I don't necessarily want to use UUIDs since they are fairly long.
The file just needs to be unique within its directory.
One thought which comes to mind is to use File.createTempFile(String prefix, String suffix), but that seems wrong because the file is not temporary.
The case of two files created in the same millisecond needs to be handled.
I use the timestamp
i.e
new File( simpleDateFormat.format( new Date() ) );
And have the simpleDateFormat initialized to something like as:
new SimpleDateFormat("File-ddMMyy-hhmmss.SSS.txt");
EDIT
What about
new File(String.format("%s.%s", sdf.format( new Date() ),
random.nextInt(9)));
Unless the number of files created in the same second is too high.
If that's the case and the name doesn't matters
new File( "file."+count++ );
:P
Well, you could use the 3-argument version: File.createTempFile(String prefix, String suffix, File directory) which will let you put it where you'd like. Unless you tell it to, Java won't treat it differently than any other file. The only drawback is that the filename is guaranteed to be at least 8 characters long (minimum of 3 characters for the suffix, plus 5 or more characters generated by the function).
If that's too long for you, I suppose you could always just start with the filename "a", and loop through "b", "c", etc until you find one that doesn't already exist.
Have a look at my answer to this similar (but .NET) question.
Basically, create a random filename in any normal way, using an instance of java.util.Random or whatever other RNG you want to use. Then attempt to create the file - if it fails, repeat until it works.
Look at the File javadoc, the method createNewFile will create the file only if it doesn't exist, and will return a boolean to say if the file was created.
You may also use the exists() method:
int i = 0;
String filename = Integer.toString(i);
File f = new File(filename);
while (f.exists()) {
i++;
filename = Integer.toString(i);
f = new File(filename);
}
f.createNewFile();
System.out.println("File in use: " + f);
How about generate based on time stamp rounded to the nearest millisecond, or whatever accuracy you need... then use a lock to synchronize access to the function.
If you store the last generated file name, you can append sequential letters or further digits to it as needed to make it unique.
Or if you'd rather do it without locks, use a time step plus a thread ID, and make sure that the function takes longer than a millisecond, or waits so that it does.
Combining other answers, why not use the ms timestamp with a random value appended; repeat until no conflict, which in practice will be almost never.
For example: File-ccyymmdd-hhmmss-mmm-rrrrrr.txt
I'd use Apache Commons Lang library (http://commons.apache.org/lang).
There is a class org.apache.commons.lang.RandomStringUtils that can be used to generate random strings of given length. Very handy not only for filename generation!
Here is the example:
String ext = "dat";
File dir = new File("/home/pregzt");
String name = String.format("%s.%s", RandomStringUtils.randomAlphanumeric(8), ext);
File file = new File(dir, name);
If you have access to a database, you can create and use a sequence in the file name.
select mySequence.nextval from dual;
It will be guaranteed to be unique and shouldn't get too large (unless you are pumping out a ton of files).
It looks like you've got a handful of solutions for creating a unique filename, so I'll leave that alone. I would test the filename this way:
String filePath;
boolean fileNotFound = true;
while (fileNotFound) {
String testPath = generateFilename();
try {
RandomAccessFile f = new RandomAccessFile(
new File(testPath), "r");
} catch (Exception e) {
// exception thrown by RandomAccessFile if
// testPath doesn't exist (ie: it can't be read)
filePath = testPath;
fileNotFound = false;
}
}
//now create your file with filePath