Netty源码学习-DelimiterBasedFrameDecoder


看DelimiterBasedFrameDecoder的API,有举例:
接收到的ChannelBuffer如下:
 +--------------+
 | ABC\nDEF\r\n |
 +--------------+

经过DelimiterBasedFrameDecoder(Delimiters.lineDelimiter())之后,得到:
 +-----+-----+
 | ABC | DEF |
 +-----+-----+

而不是
 +----------+
 | ABC\nDEF |


为什么 ?

首先要明确,如果不指定,DelimiterBasedFrameDecoder默认会去掉分隔符

其次看看Delimiters.lineDelimiter(),它返回两组delimiter,分别对应windows和linux的换行符
   
 public static ChannelBuffer[] lineDelimiter() {
        return new ChannelBuffer[] {
                ChannelBuffers.wrappedBuffer(new byte[] { '\r', '\n' }),
                ChannelBuffers.wrappedBuffer(new byte[] { '\n' }),
        };
    }

考察这两组分隔符

方案一
采用“\r\n”作为分隔,则返回
frameA = "ABC\nDEF"

方案二
采用“\n”返回
frameB_0 = "ABC"
frameB_1 = "DEF\r"

由于frameB_0的长度比frameA短,因此在这个例子中,会采用方案二

但有个问题,为什么不是比较全部,而是只比较frameB_0?
要知道,length(frameA) = length(frameB_0) + length(frameB_1),两者相等

刚开始,我还以为跟split一样,方案二会一次性返回“ABCDEF\r”
实际上不是
它是遇到一个分隔符,就返回一个结果

可以通过下面的代码证明:
public class ClientHandler extends SimpleChannelUpstreamHandler {
    @Override
    public void channelConnected(ChannelHandlerContext ctx, ChannelStateEvent e)
            throws Exception {
        String msg = "ABC\nDEF\r\n";
        ChannelBuffer buff = ChannelBuffers.buffer(msg.length());
        buff.writeBytes(msg.getBytes());
        e.getChannel().write(buff);
    }
}

Server:
      
 bootstrap.setPipelineFactory(new ChannelPipelineFactory() {
            public ChannelPipeline getPipeline() throws Exception {
                ChannelPipeline pipeline = Channels.pipeline();
				
				//这里设置:不删除分隔符,方便观察
                pipeline.addLast("handler1", new DelimiterBasedFrameDecoder(8192, false, Delimiters.lineDelimiter()));
                pipeline.addLast("handler2", new ServerStringHandler());		//打印decode后的结果
                return pipeline;
            }
        });

ServerStringHandler:
public class ServerStringHandler extends SimpleChannelUpstreamHandler{

    @Override
    public void messageReceived(ChannelHandlerContext ctx, MessageEvent e)
            throws Exception {
        ChannelBuffer buff = (ChannelBuffer)e.getMessage();
        String msg = (String)buff.toString(Helper.CHARSET_UTF8);
        
        //String s = "abc\n"; 则msg_escape 会原样输出“abc\n”,而不是“abc”外加一个换行
        String msg_escape = StringEscapeUtils.escapeJava(msg);  
        System.out.println("msg = " + msg_escape);
    }
}	


结果ServerStringHandler会分两次输出:
msg = ABC\n
msg = DEF\r\n


查看源码,会更清楚:
public class DelimiterBasedFrameDecoder extends FrameDecoder {

    private final ChannelBuffer[] delimiters;
    private final int maxFrameLength;
	
	/*返回结果中,是否去掉分隔符
	通常的调用是去掉分隔符,例如
	new DelimiterBasedFrameDecoder(8192, Delimiters.lineDelimiter())
	等价于
	new DelimiterBasedFrameDecoder(8192, /*stripDelimiter=*/true, Delimiters.lineDelimiter())
	*/
    private final boolean stripDelimiter;	
	
	@Override
    protected Object decode(
            ChannelHandlerContext ctx, Channel channel, ChannelBuffer buffer) throws Exception {
        // Try all delimiters and choose the delimiter which yields the shortest frame.
        int minFrameLength = Integer.MAX_VALUE;
        ChannelBuffer minDelim = null;
		
		/*迭代每一个delimiter,都尝试进行decode,
		然后选择返回“shortest frame”的那个delimiter
		重点在indexOf这个方法
		*/
        for (ChannelBuffer delim: delimiters) {
            int frameLength = indexOf(buffer, delim);
            if (frameLength >= 0 && frameLength < minFrameLength) {
                minFrameLength = frameLength;
                minDelim = delim;
            }
        }

        if (minDelim != null) {
            int minDelimLength = minDelim.capacity();
            ChannelBuffer frame;
            if (stripDelimiter) {
                frame = buffer.readBytes(minFrameLength);
                buffer.skipBytes(minDelimLength);
            } else {
                frame = buffer.readBytes(minFrameLength + minDelimLength);
            }

            return frame;
        }
    }
	
	/*
	对frame(haystack)进行搜索,找到第一个delimiter(needle),这个位置记为i
	返回 (i - haystack.readerIndex),也就是分隔后第一个sub frame的长度
	可以看到,它是“找到一个,就返回一个”
	*/
	private static int indexOf(ChannelBuffer haystack, ChannelBuffer needle) {
		//遍历haystack的每一个字节
        for (int i = haystack.readerIndex(); i < haystack.writerIndex(); i ++) {
            int haystackIndex = i;
            int needleIndex;
			
			/*haystack是否出现了delimiter,注意delimiter是一个ChannelBuffer(byte[])
			例如对于haystack="ABC\r\nDEF",needle="\r\n"
			那么当haystackIndex=3时,找到了“\r”,此时needleIndex=0
			继续执行循环,haystackIndex++,needleIndex++,
			找到了“\n”
			至此,整个needle都匹配到了
			程序然后执行到if (needleIndex == needle.capacity()),返回结果
			*/
            for (needleIndex = 0; needleIndex < needle.capacity(); needleIndex ++) {
                if (haystack.getByte(haystackIndex) != needle.getByte(needleIndex)) {
                    break;
                } else {
                    haystackIndex ++;
                    if (haystackIndex == haystack.writerIndex() &&
                        needleIndex != needle.capacity() - 1) {
                        return -1;
                    }
                }
            }

            if (needleIndex == needle.capacity()) {
                // Found the needle from the haystack!
                return i - haystack.readerIndex();
            }
        }
        return -1;
    }

	
}

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