6.2 Copying Files

Example 6-2 is a New I/O version of the FileCopy class of Example 3-2. It copies the file named in its first command-line argument to the file named second or, if there is no second argument, to standard output. This example demonstrates a special bulk-transfer method that is unique to the FileChannel class: the transferTo( ) method copies the entire contents of the file to the specified channel without the need for any explicitly allocated ByteBuffer objects. This method is particularly useful for web servers and other applications that transfer file contents. The transferFrom( ) method performs the reverse operation and can be useful for programs such as FTP clients.

Note that this example omits the safety features of the original FileCopy program and can overwrite existing files, so be careful when using it.

Example 6-2. FileCopy2.java

package je3.nio;
import java.io.*;
import java.nio.channels.*;

/**
 * FileCopy2.java: this program copies the file named in its first argument
 * to the file named in its second argument, or to standard output if there
 * is no second argument.
 **/
public class FileCopy2 {
    public static void main(String[  ] args) {
        FileInputStream fin = null;   // Streams to the two files.
        FileOutputStream fout = null; // These are closed in the finally block.
        try {
            // Open a stream to the input file and get a channel from it
            fin = new FileInputStream(args[0]);
            FileChannel in = fin.getChannel( );

            // Now get the output channel
            WritableByteChannel out;
            if (args.length > 1) { // If there is a second filename
                fout = new FileOutputStream(args[1]);  // open file stream
                out = fout.getChannel( );               // get its channel
            }
            else { // There is no destination filename
                out = Channels.newChannel(System.out); // wrap stdout stream
            }

            // Query the size of the input file
            long numbytes = in.size( );

            // Bulk-transfer all bytes from one channel to the other.
            // This is a special feature of FileChannel channels.
            // See also FileChannel.transferFrom( )
            in.transferTo(0, numbytes, out); 
        }
        catch(IOException e) {
            // IOExceptions usually have useful informative messages.
            // Display the message if anything goes wrong.
            System.out.println(e);
        }
        finally {
            // Always close input and output streams.  Doing this closes
            // the channels associated with them as well.
            try {
                if (fin != null) fin.close( );
                if (fout != null) fout.close( );
            }
            catch(IOException e) {  }
        }
    }
}