hadoop文件系统与I/O流

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hadoop借鉴了Linux虚拟文件系统的概念，引入了hadoop抽象文件系统，并在此基础上，提供了大量的具体文件系统的实现，满足构建于hadoop上应用的各种数据访问需求

hadoop文件系统API

hadoop提供一个抽象的文件系统，HDFS只是这个抽象文件系统的一个具体的实现。hadoop文件系统的抽象类org.apache.hadoop.fs.FileSystem

hadoop抽象文件系统的方法可以分为两部分：

1、用于处理文件和目录的相关事务

2、用于读写文件数据

hadoop抽象文件系统的操作

通过FileSystem.getFileStatus()方法，Hadoop抽象文件系统可以一次获得文件/目录的所有属性，这些属性被保存在类FileStatus中

public class FileStatus implements Writable, Comparable {

      private Path path; //文件路径
      private long length;  //文件长度
      private boolean isdir;  //是否是目录
      private short block_replication;  //副本数(为HDFS而准的特殊参数)
      private long blocksize;    //块大小(为HDFS而准的特殊参数)
      private long modification_time;    //最后修改时间
      private long access_time;        //最后访问时间
      private FsPermission permission;    //许可信息
      private String owner;            //文件所有者
      private String group;            //用户组
      ……
}

FileStatus实现了Writable接口，也就是说，FileStatus可以被序列化后在网络上传输，同时一次性将文件的所有属性读出并返回到客户端，可以减少在分布式系统中进行网络传输的次数

完整的FileStatus类的源代码如下：

/**
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package org.apache.hadoop.fs;

import java.io.DataInput;
import java.io.DataOutput;
import java.io.IOException;

import org.apache.hadoop.fs.permission.FsPermission;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.io.Writable;

/** Interface that represents the client side information for a file.
 */
public class FileStatus implements Writable, Comparable {

  private Path path;
  private long length;
  private boolean isdir;
  private short block_replication;
  private long blocksize;
  private long modification_time;
  private long access_time;
  private FsPermission permission;
  private String owner;
  private String group;

  public FileStatus() { this(0, false, 0, 0, 0, 0, null, null, null, null); }

  //We should deprecate this soon?
  public FileStatus(long length, boolean isdir, int block_replication,
                    long blocksize, long modification_time, Path path) {

    this(length, isdir, block_replication, blocksize, modification_time,
         0, null, null, null, path);
  }

  public FileStatus(long length, boolean isdir, int block_replication,
                    long blocksize, long modification_time, long access_time,
                    FsPermission permission, String owner, String group,
                    Path path) {
    this.length = length;
    this.isdir = isdir;
    this.block_replication = (short)block_replication;
    this.blocksize = blocksize;
    this.modification_time = modification_time;
    this.access_time = access_time;
    this.permission = (permission == null) ?
                      FsPermission.getDefault() : permission;
    this.owner = (owner == null) ? "" : owner;
    this.group = (group == null) ? "" : group;
    this.path = path;
  }

  /*
   * @return the length of this file, in blocks
   */
  public long getLen() {
    return length;
  }

  /**
   * Is this a directory?
   * @return true if this is a directory
   */
  public boolean isDir() {
    return isdir;
  }

  /**
   * Get the block size of the file.
   * @return the number of bytes
   */
  public long getBlockSize() {
    return blocksize;
  }

  /**
   * Get the replication factor of a file.
   * @return the replication factor of a file.
   */
  public short getReplication() {
    return block_replication;
  }

  /**
   * Get the modification time of the file.
   * @return the modification time of file in milliseconds since January 1, 1970 UTC.
   */
  public long getModificationTime() {
    return modification_time;
  }

  /**
   * Get the access time of the file.
   * @return the access time of file in milliseconds since January 1, 1970 UTC.
   */
  public long getAccessTime() {
    return access_time;
  }

  /**
   * Get FsPermission associated with the file.
   * @return permssion. If a filesystem does not have a notion of permissions
   *         or if permissions could not be determined, then default
   *         permissions equivalent of "rwxrwxrwx" is returned.
   */
  public FsPermission getPermission() {
    return permission;
  }

  /**
   * Get the owner of the file.
   * @return owner of the file. The string could be empty if there is no
   *         notion of owner of a file in a filesystem or if it could not
   *         be determined (rare).
   */
  public String getOwner() {
    return owner;
  }

  /**
   * Get the group associated with the file.
   * @return group for the file. The string could be empty if there is no
   *         notion of group of a file in a filesystem or if it could not
   *         be determined (rare).
   */
  public String getGroup() {
    return group;
  }

  public Path getPath() {
    return path;
  }

  /* These are provided so that these values could be loaded lazily
   * by a filesystem (e.g. local file system).
   */

  /**
   * Sets permission.
   * @param permission if permission is null, default value is set
   */
  protected void setPermission(FsPermission permission) {
    this.permission = (permission == null) ?
                      FsPermission.getDefault() : permission;
  }

  /**
   * Sets owner.
   * @param owner if it is null, default value is set
   */
  protected void setOwner(String owner) {
    this.owner = (owner == null) ? "" : owner;
  }

  /**
   * Sets group.
   * @param group if it is null, default value is set
   */
  protected void setGroup(String group) {
    this.group = (group == null) ? "" :  group;
  }

  //////////////////////////////////////////////////
  // Writable
  //////////////////////////////////////////////////
  public void write(DataOutput out) throws IOException {
    Text.writeString(out, getPath().toString());
    out.writeLong(length);
    out.writeBoolean(isdir);
    out.writeShort(block_replication);
    out.writeLong(blocksize);
    out.writeLong(modification_time);
    out.writeLong(access_time);
    permission.write(out);
    Text.writeString(out, owner);
    Text.writeString(out, group);
  }

  public void readFields(DataInput in) throws IOException {
    String strPath = Text.readString(in);
    this.path = new Path(strPath);
    this.length = in.readLong();
    this.isdir = in.readBoolean();
    this.block_replication = in.readShort();
    blocksize = in.readLong();
    modification_time = in.readLong();
    access_time = in.readLong();
    permission.readFields(in);
    owner = Text.readString(in);
    group = Text.readString(in);
  }

  /**
   * Compare this object to another object
   *
   * @param   o the object to be compared.
   * @return  a negative integer, zero, or a positive integer as this object
   *   is less than, equal to, or greater than the specified object.
   *
   * @throws ClassCastException if the specified object‘s is not of
   *         type FileStatus
   */
  public int compareTo(Object o) {
    FileStatus other = (FileStatus)o;
    return this.getPath().compareTo(other.getPath());
  }

  /** Compare if this object is equal to another object
   * @param   o the object to be compared.
   * @return  true if two file status has the same path name; false if not.
   */
  public boolean equals(Object o) {
    if (o == null) {
      return false;
    }
    if (this == o) {
      return true;
    }
    if (!(o instanceof FileStatus)) {
      return false;
    }
    FileStatus other = (FileStatus)o;
    return this.getPath().equals(other.getPath());
  }

  /**
   * Returns a hash code value for the object, which is defined as
   * the hash code of the path name.
   *
   * @return  a hash code value for the path name.
   */
  public int hashCode() {
    return getPath().hashCode();
  }
}

FileStatus

出现在FileSystem中的，但在java文件API中找不到对应的方法有：setReplication()、getReplication()、getContentSummary()，其声明如下：

public boolean setReplication(Path src, short replication)
    throws IOException {
    return true;
}
public short getReplication(Path src) throws IOException {
    return getFileStatus(src).getReplication();
}
public ContentSummary getContentSummary(Path f) throws IOException {
    FileStatus status = getFileStatus(f);
    if (!status.isDir()) {
      // f is a file
      return new ContentSummary(status.getLen(), 1, 0);
    }
    // f is a directory
    long[] summary = {0, 0, 1};
    for(FileStatus s : listStatus(f)) {
      ContentSummary c = s.isDir() ? getContentSummary(s.getPath()) :
                                     new ContentSummary(s.getLen(), 1, 0);
      summary[0] += c.getLength();
      summary[1] += c.getFileCount();
      summary[2] += c.getDirectoryCount();
    }
    return new ContentSummary(summary[0], summary[1], summary[2]);
}

实现一个Hadoop具体文件系统，需要实现的功能有哪些？下面整理org.apache.hadoop.fs.FileSystem中的抽象方法：

//获取文件系统URI
public abstract URI getUri();

