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 a File 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 a RandomAccessFile object to encapsulate the file object created above and open it for read only or read-write operations. Then use the
getChannel()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 the
map(MapMode,int,int)API method of the FileChannel class. This method returns a handle to a ByteBuffer class for reading/writing data - The MapMode.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.
Want to be a Java NIO Master ?
Subscribe to our newsletter and download the JDBC Ultimate Guide right now!
In order to help you master Java NIO Library, we have compiled a kick-ass guide with all the major Java NIO features and use cases! Besides studying them online you may download the eBook in PDF format!
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.
