public abstract MappedByteBuffer map (MapMode mode, long position, long size) throws IOException

Maps a region of this channel's file directly into memory.

A region of a file may be mapped into memory in one of three modes:

For a read-only mapping, this channel must have been opened for reading; for a read/write or private mapping, this channel must have been opened for both reading and writing.

The mapped byte buffer returned by this method will have a position of zero and a limit and capacity of size; its mark will be undefined. The buffer and the mapping that it represents will remain valid until the buffer itself is garbage-collected.

A mapping, once established, is not dependent upon the file channel that was used to create it. Closing the channel, in particular, has no effect upon the validity of the mapping.

Many of the details of memory-mapped files are inherently dependent upon the underlying operating system and are therefore unspecified. The behavior of this method when the requested region is not completely contained within this channel's file is unspecified. Whether changes made to the content or size of the underlying file, by this program or another, are propagated to the buffer is unspecified. The rate at which changes to the buffer are propagated to the file is unspecified.

For most operating systems, mapping a file into memory is more expensive than reading or writing a few tens of kilobytes of data via the usual read and write methods. From the standpoint of performance it is generally only worth mapping relatively large files into memory.

mode     One of the constants READ_ONLY, READ_WRITE, or PRIVATE defined in the MapMode class, according to whether the file is to be mapped read-only, read/write, or privately (copy-on-write), respectively
position     The position within the file at which the mapped region is to start; must be non-negative
size     The size of the region to be mapped; must be non-negative and no greater than java.lang.Integer.MAX_VALUE

Returns:  The mapped byte buffer

NonReadableChannelException     If the mode is READ_ONLY but this channel was not opened for reading
NonWritableChannelException     If the mode is READ_WRITE or PRIVATE but this channel was not opened for both reading and writing
IllegalArgumentException     If the preconditions on the parameters do not hold
IOException     If some other I/O error occurs

See also:
java.nio.channels.FileChannel.MapMode, java.nio.MappedByteBuffer