//为读打开一个文件，并返回一个输入流
public abstract FSDataInputStream open(Path f, int bufferSize) throws IOException;

//创建一个文件，并返回一个输出流
public abstract FSDataOutputStream create(Path f,
          FsPermission permission,
          boolean overwrite,
          int bufferSize,
          short replication,
          long blockSize,
          Progressable progress) throws IOException;

//在一个已经存在的文件中追加数据
public abstract FSDataOutputStream append(Path f, int bufferSize,
          Progressable progress) throws IOException;

//修改文件名或目录名
public abstract boolean rename(Path src, Path dst) throws IOException;

//删除文件
public abstract boolean delete(Path f) throws IOException;
public abstract boolean delete(Path f, boolean recursive) throws IOException;

//如果Path是一个目录，读取一个目录下的所有项目和项目属性
//如果Path是一个文件，获取文件属性
public abstract FileStatus[] listStatus(Path f) throws IOException;

//设置当前的工作目录
public abstract void setWorkingDirectory(Path new_dir);

//获取当前的工作目录
public abstract Path getWorkingDirectory();

//如果Path是一个文件，获取文件属性
public abstract boolean mkdirs(Path f, FsPermission permission
          ) throws IOException;

//获取文件或目录的属性
public abstract FileStatus getFileStatus(Path f) throws IOException;

实现一个具体的文件系统，至少需要实现上面的这些抽象方法

hadoop完整的FileSystem类的源代码如下：

/**
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package org.apache.hadoop.fs;

import java.io.Closeable;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.net.URI;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collection;
import java.util.HashMap;
import java.util.IdentityHashMap;
import java.util.Iterator;
import java.util.List;
import java.util.Map;
import java.util.Set;
import java.util.TreeSet;
import java.util.concurrent.atomic.AtomicLong;
import java.util.regex.Pattern;

import javax.security.auth.login.LoginException;

import org.apache.commons.logging.*;

import org.apache.hadoop.conf.*;
import org.apache.hadoop.net.NetUtils;
import org.apache.hadoop.util.*;
import org.apache.hadoop.fs.permission.FsPermission;
import org.apache.hadoop.io.MultipleIOException;
import org.apache.hadoop.security.UserGroupInformation;

/****************************************************************
 * An abstract base class for a fairly generic filesystem.  It
 * may be implemented as a distributed filesystem, or as a "local"
 * one that reflects the locally-connected disk.  The local version
 * exists for small Hadoop instances and for testing.
 *
 * <p>
 *
 * All user code that may potentially use the Hadoop Distributed
 * File System should be written to use a FileSystem object.  The
 * Hadoop DFS is a multi-machine system that appears as a single
 * disk.  It‘s useful because of its fault tolerance and potentially
 * very large capacity.
 *
 * <p>
 * The local implementation is {@link LocalFileSystem} and distributed
 * implementation is DistributedFileSystem.
 *****************************************************************/
public abstract class FileSystem extends Configured implements Closeable {
  private static final String FS_DEFAULT_NAME_KEY = "fs.default.name";

  public static final Log LOG = LogFactory.getLog(FileSystem.class);

  /** FileSystem cache */
  private static final Cache CACHE = new Cache();

  /** The key this instance is stored under in the cache. */
  private Cache.Key key;

  /** Recording statistics per a FileSystem class */
  private static final Map<Class<? extends FileSystem>, Statistics>
    statisticsTable =
      new IdentityHashMap<Class<? extends FileSystem>, Statistics>();

  /**
   * The statistics for this file system.
   */
  protected Statistics statistics;

  /**
   * A cache of files that should be deleted when filsystem is closed
   * or the JVM is exited.
   */
  private Set<Path> deleteOnExit = new TreeSet<Path>();

  /** Returns the configured filesystem implementation.*/
  public static FileSystem get(Configuration conf) throws IOException {
    return get(getDefaultUri(conf), conf);
  }

  /** Get the default filesystem URI from a configuration.
   * @param conf the configuration to access
   * @return the uri of the default filesystem
   */
  public static URI getDefaultUri(Configuration conf) {
    return URI.create(fixName(conf.get(FS_DEFAULT_NAME_KEY, "file:///")));
  }

  /** Set the default filesystem URI in a configuration.
   * @param conf the configuration to alter
   * @param uri the new default filesystem uri
   */
  public static void setDefaultUri(Configuration conf, URI uri) {
    conf.set(FS_DEFAULT_NAME_KEY, uri.toString());
  }

  /** Set the default filesystem URI in a configuration.
   * @param conf the configuration to alter
   * @param uri the new default filesystem uri
   */
  public static void setDefaultUri(Configuration conf, String uri) {
    setDefaultUri(conf, URI.create(fixName(uri)));
  }

  /** Called after a new FileSystem instance is constructed.
   * @param name a uri whose authority section names the host, port, etc.
   *   for this FileSystem
   * @param conf the configuration
   */
  public void initialize(URI name, Configuration conf) throws IOException {
    statistics = getStatistics(name.getScheme(), getClass());
  }

  /** Returns a URI whose scheme and authority identify this FileSystem.*/
  public abstract URI getUri();

  /** @deprecated call #getUri() instead.*/
  public String getName() { return getUri().toString(); }

  /** @deprecated call #get(URI,Configuration) instead. */
  public static FileSystem getNamed(String name, Configuration conf)
    throws IOException {
    return get(URI.create(fixName(name)), conf);
  }

  /** Update old-format filesystem names, for back-compatibility.  This should
   * eventually be replaced with a checkName() method that throws an exception
   * for old-format names. */
  private static String fixName(String name) {
    // convert old-format name to new-format name
    if (name.equals("local")) {         // "local" is now "file:///".
      LOG.warn("\"local\" is a deprecated filesystem name."
               +" Use \"file:///\" instead.");
      name = "file:///";
    } else if (name.indexOf(‘/‘)==-1) {   // unqualified is "hdfs://"
      LOG.warn("\""+name+"\" is a deprecated filesystem name."
               +" Use \"hdfs://"+name+"/\" instead.");
      name = "hdfs://"+name;
    }
    return name;
  }

  /**
   * Get the local file syste
   * @param conf the configuration to configure the file system with
   * @return a LocalFileSystem
   */
  public static LocalFileSystem getLocal(Configuration conf)
    throws IOException {
    return (LocalFileSystem)get(LocalFileSystem.NAME, conf);
  }

  /** Returns the FileSystem for this URI‘s scheme and authority.  The scheme
   * of the URI determines a configuration property name,
   * <tt>fs.<i>scheme</i>.class</tt> whose value names the FileSystem class.
   * The entire URI is passed to the FileSystem instance‘s initialize method.
   */
  public static FileSystem get(URI uri, Configuration conf) throws IOException {
    String scheme = uri.getScheme();
    String authority = uri.getAuthority();

    if (scheme == null) {                       // no scheme: use default FS
      return get(conf);
    }

    if (authority == null) {                       // no authority
      URI defaultUri = getDefaultUri(conf);
      if (scheme.equals(defaultUri.getScheme())    // if scheme matches default
          && defaultUri.getAuthority() != null) {  // & default has authority
        return get(defaultUri, conf);              // return default
      }
    }

    String disableCacheName = String.format("fs.%s.impl.disable.cache", scheme);
    if (conf.getBoolean(disableCacheName, false)) {
      return createFileSystem(uri, conf);
    }

    return CACHE.get(uri, conf);
  }

  private static class ClientFinalizer extends Thread {
    public synchronized void run() {
      try {
        FileSystem.closeAll();
      } catch (IOException e) {
        LOG.info("FileSystem.closeAll() threw an exception:\n" + e);
      }
    }
  }
  private static final ClientFinalizer clientFinalizer = new ClientFinalizer();

  /**
   * Close all cached filesystems. Be sure those filesystems are not
   * used anymore.
   *
   * @throws IOException
   */
  public static void closeAll() throws IOException {
    CACHE.closeAll();
  }

  /** Make sure that a path specifies a FileSystem. */
  public Path makeQualified(Path path) {
    checkPath(path);
    return path.makeQualified(this);
  }

