Java中使用内存映射实现大文件上传实例

在处理大文件时，如果利用普通的FileInputStream 或者FileOutputStream 抑或RandomAccessFile 来进行频繁的读写操作，都将导致进程因频繁读写外存而降低速度.如下为一个对比实验。
复制代码 代码如下:
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,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(); 
        } 

    } 

}

测试文件为一个大小为1253244字节的文件。测试结果：
复制代码 代码如下:
sum:220152087 time:1464 
sum:220152087 time:72 
sum:220152087 time:25

说明读数据无误。删去其中的数据处理部分。
复制代码 代码如下:
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(); 
        } 

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