How to retrieve a list of directories QUICKLY in Java?

As has already been mentioned, this is basicly a hardware problem. Disk access is always slow, and most file systems aren't really designed to handle directories with that many files.

If you for some reason have to store all the files in the same directory, I think you'll have to maintain your own cache. This could be done using a local database such as sqlite, HeidiSQL or HSQL. If you want extreme performance, use a java TreeSet and cache it in memory. This means at the very least that you'll have to read the directory less often, and it could possibly be done in the background. You could reduce the need to refresh the list even further by using your systems native file update notification API (inotify on linux) to subscribe to changes to the directory.

This doesn't seem to be possible for you, but I once solved a similiar problem by "hashing" the files into subdirectories. In my case, the challenge was to store a couple of millions images with numeric ids. I constructed the directory structure as follows:

images/[id - (id % 1000000)]/[id - (id % 1000)]/[id].jpg

This has worked well for us, and it's the solution that I would recommend. You could do something similiar to alpha-numeric filenames by simply taking the first two letters of the filename, and then the next two letters. I've done this as well once, and it did the job as well.

Do you know the finite list of possible subdirectory names? If so, use a loop over all possible names and check for directory's existence.

Otherwise, you can not get ONLY directory names in most underlying OSs (e.g. in Unix, the directory listing is merely reading contents of "directory" file, so there's no way to find "just directories" quickly without listing all the files).

However, in NIO.2 in Java7 (see http://java.sun.com/developer/technicalArticles/javase/nio/#3 ), there's a way to have a streaming directory list so you don't get a full array of file elements cluttering your memory/network.

There's actually a reason why you got the lectures: it's the correct answer to your problem. Here's the background, so that perhaps you can make some changes in your live environment.

First: directories are stored on the filesystem; think of them as files, because that's exactly what they are. When you iterate through the directory, you have to read those blocks from the disk. Each directory entry will require enough space to hold the filename, and permissions, and information on where that file is found on-disk.

Second: directories aren't stored with any internal ordering (at least, not in the filesystems where I've worked with directory files). If you have 150,000 entries and 2 sub-directories, those 2 sub-directory references could be anywhere within the 150,000. You have to iterate to find them, there's no way around that.

So, let's say that you can't avoid the big directory. Your only real option is to try to keep the blocks comprising the directory file in the in-memory cache, so that you're not hitting the disk every time you access them. You can achieve this by regularly iterating over the directory in a background thread -- but this is going to cause undue load on your disks, and interfere with other processes. Alternatively, you can scan once and keep track of the results.

The alternative is to create a tiered directory structure. If you look at commercial websites, you'll see URLs like /1/150/15023.html -- this is meant to keep the number of files per directory small. Think of it as a BTree index in a database.

Of course, you can hide that structure: you can create a filesystem abstraction layer that takes filenames and automatically generates the directory tree where those filenames can be found.

The key problem could be File.isDirectory() function called in a loop.

File.isDirectory() can be extremely slow. I saw NFS take 10 seconds to process 200 file directory.

If you can by all means prevent File.isDirectory() calls (e.g. test for extension, no extension == directory), you could improve the performance drastically.

Otherwise I would suggest doing JNA/JNI/writing a native script that does this for you.

The jCifs library lets you manipulate windows network shares more efficiently. I am not aware of a library that would do this for other network file systems.