Java Reference
In-Depth Information
Because the
MappedByteBuffer
class is a subclass of the
ByteBuffer
class, you have all the
ByteBuffer
methods available for a
MappedByteBuffer
object. This implies that you can create view buf-
fers for a
MappedByteBuffer
object, for instance.
The
MappedByteBuffer
class defines three methods of its own to add to those inherited from the
ByteBuffer
class:
TABLE 11-1:
MappedByteBuffer Class Methods
METHOD DESCRIPTION
force()
If the buffer was mapped in
MapMode.READ_WRITE
mode, this method forces any changes that you make
to the buffer's contents to be written to the file and returns a reference to the buffer. For buffers created
with other access modes, this method has no effect.
load()
Tries on a “best efforts” basis to load the contents of the buffer into memory and returns a reference to the
buffer.
isLoaded()
Returns
true
if it is likely that this buffer's contents are available in physical memory and
false
other-
wise.
The
load()
method is dependent on external operating system functions executing to achieve the desired
result, so the result cannot in general be guaranteed. Similarly, when you get a
true
return from the
isLoaded()
method, this is an indication of a probable state of affairs rather than a guarantee. This doesn't
imply any kind of problem. It just means that accessing the data in the mapped byte buffer may take longer
that you might expect in some instances.
Unless the file is large, using a mapped byte buffer is typically slower than using the
read()
and
write()
methods for a channel. Using a memory-mapped file through a
MappedByteBuffer
is simplicity itself
though, so let's try it.
TRY IT OUT: Using a Memory-Mapped File
You will access and modify the
primes_backup.bin
file using a
MappedByteBuffer
, so you might want
to rerun the file copy program to restore it to its original condition. Here's the code:
import static java.nio.file.StandardOpenOption.*;
import static java.nio.channels.FileChannel.MapMode.READ_WRITE;
import java.nio.file.*;
import java.nio.channels.FileChannel;
import java.io.IOException;
import java.nio.MappedByteBuffer;
import java.util.EnumSet;
public class MemoryMappedFile {
public static void main(String[] args) {
Path file = Paths.get(System.getProperty("user.home")).
resolve("Beginning Java
Stuff").resolve("primes_backup.bin");
if(!Files.exists(file)) {
System.out.println(file + " does not exist. Terminating
