HDFS is the primary distributed storage used by Hadoop applications. A HDFS cluster primarily consists of a NameNode that manages the file system metadata and DataNodes that store the actual data.

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 * 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

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import java.io.File;

import java.io.IOException;

import org.apache.hadoop.conf.Configuration;

import org.apache.hadoop.fs.FileSystem;

import org.apache.hadoop.fs.FSDataInputStream;

import org.apache.hadoop.fs.FSDataOutputStream;

import org.apache.hadoop.fs.Path;

public class HadoopDFSFileReadWrite {

  static void usage () {

    System.out.println("Usage : HadoopDFSFileReadWrite <inputfile> <output file>");

    System.exit(1);

  }

  static void printAndExit(String str) {

    System.err.println(str);

    System.exit(1);

  }

  public static void main (String[] argv) throws IOException {

    Configuration conf = new Configuration();

    FileSystem fs = FileSystem.get(conf);

    if (argv.length != 2)

      usage();

    // Hadoop DFS deals with Path

    Path inFile = new Path(argv[0]);

    Path outFile = new Path(argv[1]);

    // Check if input/output are valid

    if (!fs.exists(inFile))

      printAndExit("Input file not found");

    if (!fs.isFile(inFile))

      printAndExit("Input should be a file");

    if (fs.exists(outFile))

      printAndExit("Output already exists");

    // Read from and write to new file

    FSDataInputStream in = fs.open(inFile);

    FSDataOutputStream out = fs.create(outFile);

    byte buffer[] = new byte[256];

    try {

      int bytesRead = 0;

      while ((bytesRead = in.read(buffer)) > 0) {

        out.write(buffer, 0, bytesRead);

      }

    } catch (IOException e) {

      System.out.println("Error while copying file");

    } finally {

      in.close();

      out.close();

    }

  }

}

Configuration conf = new Configuration();

FileSystem fs = FileSystem.get(conf);

// Hadoop DFS deals with Path

    Path inFile = new Path(argv[0]);

    Path outFile = new Path(argv[1]);

    // Check if input/output are valid

    if (!fs.exists(inFile))

      printAndExit("Input file not found");

    if (!fs.isFile(inFile))

      printAndExit("Input should be a file");

    if (fs.exists(outFile))

      printAndExit("Output already exists");

    // Read from and write to new file

    FSDataInputStream in = fs.open(inFile);

    FSDataOutputStream out = fs.create(outFile);

    byte buffer[] = new byte[256];

    try {

      int bytesRead = 0;

      while ((bytesRead = in.read(buffer)) > 0) {

        out.write(buffer, 0, bytesRead);

      }

    } catch (IOException e) {

      System.out.println("Error while copying file");

    } finally {

      in.close();

      out.close();

    }

  static {

    addDeprecatedKeys();

    // adds the default resources

    Configuration.addDefaultResource("hdfs-default.xml");

    Configuration.addDefaultResource("hdfs-site.xml");

  }

  public HdfsDataInputStream(DFSInputStream in) throws IOException {

    super(in);

  }

  public HdfsDataOutputStream(DFSOutputStream out, FileSystem.Statistics stats,

      long startPosition) throws IOException {

    super(out, stats, startPosition);

  }