What is the best way to generate a unique and short file name in Java

Question

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

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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

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Problem is synchronization  Separate out regions of conflict   Name the file as     server-name   thread process-name   millisecond timestamp   extension   example   aws1 t1 1447402821007 png

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I use current milliseconds with random numbers   i e  Random random new Random    String ext     jpeg   File dir   new File   home pregzt    String name   String format   s s  System currentTimeMillis   random nextInt 100000  ext   File file   new File dir  name

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Generating Unique File Name     public String getFileName             String timeStamp   new SimpleDateFormat  yyyy-MM-dd HH mm ss   format new Date             return  PNG     timeStamp      png

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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

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This also works  String logFileName   new SimpleDateFormat  yyyyMMddHHmm  txt    format new Date      logFileName    loggerFile     logFileName

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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

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Why not just use something based on a timestamp

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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

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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

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This works for me   String generateUniqueFileName         String filename           long millis   System currentTimeMillis        String datetime   new Date   toGMTString        datetime   datetime replace               datetime   datetime replace               String rndchars   RandomStringUtils randomAlphanumeric 16       filename   rndchars         datetime         millis      return filename         USE   String newFile  do  newFile generateUniqueFileName           FileExt    while new File basePath newFile  exists       Output filenames should look like    2OoBwH8OwYGKW2QE 4Sep2013061732GMT 1378275452253 Ext

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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 prefix  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

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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

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Why not use synchronized to process multi thread  here is my solution It s can generate a short file name   and it s unique   private static synchronized String generateFileName        String name   make index       index         return name    private static String make int index        if index    0  return         return String valueOf chars index   chars length     make index   chars length     private static int index   1  private static char   chars     a   b   c   d   e   f   g            h   i   j   k   l   m   n            o   p   q   r   s   t            u   v   w   x   y   z      blew is main function for test    It s work   public static void main String   args        List lt String gt  names   new ArrayList lt  gt         List lt Thread gt  threads   new ArrayList lt  gt         for  int i   0  i  lt  100  i              Thread thread   new Thread new Runnable                  Override             public void run                     for  int i   0  i  lt  1000  i                          String name   generateFileName                        names add name                                                       thread run            threads add thread              for  int i   0  i  lt  10  i              try               threads get i  join              catch  InterruptedException e                e printStackTrace                         System out println names       System out println names size

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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

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I understand that I am too late to reply on this question  But I think I should put this as it seems something different from other solution   We can concatenate threadname and current timeStamp as file name  But with this there is one issue like some thread name contains special character like     which can create problem in creating file name  So we can remove special charater from thread name and then concatenate thread name and time stamp  fileName   threadName after removing special charater    currentTimeStamp

[java] What is the best way to generate a unique and short file name in Java

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