Create memory mapped file

Ilias TsagklisNovember 11th, 2012Last Updated: March 19th, 2019

0 293 1 minute read

This is an example of how to create a memory mapped file in Java. Doing reads and writes of data using Java NIO Channels implies that you should :

Create aFile object to encapsulate an actual file in the file system
Create a random access file stream (read only, read-write). To do so you must first create aRandomAccessFile object to encapsulate the file object created above and open it for read only or read-write operations. Then use thegetChannel() API method of the RandomAccessFile object to get the file channel to read/write data from/to
Map a region of this channel’s file directly into memory by using themap(MapMode,int,int) API method of theFileChannel class. This method returns a handle to aByteBuffer class for reading/writing data
TheMapMode.PRIVATE option forces that changes made to the resulting buffer will not be propagated to the file and will not be visible to other programs that have mapped the same file; instead, they will cause private copies of the modified portions of the buffer to be created

as described in the code snippet below.

Do not forget to close the channel after you are done processing the file so as to release operating system resources.

package com.javacodegeeks.snippets.core;import java.io.File;import java.io.IOException;import java.io.RandomAccessFile;import java.nio.ByteBuffer;import java.nio.channels.FileChannel;public class CreateMemoryMappedFile {public static void main(String[] args) {try {File file = new File("myfile.dat");// create a random access file stream (for read only)    FileChannel readOnlyChannel = new RandomAccessFile(file, "r").getChannel();    // map a region of this channel's file directly into memory    ByteBuffer readOnlyBuf =    readOnlyChannel.map(FileChannel.MapMode.READ_ONLY, 0, (int) readOnlyChannel.size());    // create a random access file stream (read-write)    FileChannel readWriteChannel = new RandomAccessFile(file, "rw").getChannel();    // map a region of this channel's file directly into memory    ByteBuffer readWriteBuf =  readWriteChannel.map(FileChannel.MapMode.READ_WRITE, 0, (int) readWriteChannel.size());    // create a random access file stream (private/copy-on-write))    FileChannel privateChannel = new RandomAccessFile(file, "rw").getChannel();    // map a region of this channel's file directly into memory    ByteBuffer privateBuf =    privateChannel.map(FileChannel.MapMode.PRIVATE, 0, (int) privateChannel.size());    }catch (IOException e) {System.out.println("I/O Error: " + e.getMessage());}}}

This was an example of how to create a memory mapped file in Java.

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