  /** create a file with the provided permission
   * The permission of the file is set to be the provided permission as in
   * setPermission, not permission&~umask
   *
   * It is implemented using two RPCs. It is understood that it is inefficient,
   * but the implementation is thread-safe. The other option is to change the
   * value of umask in configuration to be 0, but it is not thread-safe.
   *
   * @param fs file system handle
   * @param file the name of the file to be created
   * @param permission the permission of the file
   * @return an output stream
   * @throws IOException
   */
  public static FSDataOutputStream create(FileSystem fs,
      Path file, FsPermission permission) throws IOException {
    // create the file with default permission
    FSDataOutputStream out = fs.create(file);
    // set its permission to the supplied one
    fs.setPermission(file, permission);
    return out;
  }

  /** create a directory with the provided permission
   * The permission of the directory is set to be the provided permission as in
   * setPermission, not permission&~umask
   *
   * @see #create(FileSystem, Path, FsPermission)
   *
   * @param fs file system handle
   * @param dir the name of the directory to be created
   * @param permission the permission of the directory
   * @return true if the directory creation succeeds; false otherwise
   * @throws IOException
   */
  public static boolean mkdirs(FileSystem fs, Path dir, FsPermission permission)
  throws IOException {
    // create the directory using the default permission
    boolean result = fs.mkdirs(dir);
    // set its permission to be the supplied one
    fs.setPermission(dir, permission);
    return result;
  }

  ///////////////////////////////////////////////////////////////
  // FileSystem
  ///////////////////////////////////////////////////////////////

  protected FileSystem() {
    super(null);
  }

  /** Check that a Path belongs to this FileSystem. */
  protected void checkPath(Path path) {
    URI uri = path.toUri();
    if (uri.getScheme() == null)                // fs is relative
      return;
    String thisScheme = this.getUri().getScheme();
    String thatScheme = uri.getScheme();
    String thisAuthority = this.getUri().getAuthority();
    String thatAuthority = uri.getAuthority();
    //authority and scheme are not case sensitive
    if (thisScheme.equalsIgnoreCase(thatScheme)) {// schemes match
      if (thisAuthority == thatAuthority ||       // & authorities match
          (thisAuthority != null &&
           thisAuthority.equalsIgnoreCase(thatAuthority)))
        return;

      if (thatAuthority == null &&                // path‘s authority is null
          thisAuthority != null) {                // fs has an authority
        URI defaultUri = getDefaultUri(getConf()); // & is the conf default
        if (thisScheme.equalsIgnoreCase(defaultUri.getScheme()) &&
            thisAuthority.equalsIgnoreCase(defaultUri.getAuthority()))
          return;
        try {                                     // or the default fs‘s uri
          defaultUri = get(getConf()).getUri();
        } catch (IOException e) {
          throw new RuntimeException(e);
        }
        if (thisScheme.equalsIgnoreCase(defaultUri.getScheme()) &&
            thisAuthority.equalsIgnoreCase(defaultUri.getAuthority()))
          return;
      }
    }
    throw new IllegalArgumentException("Wrong FS: "+path+
                                       ", expected: "+this.getUri());
  }

  /**
   * Return an array containing hostnames, offset and size of
   * portions of the given file.  For a nonexistent
   * file or regions, null will be returned.
   *
   * This call is most helpful with DFS, where it returns
   * hostnames of machines that contain the given file.
   *
   * The FileSystem will simply return an elt containing ‘localhost‘.
   */
  public BlockLocation[] getFileBlockLocations(FileStatus file,
      long start, long len) throws IOException {
    if (file == null) {
      return null;
    }

    if ( (start<0) || (len < 0) ) {
      throw new IllegalArgumentException("Invalid start or len parameter");
    }

    if (file.getLen() < start) {
      return new BlockLocation[0];

    }
    String[] name = { "localhost:50010" };
    String[] host = { "localhost" };
    return new BlockLocation[] { new BlockLocation(name, host, 0, file.getLen()) };
  }

  /**
   * Opens an FSDataInputStream at the indicated Path.
   * @param f the file name to open
   * @param bufferSize the size of the buffer to be used.
   */
  public abstract FSDataInputStream open(Path f, int bufferSize)
    throws IOException;

  /**
   * Opens an FSDataInputStream at the indicated Path.
   * @param f the file to open
   */
  public FSDataInputStream open(Path f) throws IOException {
    return open(f, getConf().getInt("io.file.buffer.size", 4096));
  }

  /**
   * Opens an FSDataOutputStream at the indicated Path.
   * Files are overwritten by default.
   */
  public FSDataOutputStream create(Path f) throws IOException {
    return create(f, true);
  }

  /**
   * Opens an FSDataOutputStream at the indicated Path.
   */
  public FSDataOutputStream create(Path f, boolean overwrite)
    throws IOException {
    return create(f, overwrite,
                  getConf().getInt("io.file.buffer.size", 4096),
                  getDefaultReplication(),
                  getDefaultBlockSize());
  }

  /**
   * Create an FSDataOutputStream at the indicated Path with write-progress
   * reporting.
   * Files are overwritten by default.
   */
  public FSDataOutputStream create(Path f, Progressable progress) throws IOException {
    return create(f, true,
                  getConf().getInt("io.file.buffer.size", 4096),
                  getDefaultReplication(),
                  getDefaultBlockSize(), progress);
  }

  /**
   * Opens an FSDataOutputStream at the indicated Path.
   * Files are overwritten by default.
   */
  public FSDataOutputStream create(Path f, short replication)
    throws IOException {
    return create(f, true,
                  getConf().getInt("io.file.buffer.size", 4096),
                  replication,
                  getDefaultBlockSize());
  }

  /**
   * Opens an FSDataOutputStream at the indicated Path with write-progress
   * reporting.
   * Files are overwritten by default.
   */
  public FSDataOutputStream create(Path f, short replication, Progressable progress)
    throws IOException {
    return create(f, true,
                  getConf().getInt("io.file.buffer.size", 4096),
                  replication,
                  getDefaultBlockSize(), progress);
  }

  /**
   * Opens an FSDataOutputStream at the indicated Path.
   * @param f the file name to open
   * @param overwrite if a file with this name already exists, then if true,
   *   the file will be overwritten, and if false an error will be thrown.
   * @param bufferSize the size of the buffer to be used.
   */
  public FSDataOutputStream create(Path f,
                                   boolean overwrite,
                                   int bufferSize
                                   ) throws IOException {
    return create(f, overwrite, bufferSize,
                  getDefaultReplication(),
                  getDefaultBlockSize());
  }

  /**
   * Opens an FSDataOutputStream at the indicated Path with write-progress
   * reporting.
   * @param f the file name to open
   * @param overwrite if a file with this name already exists, then if true,
   *   the file will be overwritten, and if false an error will be thrown.
   * @param bufferSize the size of the buffer to be used.
   */
  public FSDataOutputStream create(Path f,
                                   boolean overwrite,
                                   int bufferSize,
                                   Progressable progress
                                   ) throws IOException {
    return create(f, overwrite, bufferSize,
                  getDefaultReplication(),
                  getDefaultBlockSize(), progress);
  }

  /**
   * Opens an FSDataOutputStream at the indicated Path.
   * @param f the file name to open
   * @param overwrite if a file with this name already exists, then if true,
   *   the file will be overwritten, and if false an error will be thrown.
   * @param bufferSize the size of the buffer to be used.
   * @param replication required block replication for the file.
   */
  public FSDataOutputStream create(Path f,
                                   boolean overwrite,
                                   int bufferSize,
                                   short replication,
                                   long blockSize
                                   ) throws IOException {
    return create(f, overwrite, bufferSize, replication, blockSize, null);
  }

  /**
   * Opens an FSDataOutputStream at the indicated Path with write-progress
   * reporting.
   * @param f the file name to open
   * @param overwrite if a file with this name already exists, then if true,
   *   the file will be overwritten, and if false an error will be thrown.
   * @param bufferSize the size of the buffer to be used.
   * @param replication required block replication for the file.
   */
  public FSDataOutputStream create(Path f,
                                            boolean overwrite,
                                            int bufferSize,
                                            short replication,
                                            long blockSize,
                                            Progressable progress
                                            ) throws IOException {
    return this.create(f, FsPermission.getDefault(),
        overwrite, bufferSize, replication, blockSize, progress);
  }

  /**
   * Opens an FSDataOutputStream at the indicated Path with write-progress
   * reporting.
   * @param f the file name to open
   * @param permission
   * @param overwrite if a file with this name already exists, then if true,
   *   the file will be overwritten, and if false an error will be thrown.
   * @param bufferSize the size of the buffer to be used.
   * @param replication required block replication for the file.
   * @param blockSize
   * @param progress
   * @throws IOException
   * @see #setPermission(Path, FsPermission)
   */
  public abstract FSDataOutputStream create(Path f,
      FsPermission permission,
      boolean overwrite,
      int bufferSize,
      short replication,
      long blockSize,
      Progressable progress) throws IOException;

