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What is the best way to write large amount of data to a file?

Posted on 2004-08-02
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Hi,

I have a Java program that does queries on a table for records meeting specific criteria and put the data in a file.  I store the data that will go to the file in a byte array.  Problem is I may have a large set of results that I may need to allocate a large enough byte array that maybe too big for the computer's memory.

I know there is FileOutputStream out = new FileOutputStream(String filename, boolean append) and write to the file over and over getting some results from the database at a time.   But I was told the pointer to a place in the file in the FileOutputStream is unreliable.

It is very IMPORTANT that the data coming from the database is on the right positions in the file because the file will go to a mainframe.

What very reliable methods do you all suggest I do to write to a file several times when I am fetching large amounts of data from a database?  Thanks.
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Question by:Mero
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by:CEHJ
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Use a RandomAccessFile and use its seek method
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by:CEHJ
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>>I store the data that will go to the file in a byte array

Can you show us how?
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by:Mero
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The code is long so I'll show you snippets of it.  This is the code currently gets all the data in the byte array and puts in the file one time.

for (loop=0; loop<numberofrecords; loop++)
{
.....
                byte[] buff;  //the byte array for the file
      for ( i = 0; i < len; i++ )
                {
          c = string.charAt(i);
          buff[i] = (byte)c;
      }
.....
//write the end of the line
dataForFile[end] = 10;
end++;
dataForFile[end] = 0;  
}

processFile.writeData(dataForFile, true);
...
public void writeData(byte[] data, boolean append)
{
      try {
            FileOutputStream out = new FileOutputStream(anout, append);
            BufferedOutputStream bufout = new BufferedOutputStream(out);
            int i = 0;

            while ( data[i] != 0 )
            {
                                                ...
                  out.write(data[i]);
                  i++;
                                 }
                 }
}
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by:sciuriware
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Do not worry too much; write as much as you can in every write but not too much:

1 byte at a time is tragically slow.
> 1Mb per write is not showing any improvement over some 16 Kb.
Ideally you should write a disk cluster if you only knew what the disk situation was
and if drivers didn't interfere.
So, write 16K - 64Kb at a time and you're fine, even on a mainframe!

;JOOP!
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by:objects
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> But I was told the pointer to a place in the file in the FileOutputStream is unreliable.

I'm not aware of any problem with it. Using a FileOutputStream in append mode should be fine.
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by:objects
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to save wrting to memory, write your data directly to file:

FileOutputStream out = new FileOutputStream(anout, true);
BufferedOutputStream bufout = new BufferedOutputStream(out);
for (loop=0; loop<numberofrecords; loop++)
{
.....
     bufout.write(string.getBytes());
.....
    //write the end of the line
    bufout.write(10);
}
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by:Mero
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sciriuware,

I'm not currently writing one byte at a time.  I have it right now so that I allocate about 380K for the byte array.  I put all the data just one time provided the data does not exceed 380K.  That's why I'm trying to find another way where I don't have to allocate much more where there is a danger of running out of memory.

I might try using the RandomAccess method, CEHJ.  Thanks.

I don't think I want to use the append mode if there is some doubt about that.  The positions of the data ARE IMPORTANT on the file that is going to the mainframe.  

I'm just open to some more suggestions.  Thanks.
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by:sciuriware
sciuriware earned 55 total points
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You won't run out of memory: JAVA starts with a limit of 64Mb

You can use RandomAccessFile or FileOutputStream, both are 100% reliable.
The latter is important if the mainframe "eats" EBCDIC rather than ASCII.
The fears you have about positions or appending are superfluous:
   without programming errors there is no problem whatsoever.
;JOOP!
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Author Comment

by:Mero
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Believe me, I have.  I have had a out of memory exception trying to allocate 56MB for a byte array.
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by:sciuriware
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That's in line with what I said.
I run with a 512Mb limit.
You can, you know?

;JOOP!
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by:objects
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> That's why I'm trying to find another way where I don't have to allocate much more where there is a danger of running out of memory.

the code i posted above removes the need for the byte array altogether, by writing directly to file.

> I don't think I want to use the append mode if there is some doubt about that.

there isn't
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by:Mero
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Can anybody show me an example  of writing to the file using RandomAccessFile and seek() with the I/O buffered?  Thanks.
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by:CEHJ
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Here is an example of using the RAF i suggested. Of course, you can seek() to a different point in the file if you want:

http://javaalmanac.com/egs/java.io/UseRandomAccessFile.html
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by:
CEHJ earned 75 total points
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Your method with RAF would be:

      public void writeData(byte[] data, boolean append)
      {
            RandomAccessFile raf = null;
            try
            {
                  File f = new File("filename");
                  raf = new RandomAccessFile(f, "rw");
                  // Seek to end of file
                  raf.seek(f.length());
                  int i = 0;

                  // Append to the end
                  while ( data[i] != 0 )
                  {
                        raf.write(data[i]);
                        i++;
                  }
            }
            catch (IOException e)
            {
                  e.printStackTrace();
            }
            finally
            {

                  try { raf.close(); } catch(IOException e) { /* ignore */ }
            }
      }
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by:CEHJ
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(that assumes append == true. If not, you may want to seek elsewhere)
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by:Mero
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is this write buffered?  If not, can you show me how?  Thanks.
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by:CEHJ
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>>is this write buffered?  

No. Why would you want it to be?
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by:Mero
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Why?  It is very common in Java to buffer your I/O
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by:CEHJ
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Sometimes buffering can improve write speeds, but not necessarily. You should try both with and without before deciding.
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by:Mero
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but how do you do it in this example?
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by:CEHJ
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objects has already posted an example of that, beginning:

>>to save wrting to memory, write your data directly to file:

although it must be said that the statement quoted above is inaccurate. The reason buffering is used is precisely to write to memory rather than 'directly fo file', which is what can *sometimes* improve performance
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Assisted Solution

by:objects
objects earned 70 total points
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RAF has no real advanatages in this situation, there are no problems with opening a file in append mode that I am aware of.
Can you post a reference or bug report of any problems?

Your code is already buffered :)
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Author Comment

by:Mero
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thanks folks!
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by:CEHJ
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:-)
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by:objects
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