How to operate HDFS using Java API?

1. Traverse all files and folders in the current directory

You can use the listStatus method to achieve the above requirements.
The signature of the listStatus method is as follows

  /**
   * 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 FileNotFoundException when the path does not exist;
   *         IOException see specific implementation
   */
  public abstract FileStatus[] listStatus(Path f) throws FileNotFoundException, 
                                                         IOException;

It can be seen that listStatus only needs to pass in the parameter Path, and an array of FileStatus is returned.
FileStatus contains the following information

/** Interface that represents the client side information for a file.
 */
@InterfaceAudience.Public
@InterfaceStability.Stable
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;
  private Path symlink;
  ....

It is not difficult to see from FileStatus, including file path, size, whether it is a directory, block_replication, blocksize... and other information.

import org.apache.hadoop.fs.{FileStatus, FileSystem, Path}
import org.apache.spark.sql.SparkSession
import org.apache.spark.{SparkConf, SparkContext}
import org.slf4j.LoggerFactory

object HdfsOperation {
	
	val logger = LoggerFactory.getLogger(this.getClass)
	
	def tree(sc: SparkContext, path: String) : Unit = {
		val fs = FileSystem.get(sc.hadoopConfiguration)
		val fsPath = new Path(path)
		val status = fs.listStatus(fsPath)
		for(filestatus:FileStatus <- status) {
			logger.error("getPermission is: {}", filestatus.getPermission)
			logger.error("getOwner is: {}", filestatus.getOwner)
			logger.error("getGroup is: {}", filestatus.getGroup)
			logger.error("getLen is: {}", filestatus.getLen)
			logger.error("getModificationTime is: {}", filestatus.getModificationTime)
			logger.error("getReplication is: {}", filestatus.getReplication)
			logger.error("getBlockSize is: {}", filestatus.getBlockSize)
			if (filestatus.isDirectory) {
				val dirpath = filestatus.getPath.toString
				logger.error("文件夹名字为: {}", dirpath)
				tree(sc, dirpath)
			} else {
				val fullname = filestatus.getPath.toString
				val filename = filestatus.getPath.getName
				logger.error("全部文件名为: {}", fullname)
				logger.error("文件名为: {}", filename)
			}
		}
	}
}

If it is determined that fileStatus is a folder, the tree method is called recursively to achieve the purpose of traversing all.

2. Traverse all files

The above method is to traverse all files and folders. If you just want to iterate over files, you can use the listFiles method.

	def findFiles(sc: SparkContext, path: String) = {
		val fs = FileSystem.get(sc.hadoopConfiguration)
		val fsPath = new Path(path)
		val files = fs.listFiles(fsPath, true)
		while(files.hasNext) {
			val filestatus = files.next()
			val fullname = filestatus.getPath.toString
			val filename = filestatus.getPath.getName
			logger.error("全部文件名为: {}", fullname)
			logger.error("文件名为: {}", filename)
			logger.error("文件大小为: {}", filestatus.getLen)
		}
	}

  /**
   * List the statuses and block locations of the files in the given path.
   * 
   * If the path is a directory, 
   *   if recursive is false, returns files in the directory;
   *   if recursive is true, return files in the subtree rooted at the path.
   * If the path is a file, return the file's status and block locations.
   * 
   * @param f is the path
   * @param recursive if the subdirectories need to be traversed recursively
   *
   * @return an iterator that traverses statuses of the files
   *
   * @throws FileNotFoundException when the path does not exist;
   *         IOException see specific implementation
   */
  public RemoteIterator<LocatedFileStatus> listFiles(
      final Path f, final boolean recursive)
  throws FileNotFoundException, IOException {
  ...

As can be seen from the source code, listFiles returns an iterable objectRemoteIterator<locatedfilestatus></locatedfilestatus>, and listStatus returns an array. At the same time, listFiles returns all files.

3.Create folder

	def mkdirToHdfs(sc: SparkContext, path: String) = {
		val fs = FileSystem.get(sc.hadoopConfiguration)
		val result = fs.mkdirs(new Path(path))
		if (result) {
			logger.error("mkdirs already success!")
		} else {
			logger.error("mkdirs had failed!")
		}
	}

4.Delete folder

	def deleteOnHdfs(sc: SparkContext, path: String) = {
		val fs = FileSystem.get(sc.hadoopConfiguration)
		val result = fs.delete(new Path(path), true)
		if (result) {
			logger.error("delete already success!")
		} else {
			logger.error("delete had failed!")
		}
	}

5.Upload file

	def uploadToHdfs(sc: SparkContext, localPath: String, hdfsPath: String): Unit = {
		val fs = FileSystem.get(sc.hadoopConfiguration)
		fs.copyFromLocalFile(new Path(localPath), new Path(hdfsPath))
		fs.close()
	}

6.Download file

	def downloadFromHdfs(sc: SparkContext, localPath: String, hdfsPath: String) = {
		val fs = FileSystem.get(sc.hadoopConfiguration)
		fs.copyToLocalFile(new Path(hdfsPath), new Path(localPath))
		fs.close()
	}

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