档中TimeDecoder的decode方法:

protected Object decode(
            ChannelHandlerContext ctx, Channel channel, ChannelBuffer buffer)2 {
            
        if (buffer.readableBytes() < 4) {
            return null; 3
        }
        
        return buffer.readBytes(4);
    }




查看一下FrameDecoder 源码:

FrameDecoder继承自SimpleChannelUpstreamHandler,而SimpleChannelUpstreamHandler实现了ChannelUpstreamHandler接口
ChannelUpstreamHandler很简单,只定义了一个handleUpstream方法

要理解FrameDecoder,首先要理解Netty的event在pipeline里面是怎么流转处理的,可以参看这篇博文 http://bylijinnan.iteye.com/blog/1981763
其次,理解Upstream和Downstream
我是这样理解的,对于Handler来说,Handler接收的消息是Upstream;从Handler发出的消息,是Downstream
然后,我们就可以理解FrameDecoder的流程了:

-->ChannlPipeline 开始处理Upstream,会调用sendUpstream方法,
-->调用SimpleChannelUpstreamHandler(也就是FrameDecoder)的handleUpstream方法
而SimpleChannelUpstreamHandler的handleUpstream会触发messageReceived方法:

public void handleUpstream(
				ChannelHandlerContext ctx, ChannelEvent e) throws Exception {

			if (e instanceof MessageEvent) {
				messageReceived(ctx, (MessageEvent) e);
			} 
			/*omit others*/


-->FrameDecoder重写了messageReceived方法,在messageReceived里面,就会调用到文章开头提到的decode方法:

public void messageReceived(
            ChannelHandlerContext ctx, MessageEvent e) {
				
				/*只保留关键代码*/
				ChannelBuffer input = (ChannelBuffer) e.getMessage();
				callDecode(ctx, e.getChannel(), input, e.getRemoteAddress());
                updateCumulation(ctx, input);
		}
	


而callDecode方法的关键代码是这样的:

private void callDecode(
            ChannelHandlerContext context, Channel channel,
            ChannelBuffer cumulation, SocketAddress remoteAddress) throws Exception {

			while (cumulation.readable()) {
				Object frame = decode(context, channel, cumulation);
				unfoldAndFireMessageReceived(context, remoteAddress, frame);
			}
		}


在unfoldAndFireMessageReceived方法里面,会调用Channels.fireMessageReceived,最后会调用ctx.sendUpstream,事件会交给下一个Handler来处理
在示例中,下一个Handler就是TimeClientHandler
因此,如果TimeDecoder(extends FrameDecoder)的decode方法返回了UnixTime对象,那TimeClientHandler就可以直接拿到并强制转换成UnixTime对象:
TimeDecoder:

protected Object decode(
				ChannelHandlerContext ctx, Channel channel, ChannelBuffer buffer) {
			if (buffer.readableBytes() < 4) {
				return null;
			}
			return new UnixTime(buffer.readInt());1
		}


TimeClientHandler:

public void messageReceived(ChannelHandlerContext ctx, MessageEvent e) {
			UnixTime m = (UnixTime) e.getMessage();
		}



pipeline里面对Upstream的处理,在handler链表里面,是从head到tail,因此TimeDecoder应在TimeClientHandler之前:

bootstrap.setPipelineFactory(new ChannelPipelineFactory() {
            public ChannelPipeline getPipeline() {
                return Channels.pipeline(
                        new TimeDecoder(),
                        new TimeClientHandler());
            }
        });



开发中,只需要重写FrameDecoder.decode方法就可以了
由于Netty接收到数据后,会不断触发整个Upstream的处理流程,像上面分析的那样,因此,decode方法就会不断被调用


最后一个问题,FrameDecoder是怎样保留之前接收到的数据呢?
原来FrameDecoder维护了一个ChannelBuffer(作为它的field),源码里命名为cumulation
cumulation会一直保留数据,并按需扩容,直到所需要的数据全部接收到达为止

看看源码:
	
	//decode得到的结果可能是一个数组或者集合,如果unfold=true,则让每一个元素都触发一个fireMessageReceived事件
	private boolean unfold;
    protected ChannelBuffer cumulation;	//将每次接收到的数据累积起来,直到数据接收完全
    private volatile ChannelHandlerContext ctx;
	
	//cumulation使用了CompositeChannelBuffer来避免“memory copy”(ChannelBuffers.wrappedBuffer)
	//cumulation是否需要开辟新内存空间并复制数据,取决于两个值:
	//1.copyThreshold是从cumulation的总大小来限制
	//2.maxCumulationBufferComponents是从cumulation的component的个数来限制
	
	//如果cumulation的大小超过此值,就把cumulation复制到一个新创建的ChannelBuffer里面
    private int copyThreshold;		
	
	//如果cumulation中component的个数超过此值,则像上面那样进行复制
    private int maxCumulationBufferComponents;
	
	//这个方法体现了上面据说的第2个限制:maxCumulationBufferComponents
    protected ChannelBuffer appendToCumulation(ChannelBuffer input) {
        ChannelBuffer cumulation = this.cumulation;
        assert cumulation.readable();
        if (cumulation instanceof CompositeChannelBuffer) {
            // Make sure the resulting cumulation buffer has no more than the configured components.
            CompositeChannelBuffer composite = (CompositeChannelBuffer) cumulation;
            if (composite.numComponents() >= maxCumulationBufferComponents) {
                cumulation = composite.copy();
            }
        }

        this.cumulation = input = ChannelBuffers.wrappedBuffer(cumulation, input);
        return input;
    }

	//这个方法体现了上面据说的第1个限制:copyThreshold
    protected ChannelBuffer updateCumulation(ChannelHandlerContext ctx, ChannelBuffer input) {
        ChannelBuffer newCumulation;
        int readableBytes = input.readableBytes();
        if (readableBytes > 0) {
            int inputCapacity = input.capacity();

			//超限了
            if (readableBytes < inputCapacity && inputCapacity > copyThreshold) {
			
				//newCumulationBuffer方法新建一个指定大小的ChannelBuffer
                cumulation = newCumulation = newCumulationBuffer(ctx, input.readableBytes());
                cumulation.writeBytes(input);
            } else {
                if (input.readerIndex() != 0) {
                    cumulation = newCumulation = input.slice();
                } else {
                    cumulation = newCumulation = input;
                }
            }
        } else {
            cumulation = newCumulation = null;
        }
        return newCumulation;
    }
	
	//unfold的用途
	protected final void unfoldAndFireMessageReceived(
            ChannelHandlerContext context, SocketAddress remoteAddress, Object result) {
        if (unfold) {
            if (result instanceof Object[]) {
                for (Object r: (Object[]) result) {
                    Channels.fireMessageReceived(context, r, remoteAddress);
                }
            } else if (result instanceof Iterable<?>) {
                for (Object r: (Iterable<?>) result) {
                    Channels.fireMessageReceived(context, r, remoteAddress);
                }
            } else {
                Channels.fireMessageReceived(context, result, remoteAddress);
            }
        } else {
            Channels.fireMessageReceived(context, result, remoteAddress);
        }
    }
	






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