C++中的extern “C”用法詳解

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在處置年夜文件時，假如應用通俗的FileInputStream 或許FileOutputStream 抑或RandomAccessFile 來停止頻仍的讀寫操作，都將招致過程因頻仍讀寫外存而下降速度.以下為一個比較試驗。

package test; 
 
import java.io.BufferedInputStream; 
import java.io.FileInputStream; 
import java.io.FileNotFoundException; 
import java.io.IOException; 
import java.io.RandomAccessFile; 
import java.nio.MappedByteBuffer; 
import java.nio.channels.FileChannel; 
 
public class Test { 
 
   
  public static void main(String[] args) { 
    try { 
      FileInputStream fis=new FileInputStream("/home/tobacco/test/res.txt"); 
      int sum=0; 
      int n; 
      long t1=System.currentTimeMillis(); 
      try { 
        while((n=fis.read())>=0){ 
          sum+=n; 
        } 
      } catch (IOException e) { 
        // TODO Auto-generated catch block 
        e.printStackTrace(); 
      } 
      long t=System.currentTimeMillis()-t1; 
      System.out.println("sum:"+sum+" time:"+t); 
    } catch (FileNotFoundException e) { 
      // TODO Auto-generated catch block 
      e.printStackTrace(); 
    } 
     
    try { 
      FileInputStream fis=new FileInputStream("/home/tobacco/test/res.txt"); 
      BufferedInputStream bis=new BufferedInputStream(fis); 
      int sum=0; 
      int n; 
      long t1=System.currentTimeMillis(); 
      try { 
        while((n=bis.read())>=0){ 
          sum+=n; 
        } 
      } catch (IOException e) { 
        // TODO Auto-generated catch block 
        e.printStackTrace(); 
      } 
      long t=System.currentTimeMillis()-t1; 
      System.out.println("sum:"+sum+" time:"+t); 
    } catch (FileNotFoundException e) { 
      // TODO Auto-generated catch block 
      e.printStackTrace(); 
    } 
     
    MappedByteBuffer buffer=null; 
    try { 
      buffer=new RandomAccessFile("/home/tobacco/test/res.txt","rw").getChannel().map(FileChannel.MapMode.READ_WRITE, 0, 1253244); 
      int sum=0; 
      int n; 
      long t1=System.currentTimeMillis(); 
      for(int i=0;i<1253244;i++){ 
        n=0x000000ff&buffer.get(i); 
        sum+=n; 
      } 
      long t=System.currentTimeMillis()-t1; 
      System.out.println("sum:"+sum+" time:"+t); 
    } catch (FileNotFoundException e) { 
      // TODO Auto-generated catch block 
      e.printStackTrace(); 
    } catch (IOException e) { 
      // TODO Auto-generated catch block 
      e.printStackTrace(); 
    } 
 
  } 
 
} 

測試文件為一個年夜小為1253244字節的文件。測試成果：

sum:220152087  time:1464

sum:220152087  time:72

sum:220152087  time:25

解釋讀數據無誤。刪去個中的數據處置部門。

package test; 
 
import java.io.BufferedInputStream; 
import java.io.FileInputStream; 
import java.io.FileNotFoundException; 
import java.io.IOException; 
import java.io.RandomAccessFile; 
import java.nio.MappedByteBuffer; 
import java.nio.channels.FileChannel; 
 
public class Test { 
 
   
  public static void main(String[] args) { 
    try { 
      FileInputStream fis=new FileInputStream("/home/tobacco/test/res.txt"); 
      int sum=0; 
      int n; 
      long t1=System.currentTimeMillis(); 
      try { 
        while((n=fis.read())>=0){ 
          //sum+=n; 
        } 
      } catch (IOException e) { 
        // TODO Auto-generated catch block 
        e.printStackTrace(); 
      } 
      long t=System.currentTimeMillis()-t1; 
      System.out.println("sum:"+sum+" time:"+t); 
    } catch (FileNotFoundException e) { 
      // TODO Auto-generated catch block 
      e.printStackTrace(); 
    } 
     
    try { 
      FileInputStream fis=new FileInputStream("/home/tobacco/test/res.txt"); 
      BufferedInputStream bis=new BufferedInputStream(fis); 
      int sum=0; 
      int n; 
      long t1=System.currentTimeMillis(); 
      try { 
        while((n=bis.read())>=0){ 
          //sum+=n; 
        } 
      } catch (IOException e) { 
        // TODO Auto-generated catch block 
        e.printStackTrace(); 
      } 
      long t=System.currentTimeMillis()-t1; 
      System.out.println("sum:"+sum+" time:"+t); 
    } catch (FileNotFoundException e) { 
      // TODO Auto-generated catch block 
      e.printStackTrace(); 
    } 
     
    MappedByteBuffer buffer=null; 
    try { 
      buffer=new RandomAccessFile("/home/tobacco/test/res.txt","rw").getChannel().map(FileChannel.MapMode.READ_WRITE, 0, 1253244); 
      int sum=0; 
      int n; 
      long t1=System.currentTimeMillis(); 
      for(int i=0;i<1253244;i++){ 
        //n=0x000000ff&buffer.get(i); 
        //sum+=n; 
      } 
      long t=System.currentTimeMillis()-t1; 
      System.out.println("sum:"+sum+" time:"+t); 
    } catch (FileNotFoundException e) { 
      // TODO Auto-generated catch block 
      e.printStackTrace(); 
    } catch (IOException e) { 
      // TODO Auto-generated catch block 
      e.printStackTrace(); 
    } 
 
  } 
 
} 

測試成果：

sum:0  time:1458

sum:0  time:67

sum:0  time:8

因而可知，將文件部門或許全體映照到內存落後行讀寫，速度將進步許多。

這是由於內存映照文件起首將外存上的文件映照到內存中的一塊持續區域，被當做一個字節數組停止處置，讀寫操作直接對內存停止操作，爾後再將內存區域從新映照到外存文件，這就節儉了中央頻仍的對外存停止讀寫的時光，年夜年夜下降了讀寫時光。

以上這篇Java頂用內存映照處置年夜文件的完成代碼就是小編分享給年夜家的全體內容了，願望能給年夜家一個參考，也願望年夜家多多支撐。