Java IOException Too many open files

Question

I m doing some file I O with multiple files  writing to 19 files  it so happens    After writing to them a few hundred times I get the Java IOException  Too many open files   But I actually have only a few files opened at once   What is the problem here   I can verify that the writes were successful

User · Answer

You re obviously not closing your file descriptors before opening new ones  Are you on windows or linux

User · Answer

Although in most general cases the error is quite clearly that file handles have not been closed  I just encountered an instance with JDK7 on Linux that well    is sufficiently     ed up to explain here   The program opened a FileOutputStream  fos   a BufferedOutputStream  bos  and a DataOutputStream  dos   After writing to the dataoutputstream  the dos was closed and I thought everything went fine    Internally however  the dos  tried to flush the bos  which returned a Disk Full error  That exception was eaten by the DataOutputStream  and as a consequence the underlying bos was not closed  hence the fos was still open    At a later stage that file was then renamed from  something with a  tmp  to its real name  Thereby  the java file descriptor trackers lost track of the original  tmp  yet it was still open    To solve this  I had to first flush the DataOutputStream myself  retrieve the IOException and close the FileOutputStream myself   I hope this helps someone

User · Answer

Recently  I had a program batch processing files  I have certainly closed each file in the loop  but the error still there   And later  I resolved this problem by garbage collect eagerly every hundreds of files   int index  while          try              do with outputStream          finally           out close              if  index     100   0          System gc

User · Answer

On Linux and other UNIX   UNIX-like platforms  the OS places a limit on the number of open file descriptors that a process may have at any given time   In the old days  this limit used to be hardwired1  and relatively small   These days it is much larger  hundreds   thousands   and subject to a  soft  per-process configurable resource limit    Look up the ulimit shell builtin       Your Java application must be exceeding the per-process file descriptor limit   You say that you have 19 files open  and that after a few hundred times you get an IOException saying  too many files open    Now this particular exception can ONLY happen when a new file descriptor is requested  i e  when you are opening a file  or a pipe or a socket    You can verify this by printing the stacktrace for the IOException   Unless your application is being run with a small resource limit  which seems unlikely   it follows that it must be repeatedly opening files   sockets   pipes  and failing to close them   Find out why that is happening and you should be able to figure out what to do about it   FYI  the following pattern is a safe way to write to files that is guaranteed not to leak file descriptors   Writer w   new FileWriter       try          write stuff to the file   finally       try           w close          catch  IOException ex               Log error writing file and bail out              1 - Hardwired  as in compiled into the kernel   Changing the number of available fd slots required a recompilation     and could result in less memory being available for other things   In the days when Unix commonly ran on 16-bit machines  these things really mattered   UPDATE  The Java 7 way is more concise   try  Writer w   new FileWriter                write stuff to the file      the  w  resource is automatically closed      UPDATE 2  Apparently you can also encounter a  too many files open  while attempting to run an external program   The basic cause is as described above   However  the reason that you encounter this in exec      is that the JVM is attempting to create  pipe  file descriptors that will be connected to the external application s standard input   output   error

User · Answer

For UNIX   As Stephen C has suggested  changing the maximum file descriptor value to a higher value avoids this problem   Try looking at your present file descriptor capacity        ulimit -n   Then change the limit according to your requirements        ulimit -n  lt value gt    Note that this just changes the limits in the current shell and any child   descendant process   To make the change  stick  you need to put it into the relevant shell script or initialization file

[java] Java IOException "Too many open files"

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