【JAVA核心技術】新I/O__JAVA

以下程式清單用於計算檔案的32位的迴圈冗餘校正和(CRC32)這個數值就是經常用來判斷一個檔案是否已損壞的校正和，因為檔案損壞極有可能導致校正和改變。java.util.zip包

包含一個CRC32類，可以使用迴圈來計算一個位元組序列的校正和。

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 FileInputStream(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 BufferedInputStream(new FileInputStream(filename));      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 = new 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 FileInputStream(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");   }}

實驗結果如下：

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

記憶體映射要比帶緩衝的順序讀取還要快。