FileChannel
In this chapter you will learn:
- How to use FileChannel
- How to create a FileChannel for reading and writing
- How to get the size of a FileChannel
Use FileChannel
A channel for reading, writing, mapping, and manipulating a file.
A file channel has a current position within its file which can be both queried and modified. File channels are safe for use by multiple concurrent threads.
FileChannel.MapMode.READ_ONLY defines a mode for a read-only mapping.
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.nio.MappedByteBuffer;
import java.nio.channels.FileChannel;
/*from j a va2 s. c o m*/
public class MainClass {
public static void main(String args[]) {
FileInputStream fIn;
FileOutputStream fOut;
FileChannel fIChan, fOChan;
long fSize;
MappedByteBuffer mBuf;
try {
fIn = new FileInputStream(args[0]);
fOut = new FileOutputStream(args[1]);
fIChan = fIn.getChannel();
fOChan = fOut.getChannel();
fSize = fIChan.size();
mBuf = fIChan.map(FileChannel.MapMode.READ_ONLY, 0, fSize);
fOChan.write(mBuf); // this copies the file
fIChan.close();
fIn.close();
fOChan.close();
fOut.close();
} catch (IOException exc) {
System.out.println(exc);
System.exit(1);
} catch (ArrayIndexOutOfBoundsException exc) {
System.out.println("Usage: Copy from to");
System.exit(1);
}
}
}
Create a FileChannel for reading and writing
FileChannel.MapMode.READ_WRITE defines the mode for a read/write mapping.
import java.io.File;
import java.io.IOException;
import java.io.RandomAccessFile;
import java.nio.IntBuffer;
import java.nio.channels.FileChannel;
//from ja va 2s . c o m
public class Main {
public static void main(String[] args) throws IOException {
FileChannel fc = new RandomAccessFile(new File("temp.tmp"), "rw").getChannel();
IntBuffer ib = fc.map(FileChannel.MapMode.READ_WRITE, 0, fc.size()).asIntBuffer();
ib.put(0);
for (int i = 1; i < 10; i++)
ib.put(ib.get(i - 1));
fc.close();
}
}
Get the size of a FileChannel
abstract long size()
returns the current size of this channel's file.
import java.io.FileInputStream;
import java.io.IOException;
import java.nio.ByteBuffer;
import java.nio.channels.FileChannel;
//ja v a 2 s.c o m
public class MainClass {
public static void main(String args[]) {
FileInputStream fIn;
FileChannel fChan;
long fSize;
ByteBuffer mBuf;
try {
fIn = new FileInputStream("test.txt");
fChan = fIn.getChannel();
fSize = fChan.size();
mBuf = ByteBuffer.allocate((int) fSize);
fChan.read(mBuf);
mBuf.rewind();
for (int i = 0; i < fSize; i++)
System.out.print((char) mBuf.get());
fChan.close();
fIn.close();
} catch (IOException exc) {
System.out.println(exc);
}
}
}
Next chapter...
What you will learn in the next chapter: