Reading Files - Beginning Java 2 SDK - page 457

force()

If the buffer was mapped in MAP _ RW mode this method forces any changes
made to the buffer's contents to be written to the file and returns a reference to
the buffer. For buffers created with MAP _ RO or MAP _ COW mode this method has
no effect.

load() Loads the contents of the buffer into memory and returns a reference to the

buffer.

isLoaded() Returns true if this buffer's contents is available in physical memory and false

otherwise.

Using a memory-mapped file through a MappedByteBuffer is simplicity itself, so let's try it.

Try It Out - Using a Memory-Mapped File
We will access and modify the primes.bin file using a MappedByteBuffer . Here's the code:

import java.io.*;
import java.nio.*;
import java.nio.channels.FileChannel;

public class MemoryMappedFile {
public static void main(String[] args) {
File aFile = new File("C:/Beg Java Stuff/primes.bin");
RandomAccessFile ioFile = null;

try {
ioFile = new RandomAccessFile(aFile,"rw");

} catch(FileNotFoundException e) {
e.printStackTrace(System.err);
System.exit(1);
}
FileChannel ioChannel = ioFile.getChannel();

final int PRIMESREQUIRED = 10;
long[] primes = new long[PRIMESREQUIRED];

int index = 0; // Position for a prime in the file
final long REPLACEMENT = 999999L; // Replacement for a selected prime

try {
final int PRIMECOUNT = (int)ioChannel.size()/8;
MappedByteBuffer buf =
ioChannel.map(ioChannel.MAP _ RW, 0L, ioChannel.size()).load();

for(int i = 0 ; i<PRIMESREQUIRED ; i++) {
index = 8*(int)(PRIMECOUNT*Math.random());
primes[i] = buf.getLong(index);
buf.putLong(index, REPLACEMENT );
}