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.

/*** 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.*/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 PathPath inFile = new Path(argv[0]);Path outFile = new Path(argv[1]);// Check if input/output are validif (!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 fileFSDataInputStream 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 PathPath inFile = new Path(argv[0]);Path outFile = new Path(argv[1]);// Check if input/output are validif (!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 fileFSDataInputStream 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 resourcesConfiguration.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);}