How to create a temporary directory folder in Java

Question

Is there a standard and reliable way of creating a temporary directory inside a Java application  There s an entry in Java s issue database  which has a bit of code in the comments  but I wonder if there is a standard solution to be found in one of the usual libraries  Apache Commons etc

User · Answer

I like the multiple attempts at creating a unique name but even this solution does not rule out a race condition   Another process can slip in after the test for exists   and the if newTempDir mkdirs    method invocation   I have no idea how to completely make this safe without resorting to native code  which I presume is what s buried inside File createTempFile

User · Answer

Do not use deleteOnExit   even if you explicitly delete it later   Google  deleteonexit is evil  for more info  but the gist of the problem is    deleteOnExit   only deletes for normal JVM shutdowns  not crashes or killing the JVM process  deleteOnExit   only deletes on JVM shutdown - not good for long running server processes because  The most evil of all - deleteOnExit   consumes memory for each temp file entry  If your process is running for months  or creates a lot of temp files in a short time  you consume memory and never release it until the JVM shuts down

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Try this small example   Code   try       Path tmpDir   Files createTempDirectory  tmpDir        System out println tmpDir toString         Files delete tmpDir     catch  IOException e        e printStackTrace         Imports  java io IOException java nio file Files java nio file Path  Console output on Windows machine  C  Users userName AppData Local Temp tmpDir2908538301081367877  Comment  Files createTempDirectory generates unique ID atomatically - 2908538301081367877    Note  Read the following for deleting directories recursively  Delete directories recursively in Java

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Before Java 7 you could also   File folder   File createTempFile  testFileUtils           no suffix folder delete    folder mkdirs    folder deleteOnExit

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If you need a temporary directory for testing and you are using jUnit   Rule together with TemporaryFolder solves your problem    Rule public TemporaryFolder folder   new TemporaryFolder      From the documentation      The TemporaryFolder Rule allows creation of files and folders that are guaranteed to be deleted when the test method finishes  whether it passes or fails       Update    If you are using JUnit Jupiter  version 5 1 1 or greater   you have the option to use JUnit Pioneer which is the JUnit 5 Extension Pack    Copied from the project documentation      For example  the following test registers the extension for a single test method  creates and writes a file to the temporary directory and checks its content     Test  ExtendWith TempDirectory class  void test  TempDir Path tempDir        Path file   tempDir resolve  test txt        writeFile file       assertExpectedFileContent file       More info in the JavaDoc and the JavaDoc of TempDirectory  Gradle   dependencies       testImplementation  org junit-pioneer junit-pioneer 0 1 2      Maven    lt dependency gt      lt groupId gt org junit-pioneer lt  groupId gt      lt artifactId gt junit-pioneer lt  artifactId gt      lt version gt 0 1 2 lt  version gt      lt scope gt test lt  scope gt   lt  dependency gt      Update 2   The  TempDir annotation was added to the JUnit Jupiter 5 4 0 release as an experimental feature  Example copied from the JUnit 5 User Guide    Test void writeItemsToFile  TempDir Path tempDir  throws IOException       Path file   tempDir resolve  test txt         new ListWriter file  write  a    b    c         assertEquals singletonList  a b c    Files readAllLines file

User · Answer

Using File createTempFile and delete to create a unique name for the directory seems ok  You should add a ShutdownHook to delete the directory  recursively  on JVM shutdown

