In this example we shall show you how to write data to a file using a memory mapped ByteBuffer and a NIO FileChannel 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-write). To do so you must first create a RandomAccessFile object to encapsulate the file object created above and open it for 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 MappedByteBuffer class for reading/writing data - Write data into this buffer at the current position using the
put(byte)API method of the MappedByteBuffer class. Do not forget toforce()any changes made to this buffer’s contents so as to be written to the storage device containing the mapped file
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.MappedByteBuffer;
import java.nio.channels.FileChannel;
public class SaveChangesToMemoryMappedByteBufferFromFileChannel {
public static void main(String[] args) {
try {
File file = new File("myfile.dat");
// create a random access file stream (read-write)
FileChannel channel = new RandomAccessFile(file, "rw").getChannel();
// map a region of this channel's file directly into memory
MappedByteBuffer buf = channel.map(FileChannel.MapMode.READ_WRITE, 0, (int) channel.size());
// write the given byte into this buffer at the current
// position, and then increment the position
buf.put((byte)0x01);
// force any changes made to this buffer's content to be written
// to the storage device containing the mapped file
buf.force();
// close the channel
channel.close();
}
catch (IOException e) {
System.out.println("I/O Error: " + e.getMessage());
}
}
}
This was an example of how to write data to a file using a memory mapped ByteBuffer and a NIO FileChannel in Java.