  /**
   * Creates the given Path as a brand-new zero-length file.  If
   * create fails, or if it already existed, return false.
   */
  public boolean createNewFile(Path f) throws IOException {
    if (exists(f)) {
      return false;
    } else {
      create(f, false, getConf().getInt("io.file.buffer.size", 4096)).close();
      return true;
    }
  }

  /**
   * Append to an existing file (optional operation).
   * Same as append(f, getConf().getInt("io.file.buffer.size", 4096), null)
   * @param f the existing file to be appended.
   * @throws IOException
   */
  public FSDataOutputStream append(Path f) throws IOException {
    return append(f, getConf().getInt("io.file.buffer.size", 4096), null);
  }
  /**
   * Append to an existing file (optional operation).
   * Same as append(f, bufferSize, null).
   * @param f the existing file to be appended.
   * @param bufferSize the size of the buffer to be used.
   * @throws IOException
   */
  public FSDataOutputStream append(Path f, int bufferSize) throws IOException {
    return append(f, bufferSize, null);
  }

  /**
   * Append to an existing file (optional operation).
   * @param f the existing file to be appended.
   * @param bufferSize the size of the buffer to be used.
   * @param progress for reporting progress if it is not null.
   * @throws IOException
   */
  public abstract FSDataOutputStream append(Path f, int bufferSize,
      Progressable progress) throws IOException;

  /**
   * Get replication.
   *
   * @deprecated Use getFileStatus() instead
   * @param src file name
   * @return file replication
   * @throws IOException
   */
  @Deprecated
  public short getReplication(Path src) throws IOException {
    return getFileStatus(src).getReplication();
  }

  /**
   * Set replication for an existing file.
   *
   * @param src file name
   * @param replication new replication
   * @throws IOException
   * @return true if successful;
   *         false if file does not exist or is a directory
   */
  public boolean setReplication(Path src, short replication)
    throws IOException {
    return true;
  }

  /**
   * Renames Path src to Path dst.  Can take place on local fs
   * or remote DFS.
   */
  public abstract boolean rename(Path src, Path dst) throws IOException;

  /** Delete a file. */
  /** @deprecated Use delete(Path, boolean) instead */ @Deprecated
  public abstract boolean delete(Path f) throws IOException;

  /** Delete a file.
   *
   * @param f the path to delete.
   * @param recursive if path is a directory and set to
   * true, the directory is deleted else throws an exception. In
   * case of a file the recursive can be set to either true or false.
   * @return  true if delete is successful else false.
   * @throws IOException
   */
  public abstract boolean delete(Path f, boolean recursive) throws IOException;

  /**
   * Mark a path to be deleted when FileSystem is closed.
   * When the JVM shuts down,
   * all FileSystem objects will be closed automatically.
   * Then,
   * the marked path will be deleted as a result of closing the FileSystem.
   *
   * The path has to exist in the file system.
   *
   * @param f the path to delete.
   * @return  true if deleteOnExit is successful, otherwise false.
   * @throws IOException
   */
  public boolean deleteOnExit(Path f) throws IOException {
    if (!exists(f)) {
      return false;
    }
    synchronized (deleteOnExit) {
      deleteOnExit.add(f);
    }
    return true;
  }

  /**
   * Delete all files that were marked as delete-on-exit. This recursively
   * deletes all files in the specified paths.
   */
  protected void processDeleteOnExit() {
    synchronized (deleteOnExit) {
      for (Iterator<Path> iter = deleteOnExit.iterator(); iter.hasNext();) {
        Path path = iter.next();
        try {
          delete(path, true);
        }
        catch (IOException e) {
          LOG.info("Ignoring failure to deleteOnExit for path " + path);
        }
        iter.remove();
      }
    }
  }

  /** Check if exists.
   * @param f source file
   */
  public boolean exists(Path f) throws IOException {
    try {
      return getFileStatus(f) != null;
    } catch (FileNotFoundException e) {
      return false;
    }
  }

  /** True iff the named path is a directory. */
  /** @deprecated Use getFileStatus() instead */ @Deprecated
  public boolean isDirectory(Path f) throws IOException {
    try {
      return getFileStatus(f).isDir();
    } catch (FileNotFoundException e) {
      return false;               // f does not exist
    }
  }

  /** True iff the named path is a regular file. */
  public boolean isFile(Path f) throws IOException {
    try {
      return !getFileStatus(f).isDir();
    } catch (FileNotFoundException e) {
      return false;               // f does not exist
    }
  }

  /** The number of bytes in a file. */
  /** @deprecated Use getFileStatus() instead */ @Deprecated
  public long getLength(Path f) throws IOException {
    return getFileStatus(f).getLen();
  }

  /** Return the {@link ContentSummary} of a given {@link Path}. */
  public ContentSummary getContentSummary(Path f) throws IOException {
    FileStatus status = getFileStatus(f);
    if (!status.isDir()) {
      // f is a file
      return new ContentSummary(status.getLen(), 1, 0);
    }
    // f is a directory
    long[] summary = {0, 0, 1};
    for(FileStatus s : listStatus(f)) {
      ContentSummary c = s.isDir() ? getContentSummary(s.getPath()) :
                                     new ContentSummary(s.getLen(), 1, 0);
      summary[0] += c.getLength();
      summary[1] += c.getFileCount();
      summary[2] += c.getDirectoryCount();
    }
    return new ContentSummary(summary[0], summary[1], summary[2]);
  }

  final private static PathFilter DEFAULT_FILTER = new PathFilter() {
      public boolean accept(Path file) {
        return true;
      }
    };

  /**
   * List the statuses of the files/directories in the given path if the path is
   * a directory.
   *
   * @param f
   *          given path
   * @return the statuses of the files/directories in the given patch
   * @throws IOException
   */
  public abstract FileStatus[] listStatus(Path f) throws IOException;

  /*
   * Filter files/directories in the given path using the user-supplied path
   * filter. Results are added to the given array <code>results</code>.
   */
  private void listStatus(ArrayList<FileStatus> results, Path f,
      PathFilter filter) throws IOException {
    FileStatus listing[] = listStatus(f);
    if (listing != null) {
      for (int i = 0; i < listing.length; i++) {
        if (filter.accept(listing[i].getPath())) {
          results.add(listing[i]);
        }
      }
    }
  }

  /**
   * Filter files/directories in the given path using the user-supplied path
   * filter.
   *
   * @param f
   *          a path name
   * @param filter
   *          the user-supplied path filter
   * @return an array of FileStatus objects for the files under the given path
   *         after applying the filter
   * @throws IOException
   *           if encounter any problem while fetching the status
   */
  public FileStatus[] listStatus(Path f, PathFilter filter) throws IOException {
    ArrayList<FileStatus> results = new ArrayList<FileStatus>();
    listStatus(results, f, filter);
    return results.toArray(new FileStatus[results.size()]);
  }

  /**
   * Filter files/directories in the given list of paths using default
   * path filter.
   *
   * @param files
   *          a list of paths
   * @return a list of statuses for the files under the given paths after
   *         applying the filter default Path filter
   * @exception IOException
   */
  public FileStatus[] listStatus(Path[] files)
      throws IOException {
    return listStatus(files, DEFAULT_FILTER);
  }

  /**
   * Filter files/directories in the given list of paths using user-supplied
   * path filter.
   *
   * @param files
   *          a list of paths
   * @param filter
   *          the user-supplied path filter
   * @return a list of statuses for the files under the given paths after
   *         applying the filter
   * @exception IOException
   */
  public FileStatus[] listStatus(Path[] files, PathFilter filter)
      throws IOException {
    ArrayList<FileStatus> results = new ArrayList<FileStatus>();
    for (int i = 0; i < files.length; i++) {
      listStatus(results, files[i], filter);
    }
    return results.toArray(new FileStatus[results.size()]);
  }

