How can I lock a file using java if possible

Question

I have a Java process that opens a file using a FileReader  How can I prevent another  Java  process from opening this file  or at least notify that second process that the file is already opened  Does this automatically make the second process get an exception if the file is open  which solves my problem  or do I have to explicitly open it in the first process with some sort of flag or argument  To clarify  I have a Java app that lists a folder and opens each file in the listing for processing it  It processes each file after the other  The processing of each file consists of reading it and doing some calculations based on the contents and it takes about 2 minutes  I also have another Java app that does the same thing but instead writes on the file  What I want is to be able to run these apps at the same time so the scenario goes like this  ReadApp lists the folder and finds files A  B  C  It opens file A and starts the reading  WriteApp lists the folder and finds files A  B  C  It opens file A  sees that is is open  by an exception or whatever way  and goes to file B  ReadApp finishes file A and continues to B  It sees that it is open and continues to C  It is crucial that WriteApp doesn t write while ReadApp is reading the same file or vice versa  They are different processes

User · Answer

If you can use Java NIO  JDK 1 4 or greater   then I think you re looking for java nio channels FileChannel lock    FileChannel lock

User · Answer

Use a RandomAccessFile  get it s channel  then call lock     The channel provided by input or output streams does not have sufficient privileges to lock properly   Be sure to call unlock   in the finally block  closing the file doesn t necessarily release the lock

User · Answer

FileChannel lock is probably what you want   try       FileInputStream in   new FileInputStream file       java nio channels FileLock lock   in getChannel   lock        Reader reader   new InputStreamReader in  charset                   Disclaimer  Code not compiled and certainly not tested    Note the section entitled  platform dependencies  in the API doc for FileLock

User · Answer

Use this for unix if you are transferring using winscp or ftp  public static void isFileReady File entry  throws Exception           long realFileSize   entry length            long currentFileSize   0          do               try  FileInputStream fis   new FileInputStream entry                      currentFileSize   0                  while  fis available    gt  0                        byte   b   new byte 1024                       int nResult   fis read b                       currentFileSize    nResult                      if  nResult    -1                          break                                  catch  Exception e                    e printStackTrace                              System out println  quot currentFileSize  quot    currentFileSize    quot   realFileSize  quot    realFileSize             while  currentFileSize    realFileSize

User · Answer

use java nio channels FileLock in conjunction with java nio channels FileChannel

User · Answer

Below is a sample snippet code to lock a file until it s process is done by JVM      public static void main String   args  throws InterruptedException       File file   new File FILE FULL PATH NAME       RandomAccessFile in   null      try           in   new RandomAccessFile file   rw            FileLock lock   in getChannel   lock            try                while  in read      -1                    System out println in readLine                             finally               lock release                    catch  FileNotFoundException e            e printStackTrace          catch  IOException e            e printStackTrace         finally           try               in close              catch  IOException e                e printStackTrace

User · Answer

This may not be what you are looking for  but in the interest of coming at a problem from another angle      Are these two Java processes that might want to access the same file in the same application  Perhaps you can just filter all access to the file through a single  synchronized method  or  even better  using JSR-166    That way  you can control access to the file  and perhaps even queue access requests

User · Answer

Don t use  the classes in thejava io package  instead  use the java nio package   The latter has a FileLock class  You can apply a lock to a FileChannel    try              Get a file channel for the file         File file   new File  filename            FileChannel channel   new RandomAccessFile file   rw   getChannel                Use the file channel to create a lock on the file             This method blocks until it can retrieve the lock          FileLock lock   channel lock                           use channel lock OR channel tryLock                           Try acquiring the lock without blocking  This method returns            null or throws an exception if the file is already locked          try               lock   channel tryLock              catch  OverlappingFileLockException e                   File is already locked in this thread or virtual machine                       Release the lock - if it is not null          if  lock    null                 lock release                          Close the file         channel close          catch  Exception e

[java] How can I lock a file using java (if possible)

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