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This is what I decided to do for my own code          Create a new temporary directory  Use something like      link  recursiveDelete File   to clean this directory up since it isn t    deleted automatically     return  the new directory     throws IOException if there is an error creating the temporary directory     public static File createTempDir   throws IOException       final File sysTempDir   new File System getProperty  java io tmpdir         File newTempDir      final int maxAttempts   9      int attemptCount   0      do               attemptCount            if attemptCount  gt  maxAttempts                        throw new IOException                       The highly improbable has occurred  Failed to                          create a unique temporary directory after                         maxAttempts     attempts                       String dirName   UUID randomUUID   toString            newTempDir   new File sysTempDir  dirName         while newTempDir exists          if newTempDir mkdirs                  return newTempDir            else               throw new IOException                   Failed to create temp dir named                     newTempDir getAbsolutePath                     Recursively delete file or directory     param fileOrDir             the file or dir to delete     return             true iff all files are successfully deleted     public static boolean recursiveDelete File fileOrDir        if fileOrDir isDirectory                     recursively delete contents         for File innerFile  fileOrDir listFiles                          if  FileUtilities recursiveDelete innerFile                                 return false                                     return fileOrDir delete

User · Answer

As you can see in the other answers  no standard approach has arisen  Hence you already mentioned Apache Commons  I propose the following approach using FileUtils from Apache Commons IO          Creates a temporary subdirectory in the standard temporary directory     This will be automatically deleted upon exit          param prefix               the prefix used to create the directory  completed by a               current timestamp  Use for instance your application s name     return the directory     public static File createTempDirectory String prefix         final File tmp   new File FileUtils getTempDirectory   getAbsolutePath                       prefix   System currentTimeMillis         tmp mkdir        Runtime getRuntime   addShutdownHook new Thread               Override         public void run                  try                   FileUtils deleteDirectory tmp                 catch  IOException e                    e printStackTrace                                        return tmp       This is preferred since apache commons the library that comes as closest to the asked  standard  and works with both JDK 7 and older versions  This also returns an  old  File instance  which is stream based  and not a  new  Path instance  which is buffer based and would be the result of JDK7 s getTemporaryDirectory   method  -  Therefore it returns what most people need when they want to create a temporary directory

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This code should work reasonably well   public static File createTempDir         final String baseTempPath   System getProperty  java io tmpdir         Random rand   new Random        int randomInt   1   rand nextInt         File tempDir   new File baseTempPath   File separator    tempDir    randomInt       if  tempDir exists      false            tempDir mkdir               tempDir deleteOnExit         return tempDir

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I got the same problem so this is just another answer for those who are interested  and it s similar to one of the above   public static final String tempDir   System getProperty  java io tmpdir    tmp  System nanoTime    static       File f   new File tempDir       if  f exists            f mkdir        And for my application  I decided that to add in a option to clear the temp on exit so I added in a shut-down hook   Runtime getRuntime   addShutdownHook new Thread              Override         public void run                   stackless deletion             String root   MainWindow tempDir              Stack lt String gt  dirStack   new Stack lt String gt                 dirStack push root               while  dirStack empty                      String dir   dirStack pop                    File f   new File dir                   if f listFiles   length  0                      f delete                    else                       dirStack push dir                       for File ff  f listFiles                              if ff isFile                                ff delete                            else if ff isDirectory                                dirStack push ff getPath                                                                               The method delete all subdirs and files before deleting the temp  without using the callstack  which is totally optional and you could do it with recursion at this point   but I want to be on the safe side

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The Google Guava library has a ton of helpful utilities  One of note here is the Files class  It has a bunch of useful methods including   File myTempDir   Files createTempDir      This does exactly what you asked for in one line  If you read the documentation here you ll see that the proposed adaptation of File createTempFile  install    dir   typically introduces security vulnerabilities

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This code should work reasonably well   public static File createTempDir         final String baseTempPath   System getProperty  java io tmpdir         Random rand   new Random        int randomInt   1   rand nextInt         File tempDir   new File baseTempPath   File separator    tempDir    randomInt       if  tempDir exists      false            tempDir mkdir               tempDir deleteOnExit         return tempDir

User · Answer

Well   createTempFile  actually creates the file   So why not just delete it first  and then do the mkdir on it

User · Answer

As discussed in this RFE and its comments  you could call tempDir delete   first  Or you could use System getProperty  java io tmpdir   and create a directory there  Either way  you should remember to call tempDir deleteOnExit    or the file won t be deleted after you re done