  /**
   * <p>Return all the files that match filePattern and are not checksum
   * files. Results are sorted by their names.
   *
   * <p>
   * A filename pattern is composed of <i>regular</i> characters and
   * <i>special pattern matching</i> characters, which are:
   *
   * <dl>
   *  <dd>
   *   <dl>
   *    <p>
   *    <dt> <tt> ? </tt>
   *    <dd> Matches any single character.
   *
   *    <p>
   *    <dt> <tt> * </tt>
   *    <dd> Matches zero or more characters.
   *
   *    <p>
   *    <dt> <tt> [<i>abc</i>] </tt>
   *    <dd> Matches a single character from character set
   *     <tt>{<i>a,b,c</i>}</tt>.
   *
   *    <p>
   *    <dt> <tt> [<i>a</i>-<i>b</i>] </tt>
   *    <dd> Matches a single character from the character range
   *     <tt>{<i>a...b</i>}</tt>.  Note that character <tt><i>a</i></tt> must be
   *     lexicographically less than or equal to character <tt><i>b</i></tt>.
   *
   *    <p>
   *    <dt> <tt> [^<i>a</i>] </tt>
   *    <dd> Matches a single character that is not from character set or range
   *     <tt>{<i>a</i>}</tt>.  Note that the <tt>^</tt> character must occur
   *     immediately to the right of the opening bracket.
   *
   *    <p>
   *    <dt> <tt> \<i>c</i> </tt>
   *    <dd> Removes (escapes) any special meaning of character <i>c</i>.
   *
   *    <p>
   *    <dt> <tt> {ab,cd} </tt>
   *    <dd> Matches a string from the string set <tt>{<i>ab, cd</i>} </tt>
   *
   *    <p>
   *    <dt> <tt> {ab,c{de,fh}} </tt>
   *    <dd> Matches a string from the string set <tt>{<i>ab, cde, cfh</i>}</tt>
   *
   *   </dl>
   *  </dd>
   * </dl>
   *
   * @param pathPattern a regular expression specifying a pth pattern

   * @return an array of paths that match the path pattern
   * @throws IOException
   */
  public FileStatus[] globStatus(Path pathPattern) throws IOException {
    return globStatus(pathPattern, DEFAULT_FILTER);
  }

  /**
   * Return an array of FileStatus objects whose path names match pathPattern
   * and is accepted by the user-supplied path filter. Results are sorted by
   * their path names.
   * Return null if pathPattern has no glob and the path does not exist.
   * Return an empty array if pathPattern has a glob and no path matches it.
   *
   * @param pathPattern
   *          a regular expression specifying the path pattern
   * @param filter
   *          a user-supplied path filter
   * @return an array of FileStatus objects
   * @throws IOException if any I/O error occurs when fetching file status
   */
  public FileStatus[] globStatus(Path pathPattern, PathFilter filter)
      throws IOException {
    String filename = pathPattern.toUri().getPath();
    List<String> filePatterns = GlobExpander.expand(filename);
    if (filePatterns.size() == 1) {
      return globStatusInternal(pathPattern, filter);
    } else {
      List<FileStatus> results = new ArrayList<FileStatus>();
      for (String filePattern : filePatterns) {
        FileStatus[] files = globStatusInternal(new Path(filePattern), filter);
        for (FileStatus file : files) {
          results.add(file);
        }
      }
      return results.toArray(new FileStatus[results.size()]);
    }
  }

  private FileStatus[] globStatusInternal(Path pathPattern, PathFilter filter)
      throws IOException {
    Path[] parents = new Path[1];
    int level = 0;
    String filename = pathPattern.toUri().getPath();

    // path has only zero component
    if ("".equals(filename) || Path.SEPARATOR.equals(filename)) {
      return getFileStatus(new Path[]{pathPattern});
    }

    // path has at least one component
    String[] components = filename.split(Path.SEPARATOR);
    // get the first component
    if (pathPattern.isAbsolute()) {
      parents[0] = new Path(Path.SEPARATOR);
      level = 1;
    } else {
      parents[0] = new Path(Path.CUR_DIR);
    }

    // glob the paths that match the parent path, i.e., [0, components.length-1]
    boolean[] hasGlob = new boolean[]{false};
    Path[] parentPaths = globPathsLevel(parents, components, level, hasGlob);
    FileStatus[] results;
    if (parentPaths == null || parentPaths.length == 0) {
      results = null;
    } else {
      // Now work on the last component of the path
      GlobFilter fp = new GlobFilter(components[components.length - 1], filter);
      if (fp.hasPattern()) { // last component has a pattern
        // list parent directories and then glob the results
        results = listStatus(parentPaths, fp);
        hasGlob[0] = true;
      } else { // last component does not have a pattern
        // get all the path names
        ArrayList<Path> filteredPaths = new ArrayList<Path>(parentPaths.length);
        for (int i = 0; i < parentPaths.length; i++) {
          parentPaths[i] = new Path(parentPaths[i],
            components[components.length - 1]);
          if (fp.accept(parentPaths[i])) {
            filteredPaths.add(parentPaths[i]);
          }
        }
        // get all their statuses
        results = getFileStatus(
            filteredPaths.toArray(new Path[filteredPaths.size()]));
      }
    }

    // Decide if the pathPattern contains a glob or not
    if (results == null) {
      if (hasGlob[0]) {
        results = new FileStatus[0];
      }
    } else {
      if (results.length == 0 ) {
        if (!hasGlob[0]) {
          results = null;
        }
      } else {
        Arrays.sort(results);
      }
    }
    return results;
  }

  /*
   * For a path of N components, return a list of paths that match the
   * components [<code>level</code>, <code>N-1</code>].
   */
  private Path[] globPathsLevel(Path[] parents, String[] filePattern,
      int level, boolean[] hasGlob) throws IOException {
    if (level == filePattern.length - 1)
      return parents;
    if (parents == null || parents.length == 0) {
      return null;
    }
    GlobFilter fp = new GlobFilter(filePattern[level]);
    if (fp.hasPattern()) {
      parents = FileUtil.stat2Paths(listStatus(parents, fp));
      hasGlob[0] = true;
    } else {
      for (int i = 0; i < parents.length; i++) {
        parents[i] = new Path(parents[i], filePattern[level]);
      }
    }
    return globPathsLevel(parents, filePattern, level + 1, hasGlob);
  }

  /* A class that could decide if a string matches the glob or not */
  private static class GlobFilter implements PathFilter {
    private PathFilter userFilter = DEFAULT_FILTER;
    private Pattern regex;
    private boolean hasPattern = false;

    /** Default pattern character: Escape any special meaning. */
    private static final char  PAT_ESCAPE = ‘\\‘;
    /** Default pattern character: Any single character. */
    private static final char  PAT_ANY = ‘.‘;
    /** Default pattern character: Character set close. */
    private static final char  PAT_SET_CLOSE = ‘]‘;

    GlobFilter() {
    }

    GlobFilter(String filePattern) throws IOException {
      setRegex(filePattern);
    }

    GlobFilter(String filePattern, PathFilter filter) throws IOException {
      userFilter = filter;
      setRegex(filePattern);
    }

    private boolean isJavaRegexSpecialChar(char pChar) {
      return pChar == ‘.‘ || pChar == ‘$‘ || pChar == ‘(‘ || pChar == ‘)‘ ||
             pChar == ‘|‘ || pChar == ‘+‘;
    }
    void setRegex(String filePattern) throws IOException {
      int len;
      int setOpen;
      int curlyOpen;
      boolean setRange;

      StringBuilder fileRegex = new StringBuilder();

      // Validate the pattern
      len = filePattern.length();
      if (len == 0)
        return;

      setOpen = 0;
      setRange = false;
      curlyOpen = 0;

      for (int i = 0; i < len; i++) {
        char pCh;

