Question

How do I count the number of files in a directory using Java ? For simplicity, lets assume that the directory doesn’t have any sub-directories.

I know the standard method of :

new File(<directory path>).listFiles().length 

But this will effectively go through all the files in the directory, which might take long if the number of files is large. Also, I don’t care about the actual files in the directory unless their number is greater than some fixed large number (say 5000).

I am guessing, but doesn’t the directory (or its i-node in case of Unix) store the number of files contained in it? If I could get that number straight away from the file system, it would be much faster. I need to do this check for every HTTP request on a Tomcat server before the back-end starts doing the real processing. Therefore, speed is of paramount importance.

I could run a daemon every once in a while to clear the directory. I know that, so please don’t give me that solution.

Answer 1

This might not be appropriate for your application, but you could always try a native call (using jni or jna), or exec a platform-specific command and read the output before falling back to list().length. On *nix, you could exec ls -1a | wc -l (note – that’s dash-one-a for the first command, and dash-lowercase-L for the second). Not sure what would be right on windows – perhaps just a dir and look for the summary.

Before bothering with something like this I’d strongly recommend you create a directory with a very large number of files and just see if list().length really does take too long. As this blogger suggests, you may not want to sweat this.

I’d probably go with Varkhan’s answer myself.