Why does the FTP download not work properly with Java

Question

i'm currently working on a small checklist builder program in Java. I would like to upload and download the created file to my FTP Server (ftps). I'm using the following code for downloading:

public static void downloadfile(){
    FTPSClient con = null;

    System.out.println("Download Status: 5%");
    try
    {
        System.out.println("Download Status: 20%");
        con = new FTPSClient();
        con.connect(url);

        if (con.login(user, psw))
        {
            System.out.println("Download Status: 50%");
            con.enterLocalPassiveMode(); // important!
            con.setFileType(FTP.BINARY_FILE_TYPE);
            String data = "E:\\Downloads\\Testdokument.txt";

            OutputStream out = new FileOutputStream(new File(data));
            boolean result = con.retrieveFile("Testdokument.txt", out);
            out.close();
            System.out.println(result);
            if (result) {
                System.out.println("Download Status: 100%");
            } else if(result == false) {
                System.out.println("Download won't work");
            }
            con.logout();
            con.disconnect();
        }
    }
    catch (Exception e)
    {
        System.out.println("download failed");
        e.printStackTrace();
    }

}

the problem is that the download itself works fine. But the downloaded file is empty. If i try it with an Image it isn't "readable". The upload instead works perfectly. I use the Apache Common IO Library for the FTP function.

If i download the file the console shows first status 5%, 20%, 50% and than, after adding the false statement, Download won't work ...

I have no idea why the file itself is downloading but not including any content.

Any ideas?

Answer 1

You're not doing resources in java correctly.

Anytime you create an object that represents a resource, you must close it. You open a new FileOutputStream , and that is a resource. Anything that implements AutoCloseable is definitely a resource that you must close. Try this:

try (OutputStream out = new FileOutputStream(data /* no need to wrap in File */)) {
    // do your stuff with out here
}

Second note: Your exception handling is atrocious; please stop making this common mistake. Exceptions contain 4 useful bits of information: Type, Message, Trace, and Cause. You're literally tossing 3 out of the 4 into the bin. Just add throws Exception to your main method, and your downloadFile method. It saves you typing and makes your error messages much more useful.

Answer 2

I found the solution.

i have added the following two things and it works now

con.execPBSZ(0);
con.execPROT("P");

No idea what that stands for, but I'm going to find it out.

Answer 3

I know this answer is off-topic but I like to provide an example for user rzwitserloot which shows that printStackTrace() does not hide any important information. This would not fit into a comment:

public class Main
{

    public static void doSomething()
    {
        int array[] = {1, 2, 3};
        int b = array[3];  // throws Exception
    }

    public static void test() throws Exception
    {
        try
        {
            doSomething();
        }
        catch (Exception e)
        {
            throw new Exception("something bad happened", e);
        }
    }

    public static void main(String[] args) throws Exception
    {
        try
        {
            test();
        }
        catch (Exception e)
        {
            e.printStackTrace();
        }
    }
}

Produces output:

java.lang.Exception: something bad happened
    at Main.test(Main.java:18)
    at Main.main(Main.java:26)
Caused by: java.lang.ArrayIndexOutOfBoundsException: Index 3 out of bounds for length 3
    at Main.doSomething(Main.java:7)
    at Main.test(Main.java:14)
    ... 1 more

See also the documentation of Oracle: