Java NIO 内存映射文件测试

　　内存映射文件是java nio的一个新特性。它是利用虚拟内存将一个文件或者文件的一部分映射到内存。这样，这个文件就可以当作内存数组一样访问，它比普通的文件操作要快很多。抄了个例子测试一下普通输入流、带缓冲的输入流、随机访问文件、内存映射文件在不同文件大小的访问时的情况。

如下是测试代码：

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(args[0]);
      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(args[0]);
      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(args[0]);
      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(args[0]);
      end = System.currentTimeMillis();
      System.out.println(Long.toHexString(crcValue));
      System.out.println((end - start) + " milliseconds");
   }
}

测试结果（单位为毫秒）：

总结一下：

　　1.中小文件操作没有必要使用内存映射文件。

　　2.随机访问文件会造成一定的性能受损，这是无法避免的。

　　3.内存映射文件优于带缓冲的输入流。但内存映射最大能支持2G。在几百M以上的文件读写比较合适使用。