Alternative to File.exists() in Java

Question

I never thought it would happen to me, but I ran into my first bug in Java:

http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=5003595

I\'m pretty much in th

Answer 1

The basic problem you have with NFS is that it caches attributes, files and directories information. This means the information can be out of date. You might be able to turn off caching, you will see a significant reduction in performance.

The important thing to remember is that NFS is not a messaging service and is not designed for timely delivery of data.

Answer 2

What happens if File.exists() returns true, then someone deletes the file/your NFS mount goes away, then you try and open the file? Basically, File.exists() is useless since you need to handle the exceptions that can arise from opening the file anyway.

Answer 3

The obvious alternative is File.isFile(). Try that first.

Although it will become inaccurate when reading read-only files you could always use the File.canWrite() to check if the file exists.

If both the above fail you could use the File.length(). If it returns 0L you know that the file does not exists.

Answer 4

I experienced the same problem and solved it with a call to file.getParentFile().list(). Essentially the same as your solution, but OS agnostic.

Answer 5

I notice that Java 7's java.nio.file.Path.exists() method returns false if the file doesn't exist or it's existence can't be determined. It would seem, therefore, that false negatives will be around for a while and that your code will need to tolerate them.

Answer 6

All File.exists tells you is whether the file existed at some point in the past. It doesn't tell you:

• Whether it will exist when you try to open it
• Whether you have permission to open it
• Anything useful at all, really

So try to design your application so it can handle files that don't exist without trying to check for that in advance. (You'll have to handle various exceptions when actually working with the file.)