        // Examine a single pattern character
        pCh = filePattern.charAt(i);
        if (pCh == PAT_ESCAPE) {
          fileRegex.append(pCh);
          i++;
          if (i >= len)
            error("An escaped character does not present", filePattern, i);
          pCh = filePattern.charAt(i);
        } else if (isJavaRegexSpecialChar(pCh)) {
          fileRegex.append(PAT_ESCAPE);
        } else if (pCh == ‘*‘) {
          fileRegex.append(PAT_ANY);
          hasPattern = true;
        } else if (pCh == ‘?‘) {
          pCh = PAT_ANY;
          hasPattern = true;
        } else if (pCh == ‘{‘) {
          fileRegex.append(‘(‘);
          pCh = ‘(‘;
          curlyOpen++;
          hasPattern = true;
        } else if (pCh == ‘,‘ && curlyOpen > 0) {
          fileRegex.append(")|");
          pCh = ‘(‘;
        } else if (pCh == ‘}‘ && curlyOpen > 0) {
          // End of a group
          curlyOpen--;
          fileRegex.append(")");
          pCh = ‘)‘;
        } else if (pCh == ‘[‘ && setOpen == 0) {
          setOpen++;
          hasPattern = true;
        } else if (pCh == ‘^‘ && setOpen > 0) {
        } else if (pCh == ‘-‘ && setOpen > 0) {
          // Character set range
          setRange = true;
        } else if (pCh == PAT_SET_CLOSE && setRange) {
          // Incomplete character set range
          error("Incomplete character set range", filePattern, i);
        } else if (pCh == PAT_SET_CLOSE && setOpen > 0) {
          // End of a character set
          if (setOpen < 2)
            error("Unexpected end of set", filePattern, i);
          setOpen = 0;
        } else if (setOpen > 0) {
          // Normal character, or the end of a character set range
          setOpen++;
          setRange = false;
        }
        fileRegex.append(pCh);
      }

      // Check for a well-formed pattern
      if (setOpen > 0 || setRange || curlyOpen > 0) {
        // Incomplete character set or character range
        error("Expecting set closure character or end of range, or }",
            filePattern, len);
      }
      regex = Pattern.compile(fileRegex.toString());
    }

    boolean hasPattern() {
      return hasPattern;
    }

    public boolean accept(Path path) {
      return regex.matcher(path.getName()).matches() && userFilter.accept(path);
    }

    private void error(String s, String pattern, int pos) throws IOException {
      throw new IOException("Illegal file pattern: "
                            +s+ " for glob "+ pattern + " at " + pos);
    }
  }

  /** Return the current user‘s home directory in this filesystem.
   * The default implementation returns "/user/$USER/".
   */
  public Path getHomeDirectory() {
    return new Path("/user/"+System.getProperty("user.name"))
      .makeQualified(this);
  }

  /**
   * Set the current working directory for the given file system. All relative
   * paths will be resolved relative to it.
   *
   * @param new_dir
   */
  public abstract void setWorkingDirectory(Path new_dir);

  /**
   * Get the current working directory for the given file system
   * @return the directory pathname
   */
  public abstract Path getWorkingDirectory();

  /**
   * Call {@link #mkdirs(Path, FsPermission)} with default permission.
   */
  public boolean mkdirs(Path f) throws IOException {
    return mkdirs(f, FsPermission.getDefault());
  }

  /**
   * Make the given file and all non-existent parents into
   * directories. Has the semantics of Unix ‘mkdir -p‘.
   * Existence of the directory hierarchy is not an error.
   */
  public abstract boolean mkdirs(Path f, FsPermission permission
      ) throws IOException;

  /**
   * The src file is on the local disk.  Add it to FS at
   * the given dst name and the source is kept intact afterwards
   */
  public void copyFromLocalFile(Path src, Path dst)
    throws IOException {
    copyFromLocalFile(false, src, dst);
  }

  /**
   * The src files is on the local disk.  Add it to FS at
   * the given dst name, removing the source afterwards.
   */
  public void moveFromLocalFile(Path[] srcs, Path dst)
    throws IOException {
    copyFromLocalFile(true, true, srcs, dst);
  }

  /**
   * The src file is on the local disk.  Add it to FS at
   * the given dst name, removing the source afterwards.
   */
  public void moveFromLocalFile(Path src, Path dst)
    throws IOException {
    copyFromLocalFile(true, src, dst);
  }

  /**
   * The src file is on the local disk.  Add it to FS at
   * the given dst name.
   * delSrc indicates if the source should be removed
   */
  public void copyFromLocalFile(boolean delSrc, Path src, Path dst)
    throws IOException {
    copyFromLocalFile(delSrc, true, src, dst);
  }

  /**
   * The src files are on the local disk.  Add it to FS at
   * the given dst name.
   * delSrc indicates if the source should be removed
   */
  public void copyFromLocalFile(boolean delSrc, boolean overwrite,
                                Path[] srcs, Path dst)
    throws IOException {
    Configuration conf = getConf();
    FileUtil.copy(getLocal(conf), srcs, this, dst, delSrc, overwrite, conf);
  }

  /**
   * The src file is on the local disk.  Add it to FS at
   * the given dst name.
   * delSrc indicates if the source should be removed
   */
  public void copyFromLocalFile(boolean delSrc, boolean overwrite,
                                Path src, Path dst)
    throws IOException {
    Configuration conf = getConf();
    FileUtil.copy(getLocal(conf), src, this, dst, delSrc, overwrite, conf);
  }

  /**
   * The src file is under FS, and the dst is on the local disk.
   * Copy it from FS control to the local dst name.
   */
  public void copyToLocalFile(Path src, Path dst) throws IOException {
    copyToLocalFile(false, src, dst);
  }

  /**
   * The src file is under FS, and the dst is on the local disk.
   * Copy it from FS control to the local dst name.
   * Remove the source afterwards
   */
  public void moveToLocalFile(Path src, Path dst) throws IOException {
    copyToLocalFile(true, src, dst);
  }

  /**
   * The src file is under FS, and the dst is on the local disk.
   * Copy it from FS control to the local dst name.
   * delSrc indicates if the src will be removed or not.
   */
  public void copyToLocalFile(boolean delSrc, Path src, Path dst)
    throws IOException {
    FileUtil.copy(this, src, getLocal(getConf()), dst, delSrc, getConf());
  }

  /**
   * Returns a local File that the user can write output to.  The caller
   * provides both the eventual FS target name and the local working
   * file.  If the FS is local, we write directly into the target.  If
   * the FS is remote, we write into the tmp local area.
   */
  public Path startLocalOutput(Path fsOutputFile, Path tmpLocalFile)
    throws IOException {
    return tmpLocalFile;
  }

  /**
   * Called when we‘re all done writing to the target.  A local FS will
   * do nothing, because we‘ve written to exactly the right place.  A remote
   * FS will copy the contents of tmpLocalFile to the correct target at
   * fsOutputFile.
   */
  public void completeLocalOutput(Path fsOutputFile, Path tmpLocalFile)
    throws IOException {
    moveFromLocalFile(tmpLocalFile, fsOutputFile);
  }

  /**
   * No more filesystem operations are needed.  Will
   * release any held locks.
   */
  public void close() throws IOException {
    // delete all files that were marked as delete-on-exit.
    processDeleteOnExit();
    CACHE.remove(this.key, this);
  }

  /** Return the total size of all files in the filesystem.*/
  public long getUsed() throws IOException{
    long used = 0;
    FileStatus[] files = listStatus(new Path("/"));
    for(FileStatus file:files){
      used += file.getLen();
    }
    return used;
  }

  /**
   * Get the block size for a particular file.
   * @param f the filename
   * @return the number of bytes in a block
   */
  /** @deprecated Use getFileStatus() instead */ @Deprecated
  public long getBlockSize(Path f) throws IOException {
    return getFileStatus(f).getBlockSize();
  }

  /** Return the number of bytes that large input files should be optimally
   * be split into to minimize i/o time. */
  public long getDefaultBlockSize() {
    // default to 32MB: large enough to minimize the impact of seeks
    return getConf().getLong("fs.local.block.size", 32 * 1024 * 1024);
  }

  /**
   * Get the default replication.
   */
  public short getDefaultReplication() { return 1; }

  /**
   * Return a file status object that represents the path.
   * @param f The path we want information from
   * @return a FileStatus object
   * @throws FileNotFoundException when the path does not exist;
   *         IOException see specific implementation
   */
  public abstract FileStatus getFileStatus(Path f) throws IOException;

  /**
   * Get the checksum of a file.
   *
   * @param f The file path
   * @return The file checksum.  The default return value is null,
   *  which indicates that no checksum algorithm is implemented
   *  in the corresponding FileSystem.
   */
  public FileChecksum getFileChecksum(Path f) throws IOException {
    return null;
  }

  /**
   * Set the verify checksum flag. This is only applicable if the
   * corresponding FileSystem supports checksum. By default doesn‘t do anything.
   * @param verifyChecksum
   */
  public void setVerifyChecksum(boolean verifyChecksum) {
    //doesn‘t do anything
  }

