org.apache.hadoop.hdfs.protocol 包内类：

• AlreadyBeingCreatedException.java
• The exception that happens when you ask to create a file that already is being created, but is not closed yet.
• 当要求创建一个正在被创建且没有被关闭的文件会抛出这样的错误
• Block.java
• A Block is a Hadoop FS primitive, identified by a long.
• Block 是Hadoop FS 的基元，由long 型定义

• BlockListAsLongs.java
• This class provides an interface for accessing list of blocks that has been implemented as long[]. This class is usefull for block report. Rather than send block reports as a Block[] we can send it as a long[].
• 这个类提供了一个long[] 类型的借口用于访问一组数据块，这个类在DataNode 向NameNode 汇报块时有用。

• BlockLocalPathInfo.java
• A block and the full path information to the block data file and the metadata file stored on the local file system.
• 本地数据块和它在本地元数据的全路径。(0.20.0 版本无，故删除)

• ClientDatanodeProtocol.java
• An client-datanode protocol for block recovery
• 继承VersionedProtocol 的一个接口，用于块恢复。

• ClientProtocol.java
• ClientProtocol is used by user code via  {@link org.apache.hadoop.hdfs.DistributedFileSystem} class to communicate  with the NameNode.  User code can manipulate the directory namespace,  as well as open/close file streams, etc.
• ClientProtocol 是Client 端通过org.apache.hadoop.hdfs.DistributedFileSystem 类打开、关闭文件流操作的目录名字空间的

• DatanodeID.java
• DatanodeID is composed of the data node name (hostname:portNumber) and the data storage ID, which it currently represents.
• DatanodeID 由数据节点名称（主机名：端口号）和数据存储ID

• DatanodeInfo.java
• DatanodeInfo represents the status of a DataNode. This object is used for communication in the Datanode Protocol and the Client Protocol.
• DatanodeInfo 代表DataNode 的状态。这个对象是用来在Datanode Protocol 和Client Protocol中进行通信的。

• DataTransferProtocol
• The Client transfers data to/from datanode using a streaming protocol.
• 定义了一些public static final byte 变量，用于Client 流式数据传输协议。

• FSConstants.java 接口
• Some handy constants
• 常用的变量。（每次最小写入5 个数据块，路径长度最多8k，深度最深1k）

• LocatedBlock.java
• A LocatedBlock is a pair of Block, DatanodeInfo[] objects. It tells where to find a Block.
• LocatedBlock 是一组Block，DatanodeInfo[] 对象集合，通过它可以找到一个块（关于块，参考）

• LocatedBlocks.java
• Collection of blocks with their locations and the file length.
• 通过LocatedBlock.java 或者一组数据块的位置和文件长度。

• QuotaExceededException 继承IOException
• The message for the exception specifies the directory where the quota was violated and actual quotas.
• 消息指出异常目录和实际限制

• UnregisteredDatanodeException
• This exception is thrown when a datanode that has not previously registered is trying to access the name node.
• 这个异常是为了应对没有在NameNode 上注册的数据节点要进入NameNode 时抛出的异常，比如其他hadoop 集群部署。

UML 图：