"JAVA Core technology" new I/o__java

The following list is used to compute the 32-bit cyclic redundancy checksum (CRC32) of a file that is often used to determine whether a file is corrupted, because file corruption can lead to checksum changes. Java.util.zip Bag

Contains a CRC32 class that can use loops to compute the checksum of a sequence of bytes.

Package cn.ls;
Import java.io.*;
Import java.nio.*;
Import java.nio.channels.*;

Import java.util.zip.*; /** * This program computes the CRC checksum of a file. 
   <br> * Usage:java niotest filename * @version 1.01 2004-05-11 * @author Cay Horstmann * * public class Niotest { public static long Checksuminputstream (String filename) throws IOException {InputStream in = new FILEINPUTST
      Ream (filename);

      CRC32 CRC = New CRC32 ();
      int C;
      while ((c = in.read ())!=-1) crc.update (c);
   return Crc.getvalue (); public static long Checksumbufferedinputstream (String filename) throws IOException {InputStream in = new B
      Ufferedinputstream (filename) (new FileInputStream);

      CRC32 CRC = New CRC32 ();
      int C;
      while ((c = in.read ())!=-1) crc.update (c);
   return Crc.getvalue (); public static long Checksumrandomaccessfile (String filename) throws IOException {Randomaccessfile file = n EW RandoMaccessfile (filename, "R");
      Long length = File.length ();

      CRC32 CRC = New CRC32 ();
         for (long p = 0; p < length; p++) {File.seek (P);
         int c = File.readbyte ();
      Crc.update (c);
   return Crc.getvalue (); public static long Checksummappedfile (String filename) throws IOException {FileInputStream in = new Filein
      Putstream (filename);

      FileChannel channel = In.getchannel ();
      CRC32 CRC = New CRC32 ();
      int length = (int) channel.size ();

      Mappedbytebuffer buffer = Channel.map (FileChannel.MapMode.READ_ONLY, 0, length);
         for (int p = 0; p < length; p++) {int c = Buffer.get (p);
      Crc.update (c);
   return Crc.getvalue ();
      public static void Main (string[] args) throws IOException {System.out.println ("Input Stream:");
      Long start = System.currenttimemillis ();
      Long Crcvalue = Checksuminputstream ("f:/x.html"); Long end = System. Currenttimemillis ();
      System.out.println (long.tohexstring (Crcvalue));

      System.out.println ((End-start) + "milliseconds");
      System.out.println ("Buffered Input Stream:");
      Start = System.currenttimemillis ();
      Crcvalue = Checksumbufferedinputstream ("f:/x.html");
      End = System.currenttimemillis ();
      System.out.println (long.tohexstring (Crcvalue));

      System.out.println ((End-start) + "milliseconds");
      System.out.println ("Random Access File:");
      Start = System.currenttimemillis ();
      Crcvalue = Checksumrandomaccessfile ("f:/x.html");
      End = System.currenttimemillis ();
      System.out.println (long.tohexstring (Crcvalue));

      System.out.println ((End-start) + "milliseconds");
      SYSTEM.OUT.PRINTLN ("Mapped File:");
      Start = System.currenttimemillis ();
      Crcvalue = Checksummappedfile ("f:/x.html");
      End = System.currenttimemillis ();
      System.out.println (long.tohexstring (Crcvalue)); System.out.println ((End-start) + "milliseconds");
 }
}

The experimental results are as follows:

Input Stream:
599af5f2
2303 milliseconds
Buffered Input Stream:
599af5f2
Milliseconds
Random Access File:
599af5f2
3038 milliseconds
Mapped File:
599af5f2
Milliseconds

Memory mappings are faster than sequential reads with Buffering.