  /**
   * Return a list of file status objects that corresponds to the list of paths
   * excluding those non-existent paths.
   *
   * @param paths
   *          the list of paths we want information from
   * @return a list of FileStatus objects
   * @throws IOException
   *           see specific implementation
   */
  private FileStatus[] getFileStatus(Path[] paths) throws IOException {
    if (paths == null) {
      return null;
    }
    ArrayList<FileStatus> results = new ArrayList<FileStatus>(paths.length);
    for (int i = 0; i < paths.length; i++) {
      try {
        results.add(getFileStatus(paths[i]));
      } catch (FileNotFoundException e) { // do nothing
      }
    }
    return results.toArray(new FileStatus[results.size()]);
  }

  /**
   * Set permission of a path.
   * @param p
   * @param permission
   */
  public void setPermission(Path p, FsPermission permission
      ) throws IOException {
  }

  /**
   * Set owner of a path (i.e. a file or a directory).
   * The parameters username and groupname cannot both be null.
   * @param p The path
   * @param username If it is null, the original username remains unchanged.
   * @param groupname If it is null, the original groupname remains unchanged.
   */
  public void setOwner(Path p, String username, String groupname
      ) throws IOException {
  }

  /**
   * Set access time of a file
   * @param p The path
   * @param mtime Set the modification time of this file.
   *              The number of milliseconds since Jan 1, 1970.
   *              A value of -1 means that this call should not set modification time.
   * @param atime Set the access time of this file.
   *              The number of milliseconds since Jan 1, 1970.
   *              A value of -1 means that this call should not set access time.
   */
  public void setTimes(Path p, long mtime, long atime
      ) throws IOException {
  }

  private static FileSystem createFileSystem(URI uri, Configuration conf
      ) throws IOException {
    Class<?> clazz = conf.getClass("fs." + uri.getScheme() + ".impl", null);
    if (clazz == null) {
      throw new IOException("No FileSystem for scheme: " + uri.getScheme());
    }
    FileSystem fs = (FileSystem)ReflectionUtils.newInstance(clazz, conf);
    fs.initialize(uri, conf);
    return fs;
  }

  /** Caching FileSystem objects */
  static class Cache {
    private final Map<Key, FileSystem> map = new HashMap<Key, FileSystem>();

    synchronized FileSystem get(URI uri, Configuration conf) throws IOException{
      Key key = new Key(uri, conf);
      FileSystem fs = map.get(key);
      if (fs == null) {
        fs = createFileSystem(uri, conf);
        if (map.isEmpty() && !clientFinalizer.isAlive()) {
          Runtime.getRuntime().addShutdownHook(clientFinalizer);
        }
        fs.key = key;
        map.put(key, fs);
      }
      return fs;
    }

    synchronized void remove(Key key, FileSystem fs) {
      if (map.containsKey(key) && fs == map.get(key)) {
        map.remove(key);
        if (map.isEmpty() && !clientFinalizer.isAlive()) {
          if (!Runtime.getRuntime().removeShutdownHook(clientFinalizer)) {
            LOG.info("Could not cancel cleanup thread, though no " +
                     "FileSystems are open");
          }
        }
      }
    }

    synchronized void closeAll() throws IOException {
      List<IOException> exceptions = new ArrayList<IOException>();
      for(; !map.isEmpty(); ) {
        Map.Entry<Key, FileSystem> e = map.entrySet().iterator().next();
        final Key key = e.getKey();
        final FileSystem fs = e.getValue();

        //remove from cache
        remove(key, fs);

        if (fs != null) {
          try {
            fs.close();
          }
          catch(IOException ioe) {
            exceptions.add(ioe);
          }
        }
      }

      if (!exceptions.isEmpty()) {
        throw MultipleIOException.createIOException(exceptions);
      }
    }

    /** FileSystem.Cache.Key */
    static class Key {
      final String scheme;
      final String authority;
      final String username;

      Key(URI uri, Configuration conf) throws IOException {
        scheme = uri.getScheme()==null?"":uri.getScheme().toLowerCase();
        authority = uri.getAuthority()==null?"":uri.getAuthority().toLowerCase();
        UserGroupInformation ugi = UserGroupInformation.readFrom(conf);
        if (ugi == null) {
          try {
            ugi = UserGroupInformation.login(conf);
          } catch(LoginException e) {
            LOG.warn("uri=" + uri, e);
          }
        }
        username = ugi == null? null: ugi.getUserName();
      }

      /** {@inheritDoc} */
      public int hashCode() {
        return (scheme + authority + username).hashCode();
      }

      static boolean isEqual(Object a, Object b) {
        return a == b || (a != null && a.equals(b));
      }

      /** {@inheritDoc} */
      public boolean equals(Object obj) {
        if (obj == this) {
          return true;
        }
        if (obj != null && obj instanceof Key) {
          Key that = (Key)obj;
          return isEqual(this.scheme, that.scheme)
                 && isEqual(this.authority, that.authority)
                 && isEqual(this.username, that.username);
        }
        return false;
      }

      /** {@inheritDoc} */
      public String toString() {
        return username + "@" + scheme + "://" + authority;
      }
    }
  }

  public static final class Statistics {
    private final String scheme;
    private AtomicLong bytesRead = new AtomicLong();
    private AtomicLong bytesWritten = new AtomicLong();

    public Statistics(String scheme) {
      this.scheme = scheme;
    }

    /**
     * Increment the bytes read in the statistics
     * @param newBytes the additional bytes read
     */
    public void incrementBytesRead(long newBytes) {
      bytesRead.getAndAdd(newBytes);
    }

    /**
     * Increment the bytes written in the statistics
     * @param newBytes the additional bytes written
     */
    public void incrementBytesWritten(long newBytes) {
      bytesWritten.getAndAdd(newBytes);
    }

    /**
     * Get the total number of bytes read
     * @return the number of bytes
     */
    public long getBytesRead() {
      return bytesRead.get();
    }

    /**
     * Get the total number of bytes written
     * @return the number of bytes
     */
    public long getBytesWritten() {
      return bytesWritten.get();
    }

    public String toString() {
      return bytesRead + " bytes read and " + bytesWritten +
             " bytes written";
    }

    /**
     * Reset the counts of bytes to 0.
     */
    public void reset() {
      bytesWritten.set(0);
      bytesRead.set(0);
    }

    /**
     * Get the uri scheme associated with this statistics object.
     * @return the schema associated with this set of statistics
     */
    public String getScheme() {
      return scheme;
    }
  }

  /**
   * Get the Map of Statistics object indexed by URI Scheme.
   * @return a Map having a key as URI scheme and value as Statistics object
   * @deprecated use {@link #getAllStatistics} instead
   */
  public static synchronized Map<String, Statistics> getStatistics() {
    Map<String, Statistics> result = new HashMap<String, Statistics>();
    for(Statistics stat: statisticsTable.values()) {
      result.put(stat.getScheme(), stat);
    }
    return result;
  }

  /**
   * Return the FileSystem classes that have Statistics
   */
  public static synchronized List<Statistics> getAllStatistics() {
    return new ArrayList<Statistics>(statisticsTable.values());
  }

  /**
   * Get the statistics for a particular file system
   * @param cls the class to lookup
   * @return a statistics object
   */
  public static synchronized
  Statistics getStatistics(String scheme, Class<? extends FileSystem> cls) {
    Statistics result = statisticsTable.get(cls);
    if (result == null) {
      result = new Statistics(scheme);
      statisticsTable.put(cls, result);
    }
    return result;
  }

  public static synchronized void clearStatistics() {
    for(Statistics stat: statisticsTable.values()) {
      stat.reset();
    }
  }

  public static synchronized
  void printStatistics() throws IOException {
    for (Map.Entry<Class<? extends FileSystem>, Statistics> pair:
            statisticsTable.entrySet()) {
      System.out.println("  FileSystem " + pair.getKey().getName() +
                         ": " + pair.getValue());
    }
  }
}

FileSystem

Hadoop 输入/输出流

Hadoop抽象文件系统和java类似，也是使用流机制进行文件的读写，用于读文件数据流和写文件的抽象类分别是：FSDataInputStream和FSDataOutputStream

1、FSDataInputStream

public class FSDataInputStream extends DataInputStream
    implements Seekable, PositionedReadable {
……
}

可以看到，FSDataInputStream继承自DataInputStream类，实现了Seekable和PositionedReadable接口

Seekable接口提供在（文件）流中进行随机存取的方法，其功能类似于RandomAccessFile中的getFilePointer()和seek()方法，它提供了某种随机定位文件读取位置的能力

