dn

1.接口

1.DataTransferProtocol

readBlock ：从dn读块
writeBlock：将dn块写入管道pipeline
blockChekcum：获得指定块的校验值

2.Sender和Receiver

class DataXceiver extends Receiver implements Runnable
public abstract class Receiver implements DataTransferProtocol
public class Sender implements DataTransferProtocol
3.DataXceiver

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