操作hdfs

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20:55 2010-6-2

运行环境：

Hadoop.0.20.2

CentOS 5.4

java version "1.6.0_20-ea"

配置的是单机Hadoop环境

先看下我的运行截图

主要参考这篇文章

http://myjavanotebook.blogspot.com/2008/05/hadoop-file-system-tutorial.html

1.Copy a file from the local file system to HDFS

The srcFile variable needs to contain the full name (path + file name) ofthe file in the local file system.

The dstFile variable needs to contain the desired full name of the file inthe Hadoop file system.

Configuration config = new Configuration();
  FileSystem hdfs = FileSystem.get(config);
  Path srcPath = new Path(srcFile);
  Path dstPath = new Path(dstFile);
  hdfs.copyFromLocalFile(srcPath, dstPath);

2.Create HDFS file

The fileName variable contains the file name and path in the Hadoop filesystem.

The content of the file is the buff variable which is an array of bytes.

//byte[] buff - The content of the file

  Configuration config = new Configuration();
  FileSystem hdfs = FileSystem.get(config);
  Path path = new Path(fileName);
  FSDataOutputStream outputStream = hdfs.create(path);
  outputStream.write(buff, 0, buff.length);

3.Rename HDFS file

In order to rename a file in Hadoop file system, we need the full name(path + name) of

the file we want to rename. The rename method returns true if the file wasrenamed, otherwise false.

Configuration config = new Configuration();
  FileSystem hdfs = FileSystem.get(config);
  Path fromPath = new Path(fromFileName);
  Path toPath = new Path(toFileName);
  boolean isRenamed = hdfs.rename(fromPath, toPath);

4.Delete HDFS file

In order to delete a file in Hadoop file system, we need the full name(path + name)

of the file we want to delete. The delete method returns true if the filewas deleted, otherwise false.

Configuration config = new Configuration();
  FileSystem hdfs = FileSystem.get(config);
  Path path = new Path(fileName);
  boolean isDeleted = hdfs.delete(path, false);

Recursive delete:
  Configuration config = new Configuration();
  FileSystem hdfs = FileSystem.get(config);
  Path path = new Path(fileName);
  boolean isDeleted = hdfs.delete(path, true);

5.Get HDFS file last modification time

In order to get the last modification time of a file in Hadoop filesystem,

we need the full name (path + name) of the file.

Configuration config = new Configuration();
  FileSystem hdfs = FileSystem.get(config);
  Path path = new Path(fileName);
  FileStatus fileStatus = hdfs.getFileStatus(path);
  long modificationTime = fileStatus.getModificationTime

6.Check if a file exists in HDFS

In order to check the existance of a file in Hadoop file system,

we need the full name (path + name) of the file we want to check.

The exists methods returns true if the file exists, otherwise false.

Configuration config = new Configuration();
  FileSystem hdfs = FileSystem.get(config);
  Path path = new Path(fileName);
  boolean isExists = hdfs.exists(path);

7.Get the locations of a file in the HDFS cluster

A file can exist on more than one node in the Hadoop file systemcluster for two reasons:

Based on the HDFS cluster configuration, Hadoop saves parts of files ondifferent nodes in the cluster.

Based on the HDFS cluster configuration, Hadoop saves more than one copyof each file on different nodes for redundancy (The default is three).

Configuration config = new Configuration();
  FileSystem hdfs = FileSystem.get(config);
  Path path = new Path(fileName);
  FileStatus fileStatus = hdfs.getFileStatus(path);

  BlockLocation[] blkLocations = hdfs.getFileBlockLocations(path, 0, fileStatus.getLen());

BlockLocation[] blkLocations = hdfs.getFileBlockLocations(fileStatus, 0, fileStatus.getLen());
     //这个地方，作者写错了，需要把path改为fileStatus
  int blkCount = blkLocations.length;
  for (int i=0; i < blkCount; i++) {
    String[] hosts = blkLocations[i].getHosts();
    // Do something with the block hosts
   }

8. Get a list of all the nodes host names in the HDFS cluster

his method casts the FileSystem Object to aDistributedFileSystem Object.

This method will work only when Hadoop is configured as a cluster.

