HDFS命令实现分析

The File System (FS) shell includes various shell-like commands that directly interact with the Hadoop Distributed File System (HDFS) as well as other file systems that 
Hadoop supports, such as Local FS, WebHDFS, S3 FS, and others. The FS shell is invoked by:
bin/hadoop fs <args>
All FS shell commands take path URIs as arguments. The URI format is scheme://authority/path. For HDFS the scheme is hdfs, and for the Local FS the scheme is file. 
The scheme and authority are optional. If not specified, the default scheme specified in the configuration is used. An HDFS file or directory such as /parent/child can be specified as hdfs://namenodehost/parent/child or simply as /parent/child (given that your configuration is set to point to hdfs://namenodehost).
Most of the commands in FS shell behave like corresponding Unix commands. Differences are described with each of the commands. Error information is sent to stderr and the output is sent to stdout.
If HDFS is being used, hdfs dfs is a synonym.
Relative paths can be used. For HDFS, the current working directory is the HDFS home directory /user/<username> that often has to be created manually. The HDFS home directory can also be implicitly accessed, e.g., when using the HDFS trash folder, the .Trash directory in the home directory.

FsShell: 是整个hdfs命令的核心，负责各种操作实现类的注册（通过反射实现）和命令的分发执行。
FsCommand: 各种操作命令注册实现。
CommandFactoryL: 命令注册机制的实现。主要通过工厂模式利用反射实现。
GenericOptionsParser.java： 中间转换操作，实际执行命令的仍是FsShell.run()函数。 
GenericOptionsParser.java: 负责命令输入参数的解析。
ToolRunner.java: 执行命令操作，实际上是FsShell.run()执行。

 //org/apache/hadoop/fs/FsShell.java
protected void init() throws IOException {
    getConf().setQuietMode(true);
    UserGroupInformation.setConfiguration(getConf());
    if (commandFactory == null) {
      commandFactory = new CommandFactory(getConf());
      commandFactory.addObject(new Help(), "-help");
      commandFactory.addObject(new Usage(), "-usage");
      registerCommands(commandFactory);
    }
  }

  protected void registerCommands(CommandFactory factory) {
    // TODO: DFSAdmin subclasses FsShell so need to protect the command
    // registration.  This class should morph into a base class for
    // commands, and then this method can be abstract
    if (this.getClass().equals(FsShell.class)) {
      factory.registerCommands(FsCommand.class);
    }
  }

//org/apache/hadoop/fs/shell/CommandFactory.java
/**
   * Invokes "static void registerCommands(CommandFactory)" on the given class.
   * This method abstracts the contract between the factory and the command
   * class.  Do not assume that directly invoking registerCommands on the
   * given class will have the same effect.
   * @param registrarClass class to allow an opportunity to register
   */
  public void registerCommands(Class<?> registrarClass) {
    try {
      registrarClass.getMethod(
          "registerCommands", CommandFactory.class
      ).invoke(null, this);
    } catch (Exception e) {
      throw new RuntimeException(StringUtils.stringifyException(e));
    }
  }

 //org/apache/hadoop/fs/shell/FsCommand.java
 /**
   * Register the command classes used by the fs subcommand
   * @param factory where to register the class
   */
  public static void registerCommands(CommandFactory factory) {
    factory.registerCommands(AclCommands.class);
    factory.registerCommands(CopyCommands.class);
    factory.registerCommands(Count.class);
    factory.registerCommands(Delete.class);
    factory.registerCommands(Display.class);
    factory.registerCommands(Find.class);
    factory.registerCommands(FsShellPermissions.class);
    factory.registerCommands(FsUsage.class);
    factory.registerCommands(Ls.class);
    factory.registerCommands(Mkdir.class);
    factory.registerCommands(MoveCommands.class);
    factory.registerCommands(SetReplication.class);
    factory.registerCommands(Stat.class);
    factory.registerCommands(Tail.class);
    factory.registerCommands(Head.class);
    factory.registerCommands(Test.class);
    factory.registerCommands(Touch.class);
    factory.registerCommands(Truncate.class);
    factory.registerCommands(SnapshotCommands.class);
    factory.registerCommands(XAttrCommands.class);
  }