BlockListAsLongs 类
BlockListAsLonngs 类继承自 java.lang.Iterable 接口
CacheDirective
CacheDirective 类继承自 org.apache.hadoop.util.InstrusiveCollection 类
类注释：Namenode class that tracks state related to a cached path.
FSLimitException
FSLimitException类继承自 org.apache.hadoop.hdfs.protocol.QuotaExceededException
类注释：Abstract class for deriving exceptions related to filesystem constraints.
LayoutFlags
类注释：LayoutFlags represent features which the FSImage and edit logs can either support or not, independently of layout version.
LayoutVersion
类注释：This class tracks changes in the layout version of HDFS.
Layout version is changed for following reasons:
a. The layout of how namenode or datanode stores information on disk changes.
b. A new operation code is added to the editlog.
c. Modification such as format of a record, content of a record in editlog or fsimage.
RecoveryInProgressException
RecoveryInProgressException 类继承自 IOException
类注释：Exception indicating that a replica is already being recovery.
RollingUpgradeException
RollingUpgradeException 类继承自 IOException
类注释：Exception related to rolling upgrade.
SnapshotException
SnapshotException 类继承自 IOException
类注释：Snapshot related exception
SnapshotInfo
类注释：SnapshotInfo maintains information for a snapshot.
UnregisteredNodeException
UnregisteredNodeException 类继承自 IOException
类注释：This exception is thrown when a node that has not previously registered is trying to access the name node.