Running Hadoop on the local machine only, in a non clusterconfiguration will

cause this method to throw an Exception.

Configuration config = new Configuration();
  FileSystem fs = FileSystem.get(config);
  DistributedFileSystem hdfs = (DistributedFileSystem) fs;
  DatanodeInfo[] dataNodeStats = hdfs.getDataNodeStats();
  String[] names = new String[dataNodeStats.length];
  for (int i = 0; i < dataNodeStats.length; i++) {
      names[i] = dataNodeStats[i].getHostName();
  }

程序实例

/*
*
* 演示操作HDFS的java接口
*
* */

import org.apache.hadoop.conf.*;
import org.apache.hadoop.fs.*;
import org.apache.hadoop.hdfs.*;
import org.apache.hadoop.hdfs.protocol.*;
import java.util.Date;

public class DFSOperater {

    /**
     * @param args
     */
    public static void main(String[] args) {

        Configuration conf = new Configuration();

        try {
            // Get a list of all the nodes host names in the HDFS cluster

            FileSystem fs = FileSystem.get(conf);
            DistributedFileSystem hdfs = (DistributedFileSystem)fs;
            DatanodeInfo[] dataNodeStats = hdfs.getDataNodeStats();
            String[] names = new String[dataNodeStats.length];
            System.out.println("list of all the nodes in HDFS cluster:"); //print info

            for(int i=0; i < dataNodeStats.length; i++){
                names[i] = dataNodeStats[i].getHostName();
                System.out.println(names[i]); //print info

            }
            Path f = new Path("/user/cluster/dfs.txt");

            //check if a file exists in HDFS

            boolean isExists = fs.exists(f);
            System.out.println("The file exists? [" + isExists + "]");

            //if the file exist, delete it

            if(isExists){
                 boolean isDeleted = hdfs.delete(f, false);//fase : not recursive

                 if(isDeleted)System.out.println("now delete " + f.getName());
            }

            //create and write

            System.out.println("create and write [" + f.getName() + "] to hdfs:");
            FSDataOutputStream os = fs.create(f, true, 0);
            for(int i=0; i<10; i++){
                os.writeChars("test hdfs ");
            }
            os.writeChars("\n");
            os.close();

            //get the locations of a file in HDFS

            System.out.println("locations of file in HDFS:");
            FileStatus filestatus = fs.getFileStatus(f);
            BlockLocation[] blkLocations = fs.getFileBlockLocations(filestatus, 0,filestatus.getLen());
            int blkCount = blkLocations.length;
            for(int i=0; i < blkCount; i++){
                String[] hosts = blkLocations[i].getHosts();
                //Do sth with the block hosts

                System.out.println(hosts);
            }

            //get HDFS file last modification time

            long modificationTime = filestatus.getModificationTime(); // measured in milliseconds since the epoch

            Date d = new Date(modificationTime);
         System.out.println(d);
            //reading from HDFS

            System.out.println("read [" + f.getName() + "] from hdfs:");
     FSDataInputStream dis = fs.open(f);
     System.out.println(dis.readUTF());
     dis.close();

        } catch (Exception e) {
            // TODO: handle exception

            e.printStackTrace();
        }

    }

}

编译后拷贝到node1上面运行，杯具，不会用Eclipse插件

[cluster /opt/hadoop/source]$cp /opt/winxp/hadoop/dfs_operator.jar .
[cluster /opt/hadoop/source]$hadoop jar dfs_operator.jar DFSOperater
list of all the nodes in HDFS cluster:
node1
The file exists? [true]
now delete dfs.txt
create and write [dfs.txt] to hdfs:
locations of file in HDFS:
[Ljava.lang.String;@72ffb
Wed Jun 02 18:29:14 CST 2010
read [dfs.txt] from hdfs:
est hdfs test hdfs test hdfs test hdfs test hdfs test hdfs

运行成功！查看输出文件

[cluster /opt/hadoop/source]$hadoop fs -cat dfs.txt
test hdfs test hdfs test hdfs test hdfs test hdfs test hdfs test hdfs test hdfs test hdfs test hdfs

作者：qkshan@twitter,<myduanli@gmail.com>
来源：http://duanli.cublog.cn
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