What is the most efficient way to list all of the files in a directory (including sub-directories)?

Question

I am writing a servlet which will examine a directory on the server (external to the web container), and recursively search for certain files (by certain files, I mean files that are of a certain extension as well as a certain naming convention). Once these files are found, the servlet responds with a long list of all of the found files (including the full path to the files). My problem is that there are so many files and directories that my servlet goes extremely slow. I was wondering if there was a best practice or existing servlet for this type of problem? Would it be more efficient to simply compile the entire list of files and do the filtering via js/jquery on the client side?

Answer 1

If you really have loads of files, you might think about spawning an external process to do the searching. If you're running on a unix-like server (like linux), you might get speed gains by having the "find" command do the searching for you, and parse its output.

You can google for many examples of how to use "find".

Answer 2

i think your servlet works slow because of hard drive speed. if list of files a permanent you should load it to memory

Answer 3

Disk access is slow and as the number of files and directories increases, you'll rapidly reach a point where your servlet will be useless when using the conventional recursive search through the directory tree. You'll especially reach this limit quickly if you have a large number of concurrent users performing the same search at the same time.

It's instead, much better to use an external batch job to generate the list of files which can then be read into the servlet through a database call or even by just parsing a file containing all the file names separated by a newline character. Using "find" on Linux is a simple way to do this: e.g.

find <path_to_directory> -name '*.bin' > list_files.txt

This would list every file name that ends with .bin in a particular directory and output it into a file named list_files.txt. Your servlet could then read in that file and create the list of files from there.

Answer 4

I see two possible reasons why this process might be going slowly:

1) Disk I/O is taking too long. This'll be a real constraint that you can't do much about. Usually the operating system is pretty good at keeping structures in memory that allow it to find files in your folders much quicker. If it is too slow regardless, you might have to build an index yourself in memory. This all depends on how you're doing it.

In any case, if this is the issue (you can try measuring), then there's no way doing the filtering client side will help, as that shouldn't really take very long, no matter where you do it. Instead you're going to make the client slower by sending it more data to sort through.

2) There's something wrong with your directory traversal. You say it's "recursive". If you mean it's actually recursive, i.e. a method that calls itself whenever it encounters a new directory, then that might well be slowing you down (the overhead really adds up). There's some stuff about tree traversal on wikipedia, but basically just use a queue or stack to keep track of where you are in the traversal, instead of using your method state to do so.

Note that a file system isn't actually a tree, but I'm assuming that it is in this case. It gets a bit hairier otherwise.

I don't agree with the other posters that you can't implement it in-process. It should work pretty well up to a certain point, no need for batch jobs just yet.