Seekable接口代码以及相关注释如下:

/** 接口，用于支持在流中定位. */
public interface Seekable {
  /**
   * 将当前偏移量设置到参数位置，下次读取数据将从该位置开始
   */
  void seek(long pos) throws IOException;

  /**得到当前偏移量   */
  long getPos() throws IOException;

  /**重新选择一个副本  */
  boolean seekToNewSource(long targetPos) throws IOException;
}

完整的FSDataInputStream类源代码如下：

/**
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package org.apache.hadoop.fs;

import java.io.*;

/** Utility that wraps a {@link FSInputStream} in a {@link DataInputStream}
 * and buffers input through a {@link BufferedInputStream}. */
public class FSDataInputStream extends DataInputStream
    implements Seekable, PositionedReadable {

  public FSDataInputStream(InputStream in)
    throws IOException {
    super(in);
    if( !(in instanceof Seekable) || !(in instanceof PositionedReadable) ) {
      throw new IllegalArgumentException(
          "In is not an instance of Seekable or PositionedReadable");
    }
  }

  public synchronized void seek(long desired) throws IOException {
    ((Seekable)in).seek(desired);
  }

  public long getPos() throws IOException {
    return ((Seekable)in).getPos();
  }

  public int read(long position, byte[] buffer, int offset, int length)
    throws IOException {
    return ((PositionedReadable)in).read(position, buffer, offset, length);
  }

  public void readFully(long position, byte[] buffer, int offset, int length)
    throws IOException {
    ((PositionedReadable)in).readFully(position, buffer, offset, length);
  }

  public void readFully(long position, byte[] buffer)
    throws IOException {
    ((PositionedReadable)in).readFully(position, buffer, 0, buffer.length);
  }

  public boolean seekToNewSource(long targetPos) throws IOException {
    return ((Seekable)in).seekToNewSource(targetPos);
  }
}

FSDataInputStream

FSDataInputStream实现的另一个接口是PositionedReadable，它提供了从流中某一个位置开始读数据的一系列方法：

//接口，用于在流中进行定位读
public interface PositionedReadable {

  //从指定位置开始，读最多指定长度的数据到buffer中offset开始的缓冲区中
  //注意，该函数不改变读流的当前位置，同时，它是线程安全的
  public int read(long position, byte[] buffer, int offset, int length)
    throws IOException;

  //从指定位置开始，读指定长度的数据到buffer中offset开始的缓冲区中
  public void readFully(long position, byte[] buffer, int offset, int length)
    throws IOException;

  public void readFully(long position, byte[] buffer) throws IOException;
}

PositionedReadable中的3个读方法，都不会改变流的当前位置，而且还是线程安全的

2、FSInputStream

org.apache.hadoop.fs包中还包含抽象类FSInputStream。Seekable接口和PositionedReadable中的方法都成为这个类的抽象方法

在FSInputStream类中，通过Seekable接口的seek()方法实现了PositionedReadable接口中的read()方法

//实现PositionedReadable.read()方法
public int read(long position, byte[] buffer, int offset, int length) throws IOException {
    /**
     * 由于PositionedReadable.read()是线程安全的，所以此处要借助synchronized (this)
     * 来保证方法被调用的时候其他方法不会被调用，也保证不会有其他线程改变Seekable.getPos()保存的
     * 当前读位置
     */
    synchronized (this) {
        long oldPos = getPos();    //保存当前读的位置，调用 Seekable.getPos()
        int nread = -1;
        try {
            seek(position);        //移动读数据的位置，调用Seekable.seek()
            nread = read(buffer, offset, length);  //调用InputStream.read()读取数据
         } finally {
            seek(oldPos);        //调用Seekable.seek()恢复InputStream.read()前的位置
         }
         return nread;
    }
}

完整的FSInputStream源代码如下：

package org.apache.hadoop.fs;

import java.io.*;

/****************************************************************
 * FSInputStream is a generic old InputStream with a little bit
 * of RAF-style seek ability.
 *
 *****************************************************************/
public abstract class FSInputStream extends InputStream
    implements Seekable, PositionedReadable {
  /**
   * Seek to the given offset from the start of the file.
   * The next read() will be from that location.  Can‘t
   * seek past the end of the file.
   */
  public abstract void seek(long pos) throws IOException;

  /**
   * Return the current offset from the start of the file
   */
  public abstract long getPos() throws IOException;

  /**
   * Seeks a different copy of the data.  Returns true if
   * found a new source, false otherwise.
   */
  public abstract boolean seekToNewSource(long targetPos) throws IOException;

  public int read(long position, byte[] buffer, int offset, int length)
    throws IOException {
    synchronized (this) {
      long oldPos = getPos();
      int nread = -1;
      try {
        seek(position);
        nread = read(buffer, offset, length);
      } finally {
        seek(oldPos);
      }
      return nread;
    }
  }

  public void readFully(long position, byte[] buffer, int offset, int length)
    throws IOException {
    int nread = 0;
    while (nread < length) {
      int nbytes = read(position+nread, buffer, offset+nread, length-nread);
      if (nbytes < 0) {
        throw new EOFException("End of file reached before reading fully.");
      }
      nread += nbytes;
    }
  }

  public void readFully(long position, byte[] buffer)
    throws IOException {
    readFully(position, buffer, 0, buffer.length);
  }
}

FSInputStream

注意：hadoop中没有相对应的FSOutputStream类

3、FSDataOutputStream

FSDataOutputStream用于写数据，和FSDataInputStream类似，继承自DataOutputStream，提供writeInt()和writeChar()等方法，但是FSDataOutputStream更加的简单，没有实现Seekable接口，也就是说，Hadoop文件系统不支持随机写，用户不能在文件中重新定位写位置，并通过写数据来覆盖文件原有的内容。单用户可以通过getPos()方法获得当前流的写位置，为了实现getPos()方法，FSDataOutputStream定义了内部类PositionCache，该类继承自FilterOutputStream，并通过重载write()方法跟踪目前流的写位置.

PositionCache是一个典型的过滤流，在基础的流功能上添加了getPos()方法，同时利用FileSystem.Statistics实现了文件系统读写的一些统计。

public class FSDataOutputStream extends DataOutputStream implements Syncable {
      private OutputStream wrappedStream;

      private static class PositionCache extends FilterOutputStream {
        private FileSystem.Statistics statistics;
        long position;    //当前流的写位置

        public PositionCache(OutputStream out,
                             FileSystem.Statistics stats,
                             long pos) throws IOException {
          super(out);
          statistics = stats;
          position = pos;
        }

        public void write(int b) throws IOException {
          out.write(b);
          position++;    //跟新当前位置
          if (statistics != null) {
            statistics.incrementBytesWritten(1);    //跟新文件统计值
          }
        }

        public void write(byte b[], int off, int len) throws IOException {
          out.write(b, off, len);
          position += len;                            // update position
          if (statistics != null) {
            statistics.incrementBytesWritten(len);
          }
        }

        public long getPos() throws IOException {
          return position;          //返回当前流的写位置
        }

        public void close() throws IOException {
          out.close();
        }
      }

      @Deprecated
      public FSDataOutputStream(OutputStream out) throws IOException {
        this(out, null);
      }

      public FSDataOutputStream(OutputStream out, FileSystem.Statistics stats)
        throws IOException {
        this(out, stats, 0);
      }

      public FSDataOutputStream(OutputStream out, FileSystem.Statistics stats,
                                long startPosition) throws IOException {
        super(new PositionCache(out, stats, startPosition));  //直接生成PositionCache对象并调用父类构造方法
        wrappedStream = out;
      }

      public long getPos() throws IOException {
        return ((PositionCache)out).getPos();
      }

      public void close() throws IOException {
        out.close();         // This invokes PositionCache.close()
      }

      // Returns the underlying output stream. This is used by unit tests.
      public OutputStream getWrappedStream() {
        return wrappedStream;
      }

      /** {@inheritDoc} */
      public void sync() throws IOException {
        if (wrappedStream instanceof Syncable) {
          ((Syncable)wrappedStream).sync();
        }
      }
}

FSDataOutputStream实现了Syncable接口，该接口只有一个函数sync()，其目的和Linux中系统调用sync()类似，用于将流中保存的数据同步到设备中

/** This interface declare the sync() operation. */
public interface Syncable {
  /**
   * Synchronize all buffer with the underlying devices.
   * @throws IOException
   */
  public void sync() throws IOException;
}