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If you are using JDK 7 use the new Files createTempDirectory class to create the temporary directory   Path tempDirWithPrefix   Files createTempDirectory prefix     Before JDK 7 this should do it   public static File createTempDirectory       throws IOException       final File temp       temp   File createTempFile  temp   Long toString System nanoTime           if   temp delete                   throw new IOException  Could not delete temp file      temp getAbsolutePath                if   temp mkdir                   throw new IOException  Could not create temp directory      temp getAbsolutePath                return  temp       You could make better exceptions  subclass IOException  if you want

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Naively written code to solve this problem suffers from race conditions  including several of the answers here   Historically you could think carefully about race conditions and write it yourself  or you could use a third-party library like Google s Guava  as Spina s answer suggested   Or you could write buggy code   But as of JDK 7  there is good news  The Java standard library itself now provides a properly working  non-racy  solution to this problem  You want java nio file Files createTempDirectory    From the documentation   public static Path createTempDirectory Path dir                         String prefix                         FileAttribute lt   gt     attrs                                  throws IOException      Creates a new directory in the specified directory  using the given prefix to generate its name  The resulting Path is associated with the same FileSystem as the given directory       The details as to how the name of the directory is constructed is implementation dependent and therefore not specified  Where possible the prefix is used to construct candidate names    This effectively resolves the embarrassingly ancient bug report in the Sun bug tracker which asked for just such a function

User · Answer

As discussed in this RFE and its comments  you could call tempDir delete   first  Or you could use System getProperty  java io tmpdir   and create a directory there  Either way  you should remember to call tempDir deleteOnExit    or the file won t be deleted after you re done

User · Answer

Just for completion  this is the code from google guava library  It is not my code  but I think it is valueable to show it here in this thread         Maximum loop count when creating temp directories       private static final int TEMP DIR ATTEMPTS   10000              Atomically creates a new directory somewhere beneath the system s temporary directory  as      defined by the   code java io tmpdir  system property   and returns its name             lt p gt Use this method instead of   link File createTempFile String  String   when you wish to      create a directory  not a regular file  A common pitfall is to call   code createTempFile        delete the file and create a directory in its place  but this leads a race condition which can      be exploited to create security vulnerabilities  especially when executable files are to be      written into the directory             lt p gt This method assumes that the temporary volume is writable  has free inodes and free blocks       and that it will not be called thousands of times per second             return the newly-created directory       throws IllegalStateException if the directory could not be created         public static File createTempDir         File baseDir   new File System getProperty  java io tmpdir         String baseName   System currentTimeMillis      -        for  int counter   0  counter  lt  TEMP DIR ATTEMPTS  counter            File tempDir   new File baseDir  baseName   counter         if  tempDir mkdir              return tempDir                    throw new IllegalStateException           Failed to create directory within                 TEMP DIR ATTEMPTS                 attempts  tried                 baseName                0 to                 baseName                TEMP DIR ATTEMPTS - 1

User · Answer

This is the source code to the Guava library s Files createTempDir    It s nowhere as complex as you might think   public static File createTempDir       File baseDir   new File System getProperty  java io tmpdir       String baseName   System currentTimeMillis      -      for  int counter   0  counter  lt  TEMP DIR ATTEMPTS  counter          File tempDir   new File baseDir  baseName   counter       if  tempDir mkdir            return tempDir              throw new IllegalStateException  Failed to create directory within           TEMP DIR ATTEMPTS     attempts  tried           baseName    0 to     baseName    TEMP DIR ATTEMPTS - 1              By default   private static final int TEMP DIR ATTEMPTS   10000    See here

User · Answer

Well   createTempFile  actually creates the file   So why not just delete it first  and then do the mkdir on it

User · Answer

As of Java 1 7 createTempDirectory prefix  attrs  and createTempDirectory dir  prefix  attrs  are included in java nio file Files  Example  File tempDir   Files createTempDirectory  foobar   toFile

[java] How to create a temporary directory/folder in Java?

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