Detect underlying cause for java.io.FileNotFoundException

Question

FileNotFoundException is thrown on all sorts of occasions - not necessarily only when the file name is invalid, but also when e. g. permissions do not allow a file to be created or read:

java.io.FileNotFoundException: \\server\share\directory\test.csv (Anmeldung fehlgeschlagen: unbekannter Benutzername oder falsches Kennwort)
    at java.io.FileOutputStream.open(Native Method)
    at java.io.FileOutputStream.<init>(FileOutputStream.java:179)
    at java.io.FileOutputStream.<init>(FileOutputStream.java:131)
    at java.io.FileWriter.<init>(FileWriter.java:73)

The above example shows a German Windows complaining about invalid username or password.

Is there a way short of parsing the exceptions message to get a little finer grained information on why exactly the exception occurred? Problem with message parsing is that in different locales the messages will vary.

Tourbillion

Answer 1

Do the check for file existence/read-write permissions yourself before creating FileOutputStream.

File test_csv = new File( "\\server\share\directory\test.csv" );

if ( test_csv.exists( ) && test_csv.canWrite( ) )
{
  // Create file writer
  ...
}
else
{
  // notify user
  ...
}

Notice that sometimes you will have to check the read/write permissions on a parent of you destination file, if you need to create a new file.

File test_csv = new File( "\\server\share\directory\test.csv" );
File parent_dir = test_csv.getParentFile( )

if ( parent_dir.exists( ) && parent_dir.canWrite( ) )
{
  // Create file writer
  ...
}
else
{
  // notify user
  ...
}

Palmitin

D'Oh! Sometimes life can be sooo easy. Thanks for opening my eyes to the obvious. – Tourbillion

Be careful. There are apparently some bugs that can affect the return result of these methods. You may get invalid results if your directory is a custom or system directory or if it is on certain types of network share. – Apostrophe

Apparently I just got bit by bugs.sun.com/bugdatabase/view_bug.do?bug_id=4939819 which will tell you that the user's home directory (documents and settings...) on Windows is not writable. Workaround: Try creating a temp file in the parent folder and delete it immediately afterwards, if successful. If an IOException is thrown, you really cannot write. – Tourbillion

Is a permissions issue possible if I am running this as administrator from Intellij? – Eligible

Answer 2

You may want to look at the properties of the file using the java.io.File object before attempting to read the file. There's a canRead method on that you can use to determine whether or not the user can read the file.

Neom

Answer 3

One approach is to look at the actual type of the exception: as you can see from the docs, there are a lot of subclasses that provide finer-grained information.

However, you probably won't get far with that. As with most checked exceptions, it's usually better to log/report the exception and ask the user for choices on how to correct it.

Diatomic

Problem is I get a FileNotFoundException, not a generic IOException. I'd like to present a better user message than "File Not Found" or "Generic Error", pointing the user towards the permission problem... – Tourbillion

My mistake - I read IOException. Unfortunately, Java exceptions rarely have additional information (such as a POSIX error code), and in many cases are stretched to cover conditions that have little relationship to their name (sometimes due to assignment of POSIX code to exception type, sometimes, I think, due to laziness). – Diatomic