sum.java

jdk-6u10-docs java开发宝典

/*
 * @(#)Sum.java	1.3 01/12/13
 * Compute 16-bit checksums for a list of files, in the style of the 
 * BSD "sum" command.  Uses NIO mapped byte buffers for speed.
 *
 * Copyright 2001-2002 Sun Microsystems, Inc. All Rights Reserved.
 *
 * Redistribution and use in source and binary forms, with or 
 * without modification, are permitted provided that the following 
 * conditions are met:
 * 
 * -Redistributions of source code must retain the above copyright  
 * notice, this  list of conditions and the following disclaimer.
 * 
 * -Redistribution in binary form must reproduct the above copyright 
 * notice, this list of conditions and the following disclaimer in 
 * the documentation and/or other materials provided with the 
 * distribution.
 * 
 * Neither the name of Sun Microsystems, Inc. or the names of 
 * contributors may be used to endorse or promote products derived 
 * from this software without specific prior written permission.
 * 
 * This software is provided "AS IS," without a warranty of any 
 * kind. ALL EXPRESS OR IMPLIED CONDITIONS, REPRESENTATIONS AND 
 * WARRANTIES, INCLUDING ANY IMPLIED WARRANTY OF MERCHANTABILITY, 
 * FITNESS FOR A PARTICULAR PURPOSE OR NON-INFRINGEMENT, ARE HEREBY 
 * EXCLUDED. SUN AND ITS LICENSORS SHALL NOT BE LIABLE FOR ANY 
 * DAMAGES OR LIABILITIES  SUFFERED BY LICENSEE AS A RESULT OF  OR 
 * RELATING TO USE, MODIFICATION OR DISTRIBUTION OF THE SOFTWARE OR 
 * ITS DERIVATIVES. IN NO EVENT WILL SUN OR ITS LICENSORS BE LIABLE 
 * FOR ANY LOST REVENUE, PROFIT OR DATA, OR FOR DIRECT, INDIRECT, 
 * SPECIAL, CONSEQUENTIAL, INCIDENTAL OR PUNITIVE DAMAGES, HOWEVER 
 * CAUSED AND REGARDLESS OF THE THEORY OF LIABILITY, ARISING OUT OF 
 * THE USE OF OR INABILITY TO USE SOFTWARE, EVEN IF SUN HAS BEEN 
 * ADVISED OF THE POSSIBILITY OF SUCH DAMAGES.
 * 
 * You acknowledge that Software is not designed, licensed or 
 * intended for use in the design, construction, operation or 
 * maintenance of any nuclear facility. 
 */

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


public class Sum {

    // Compute a 16-bit checksum for all the remaining bytes
    // in the given byte buffer
    //
    private static int sum(ByteBuffer bb) {
	int sum = 0;
	while (bb.hasRemaining()) {
	    if ((sum & 1) != 0)
		sum = (sum >> 1) + 0x8000;
	    else
		sum >>= 1;
	    sum += bb.get() & 0xff;
	    sum &= 0xffff;
	}
	return sum;
    }

    // Compute and print a checksum for the given file
    //
    private static void sum(File f) throws IOException {

	// Open the file and then get a channel from the stream
	FileInputStream fis = new FileInputStream(f);
	FileChannel fc = fis.getChannel();

	// Get the file's size and then map it into memory
	int sz = (int)fc.size();
	MappedByteBuffer bb = fc.map(FileChannel.MapMode.READ_ONLY, 0, sz);

	// Compute and print the checksum
	int sum = sum(bb);
	int kb = (sz + 1023) / 1024;
	String s = Integer.toString(sum);
	System.out.println(s + "\t" + kb + "\t" + f);

	// Close the channel and the stream
	fc.close();
    }

    public static void main(String[] args) {
	if (args.length < 1) {
	    System.err.println("Usage: java Sum file...");
	    return;
	}
	for (int i = 0; i < args.length; i++) {
	    File f = new File(args[i]);
	    try {
		sum(f);
	    } catch (IOException x) {
		System.err.println(f + ": " + x);
	    }
	}
    }

}