Java 内存映射文件MappedByteBuffer

• MappedByteBuffer是java nio引入的文件内存映射方案，读写性能极高。在NIO中主要用到普通的输入流，带缓冲的输入流，RandomAccessFile和MappedByteBuffer。

现在我们来看看这四种流的效率，废话少说直接上代码。

我们采用CRC32来循环冗余校验。CRC32在java.util.zip包中。

1.普通的输入流。

 public static long checksumInputStream(Path filename) throws IOException
   {
      try (InputStream in = Files.newInputStream(filename))
      {
         CRC32 crc = new CRC32();

         int c;
         while ((c = in.read()) != -1)
            crc.update(c);
         return crc.getValue();
      }
   }

2.带缓冲的输入流

   public static long checksumBufferedInputStream(Path filename) throws IOException
   {
      try (InputStream in = new BufferedInputStream(Files.newInputStream(filename)))
      {
         CRC32 crc = new CRC32();

         int c;
         while ((c = in.read()) != -1)
            crc.update(c);
         return crc.getValue();
      }
   }

3.RandomAccessFile

 public static long checksumRandomAccessFile(Path filename) throws IOException
   {
      try (RandomAccessFile file = new RandomAccessFile(filename.toFile(), "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();
      }
   }

4.MappedByteBuffer

   public static long checksumMappedFile(Path filename) throws IOException
   {
      try (FileChannel channel = FileChannel.open(filename))
      {
         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();
      }
   }

每个流对应的方法写好之后我们来开始测试

public static void main(String[] args) throws IOException {

        String name = "txt.txt";
        Path filename = Paths.get(name);
        long start, crcValue, end;

        System.out.println("Input Stream:");
        start = System.currentTimeMillis();

        crcValue = checksumInputStream(filename);
        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(filename);
        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(filename);
        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(filename);
        end = System.currentTimeMillis();
        System.out.println(Long.toHexString(crcValue));
        System.out.println((end - start) + " milliseconds");
    }

　　为了保证每种流相互之间对结果没有影响，我们可以分别把main方法中其他流的代码注释掉。

最后可以看到MappedByteBuffer效率最高，所消耗的时间最少。