How to truncate a memory mapped file

Question

If one maps a file, the mapped size will become the file size. So the size parameter passed to the map() method of FileChannel should be carefully calculated. However, what if one can't decide beforehand the size of the file?
I tried to use truncate(), but that throws a runtime exception: truncate() can't be used on a file with user-mapped section open.
public class MapFileSizeTest extends TestCase
  public void testMapFileSize() throws Exception
    final File file=new File("testMapFileSize.data");
    FileChannel configChannel= new RandomAccessFile(file, "rw").getChannel();
    //this will result a file with size 2,097,152kb
    MappedByteBuffer configBuffer= configChannel.map(FileChannel.MapMode.READ_WRITE,
        0, 1000000000);
    configBuffer.flip();
    configBuffer.force();  
    //truncate can't be used on a file with user-mapped section open
//    configChannel.truncate( configBuffer.limit());
    configChannel.close();
}Could somebody please give some suggestions? Thank you very much.

Answer 1

The region (position/size) that you pass to the map method should be contained in the file. The spec includes this statement: "The behavior of this method when the requested region is not completely contained within this channel's file is unspecified. " In the Sun implementation, we attempt to extend the file if the requested region is not completely contained but this is not required by the specification. Once you map a region you should not attempt to truncate the file as it can lead to unspecified exceptions (see MappedByteBuffer specification). Windows prevents it; others allows it but cause access to the inaccessible region to SIGSEGV (which must be handled and converted into a runtime error).