zoukankan      html  css  js  c++  java
  • 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。

  • 相关阅读:
    C++对象模型
    GUID 转 char *
    MFC中App,Doc,MainFrame,View各指针的互相获取
    MFC中设置某个VIEW为当前视图
    MFC中如何在CMainFrame类中访问CxxxView视图类中的成员
    GetMessage 和 PeekMessage 区别
    STL 常用容器
    收发ICMP封包,实现ping
    软件弹窗广告
    python 输入一个字符串,打印出它所有的组合
  • 原文地址:https://www.cnblogs.com/zhangchaozheng/p/2493465.html
Copyright © 2011-2022 走看看