How to solve exception with “org.apache.commons.net.ftp.FTPClient”

Question

Im not sure if this is a similar problem as I faced since it doesn't state an Exception: Issue with org.apache.commons.net.ftp.FTPClient listFiles()

I'm trying to use this class FTPUtil to download the root folder: https://www.codejava.net/java-se/ftp/how-to-download-a-complete-folder-from-a-ftp-server

at the line FTPFile[] subFiles = ftpClient.listFiles(dirToList); I get this Exception:

 java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String java.net.InetAddress.getHostAddress()' on a null object reference
    at org.apache.commons.net.ftp.FTPClient._openDataConnection_(FTPClient.java:938)
    at org.apache.commons.net.ftp.FTPClient._openDataConnection_(FTPClient.java:785)
    at org.apache.commons.net.ftp.FTPClient.initiateListParsing(FTPClient.java:3409)
    at org.apache.commons.net.ftp.FTPClient.initiateListParsing(FTPClient.java:3339)
    at org.apache.commons.net.ftp.FTPClient.listFiles(FTPClient.java:3016)
    at com.censored.FTPUtil.downloadDirectory(FTPUtil.java:39)

I don't really understand which Object is null and why. As String parentDir I tried "" and "\\" both with the same result I don't think the input is wrong.
The FTP server is running fine I see that my program logs in successful and file zilla can also access the folder.

EDIT:

Here is the class from the link I'm using:

package com.censored;

import java.io.BufferedOutputStream;
import java.io.File;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.OutputStream;

import org.apache.commons.net.ftp.FTP;
import org.apache.commons.net.ftp.FTPClient;
import org.apache.commons.net.ftp.FTPFile;

/**
 * This utility class implements a method that downloads a directory completely
 * from a FTP server, using Apache Commons Net API.
 *
 * @author www.codejava.net
 *
 */
public class FTPUtil {

    /**
     * Download a whole directory from a FTP server.
     * @param ftpClient an instance of org.apache.commons.net.ftp.FTPClient class.
     * @param parentDir Path of the parent directory of the current directory being
     * downloaded.
     * @param currentDir Path of the current directory being downloaded.
     * @param saveDir path of directory where the whole remote directory will be
     * downloaded and saved.
     * @throws IOException if any network or IO error occurred.
     */
    public static void downloadDirectory(FTPClient ftpClient, String parentDir,
            String currentDir, String saveDir) throws IOException {
        String dirToList = parentDir;
        if (!currentDir.equals("")) {
            dirToList += "/" + currentDir;
        }

        FTPFile[] subFiles = ftpClient.listFiles(dirToList); //This line gives the Exception

        if (subFiles != null && subFiles.length > 0) {
            for (FTPFile aFile : subFiles) {
                String currentFileName = aFile.getName();
                if (currentFileName.equals(".") || currentFileName.equals("..")) {
                    // skip parent directory and the directory itself
                    continue;
                }
                String filePath = parentDir + "/" + currentDir + "/"
                        + currentFileName;
                if (currentDir.equals("")) {
                    filePath = parentDir + "/" + currentFileName;
                }

                String newDirPath = saveDir + parentDir + File.separator
                        + currentDir + File.separator + currentFileName;
                if (currentDir.equals("")) {
                    newDirPath = saveDir + parentDir + File.separator
                              + currentFileName;
                }

                if (aFile.isDirectory()) {
                    // create the directory in saveDir
                    File newDir = new File(newDirPath);
                    boolean created = newDir.mkdirs();
                    if (created) {
                        System.out.println("CREATED the directory: " + newDirPath);
                    } else {
                        System.out.println("COULD NOT create the directory: " + newDirPath);
                    }

                    // download the sub directory
                    downloadDirectory(ftpClient, dirToList, currentFileName,
                            saveDir);
                } else {
                    // download the file
                    boolean success = downloadSingleFile(ftpClient, filePath,
                            newDirPath);
                    if (success) {
                        System.out.println("DOWNLOADED the file: " + filePath);
                    } else {
                        System.out.println("COULD NOT download the file: "
                                + filePath);
                    }
                }
            }
        }
    }

    /**
     * Download a single file from the FTP server
     * @param ftpClient an instance of org.apache.commons.net.ftp.FTPClient class.
     * @param remoteFilePath path of the file on the server
     * @param savePath path of directory where the file will be stored
     * @return true if the file was downloaded successfully, false otherwise
     * @throws IOException if any network or IO error occurred.
     */
    public static boolean downloadSingleFile(FTPClient ftpClient,
            String remoteFilePath, String savePath) throws IOException {
        File downloadFile = new File(savePath);

        File parentDir = downloadFile.getParentFile();
        if (!parentDir.exists()) {
            parentDir.mkdir();
        }

        OutputStream outputStream = new BufferedOutputStream(
                new FileOutputStream(downloadFile));
        try {
            ftpClient.setFileType(FTP.BINARY_FILE_TYPE);
            return ftpClient.retrieveFile(remoteFilePath, outputStream);
        } catch (IOException ex) {
            throw ex;
        } finally {
            if (outputStream != null) {
                outputStream.close();
            }
        }
    }
}

And this my code calling it (censored the unrelevant stuff I dont have the permission to show the full class):

val server = "10.0.2.2"  // Because of Emulator
val port = 8080 //yeah I know not standard port, this will change
val user = "censored"
val pass = "censored"        

val remoteDirPath = ""   //also tried "/"
val saveDirPath = File(myFilesDir, "fromFTP/").absolutePath   //ignore myFilesDir the Debuger shows that its set correct and it isnt relevant before the Exception

val ftpClient = FTPClient()



    try {
        // connect and login to the server
        ftpClient.connect(server, port)
        ftpClient.login(user, pass)

        // use local passive mode to pass firewall
        ftpClient.enterLocalPassiveMode()

        println("Connected")

        FTPUtil.downloadDirectory(ftpClient, remoteDirPath, "", saveDirPath) //this line is the one giving the exception

        // log out and disconnect from the server
        ftpClient.logout()
        ftpClient.disconnect()

        println("Disconnected")
    } catch (ex: IOException) {
        ex.printStackTrace()
    }

Update: During Debuging I found that this if in class FTPClient extends FTP is true:

 if (__remoteVerificationEnabled && !verifyRemote(socket))
        {
            socket.close();

            throw new IOException(
                    "Host attempting data connection " + socket.getInetAddress().getHostAddress() +
                    " is not same as server " + getRemoteAddress().getHostAddress());
        }

        return socket;

I think socket.getInetAddress() causes an exception (inside of the outer exception) because the Socket is already closed? This would mean that the FTPClient itself contains a bug? However I think the solution for my problem is that the if (__remoteVerificationEnabled && !verifyRemote(socket)) is true, so somehow this if must become false. Why is it true? What kind of verification is this?

Answer 1

Ok I think it is just a workaround but the solution seams to be to ad this line: ftpClient.setRemoteVerificationEnabled(false);

Read the Update part in the question to see how I found this solution.

Answer 2

I think you're correct. This is a bug in FTPClient.

socket.getInetAddress() is documented to throw NullPointerException if the socket is not open. This is called right after we call socket.close().