Read & Write files from HDFS

Maven Dependencies

<dependency>
            <groupId>org.apache.hadoop</groupId>
            <artifactId>hadoop-client</artifactId>
            <version>${hadoop.version}</version>
 </dependency>

HDFS URI

HDFS connection URI have the following format : 

hdfs://namenodedns:port/user/hdfs/folder/file.csv

Default port is 8020.

Init HDFS FileSystem Object

private static String HADOOP_CONF_DIR = System.getenv("HADOOP_CONF_DIR");
// ====== Init HDFS File System Object
Configuration conf = new Configuration();
conf.addResource(new Path("file:///" + HADOOP_CONF_DIR + "/core-site.xml"));
conf.addResource(new Path("file:///" + HADOOP_CONF_DIR + "/hdfs-site.xml"));
conf.set("fs.hdfs.impl", org.apache.hadoop.hdfs.DistributedFileSystem.class.getName());
conf.set("fs.file.impl", org.apache.hadoop.fs.LocalFileSystem.class.getName());
// Set HADOOP user
System.setProperty("HADOOP_USER_NAME", "hdfs");
System.setProperty("hadoop.home.dir", "/");
//Get the filesystem - HDFS
FileSystem fs = FileSystem.get(conf);

Init Subfolders

//==== Create folder if not exists
Path workingDir=fs.getWorkingDirectory();
Path newFolderPath= new Path(path);
if(!fs.exists(newFolderPath)) {
   // Create new Directory
   fs.mkdirs(newFolderPath);
   logger.info("Path "+path+" created.");
}

How to write a file to HDFS 

//==== Write file
logger.info("Begin Write file into hdfs");
//Create a path
Path hdfswritepath = new Path(newFolderPath + "/" + fileName);
//Init output stream
FSDataOutputStream outputStream=fs.create(hdfswritepath);
//Cassical output stream usage
outputStream.writeBytes(fileContent);
outputStream.close();
logger.info("End Write file into hdfs");

How to read a file from HDFS 

//==== Read file
logger.info("Read file from hdfs");
//Create a path
Path hdfsreadpath = new Path(newFolderPath + "/" + fileName);
//Init input stream
FSDataInputStream inputStream = fs.open(hdfsreadpath);
//Classical input stream usage
String out= IOUtils.toString(inputStream, "UTF-8");
logger.info(out);
inputStream.close();
fs.close(); 

With Kerberos

Maven Dependencies

<dependency>
     <groupId>org.apache.hadoop</groupId>
     <artifactId>hadoop-hdfs</artifactId>
     <version>2.6.0-cdh5.16.1.1</version>
</dependency>
<dependency>
     <groupId>org.apache.hadoop</groupId>
     <artifactId>hadoop-common</artifactId>
     <version>2.6.0-cdh5.16.1.1</version>
</dependency>

JAAS configuration

Add a jaas.conf file under src/main/resources containing the following content : 

Main {
 com.sun.security.auth.module.Krb5LoginModule required client=TRUE;
};

Create login context function

private static final String JDBC_DRIVER_NAME = "org.apache.hive.jdbc.HiveDriver";
private static String username;
private static String password;
private static String HADOOP_CONF_DIR = System.getenv("HADOOP_CONF_DIR");

private static LoginContext kinit(String username, String password) throws LoginException {
    LoginContext lc = new LoginContext(Main.class.getSimpleName(), callbacks -> {
        for (Callback c : callbacks) {
            if (c instanceof NameCallback)
                ((NameCallback) c).setName(username);
            if (c instanceof PasswordCallback)
                ((PasswordCallback) c).setPassword(password.toCharArray());
        }
    });
    lc.login();
    return lc;
}

Init HDFS FileSystem Object

if (args.length > 0) { // If any arguments provided
    username = args[0];
    password = args[1];
} else {
    System.out.println("Usage : java -jar <username> <password>");
    System.exit(0);
}

URL url = Main.class.getClassLoader().getResource("jaas.conf");
System.setProperty("java.security.auth.login.config", url.toExternalForm());

Configuration conf = new Configuration();
conf.addResource(new Path("file:///" + HADOOP_CONF_DIR + "/core-site.xml"));
conf.addResource(new Path("file:///" + HADOOP_CONF_DIR + "/hdfs-site.xml"));

UserGroupInformation.setConfiguration(conf);

LoginContext lc = kinit(username, password);
UserGroupInformation.loginUserFromSubject(lc.getSubject());

FileSystem fs = FileSystem.get(conf);

Init Subfolders

//==== Create folder if not exists
Path workingDir=fs.getWorkingDirectory();
Path newFolderPath= new Path(path);
if(!fs.exists(newFolderPath)) {
   // Create new Directory
   fs.mkdirs(newFolderPath);
   logger.info("Path "+path+" created.");
}

How to write a file to HDFS 

//==== Write file
logger.info("Begin Write file into hdfs");
//Create a path
Path hdfswritepath = new Path(newFolderPath + "/" + fileName);
//Init output stream
FSDataOutputStream outputStream=fs.create(hdfswritepath);
//Cassical output stream usage
outputStream.writeBytes(fileContent);
outputStream.close();
logger.info("End Write file into hdfs");

How to read a file from HDFS 

//==== Read file
logger.info("Read file from hdfs");
//Create a path
Path hdfsreadpath = new Path(newFolderPath + "/" + fileName);
//Init input stream
FSDataInputStream inputStream = fs.open(hdfsreadpath);
//Classical input stream usage
String out= IOUtils.toString(inputStream, "UTF-8");
logger.info(out);
inputStream.close();
fs.close();